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SILICON.

4 LP Resurrection: Transferring LPs & Tapes To CD PC multimedia technology makes it easy to transfer your LPs and tapes to CD. You can even clean up the sound in the process — by Greg Swain

14 Biorecognition: Checking Your Identity Keys and access cards are on the way out. Biorecognition machines can check that you really are who you say you are — by Jon Reid

32 Look Mum, No Cables Wireless networks are gaining in popularity and costs are coming down. Here’s one that’s a cinch to install — by Greg Swain

LP Resurrection: Transferring LPs & Tapes To CD - Page 4.

22 The LP poctor. Came Up Clicks & Pops It cleans up clicks, pops and noise on LPs and even includes a stereo RIAA phono preamplifier — by Leo Simpson & John Clarke

36 The WaveMaker: An Arbitrary Waveform Generator Use this simple circuit with your PC to generate oddball waveforms. It does standard sine, triangle and square waves as well — by David Sibley

66 2-Channel Guitar Preamplifier, Pt.3

The LP Doctor: Cleaning Up Clicks & Pops — Page 22.

Final article shows you how to build the preamps and reverb module into a metal rack case — by John Clarke

72 Digital Reverb — The Missing Pages We messed up and two pages from last month’s article went missing. The person responsible has been sent to a Siberian salt mine.

76 PIC Programmer & TestBed Got a concept you'd like to try before building a full prototype? This unit lets you test new PIC circuits and is great for learning too — by Barry Hubble.

The WaveMaker Arbitrary Waveform Generator — Page 36.

56 Sirvicanany s Le Ain’t no mountain high enough — by the TV Serviceman

86 Vintage Radio The 32V 5-Valve Operatic Mignon — by Rodney Champness

2 53 55 61 64

Publisher’s Letter Product Showcase Electronics Showcase Circuit Notebook Mailbag

75 91 93 94 96

Subscriptions Form Ask Silicon Chip Notes & Errata Market Centre Advertising Index

PIC Programmer And Page 76. JANUARY 2001

1

PUBLISHER’S LETTER www. siliconchip. com. au Publisher & Editor-in-Chief

Leo Simpson, B. Bus., FAICD

Production Manager _ Greg Swain, B.Sc.(Hons.) :

Technical Staff | John Clarke, B.E.(Elec.) | Peter Smith

;

Ross Tester Rick Walters

— —

Reader Services | Ann Jenkinson

Advertising Enquiries. Rick Winkler Phone (02) 9979 5644 . Fax (02) 9979 6503

Mobile: 0408 34 6669

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Regular Contributors Brendan Akhurst | Louis Challis Rodney Champness Garry Cratt, VK2YBX Julian Edgar, Dip.T.(Sec.), B.Ed Mike Sheriff, B.Sc, VK2YFK Philip Watson, MIREE, VK2ZPW Bob Young

SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 003 205 490. ABN 49 003 205 490 All material copyright ©. No part of this publication may be reproduced without the written consent of the publisher. _ Printing: NSW.

Hannanprint, :

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Distribution: Network Distribution

Company. Subscription rates: $69.50 per — year in Australia. For overseas | rates, see the subscription page in. this iissue.

Editorial & advertising offices: Unit 8, 101 Darley St, Mona Vale,

NSW 2103. Postal address: PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 9979 5644. Fax (02) 9979 6503.

E-mail: silchip@ siliconchip.com.au

ISSN 1030-2662 * Recommended and maximum price only. 2

SILICON CHIP

Technology has its benefits — and its drawbacks OK, how many of you noticed the obvious mistake in the December issue? There were no prizes for spotting it and it caused a great deal of chagrin, if you can use such a mild term, in the SILICON CHIP offices. If you didn’t spot the error, I’1l put it down to having flitted over those two pages (twice) and not being particularly interested in the affected article on digital reverberation.

_ For those who spotted it and wondered just how such a painful mistake could occur, the answer is, all too easily. Nor can we fall back on the standard excuse, used countless times by publishers in years past, that it “was a printer’s error”. These days any mistakes you see are entirely ours, apart from the occasional page having poor colour because of registration problems. No, since our magazine is produced CTP — computer to plate — the printer has very little input into the final product. The entire editorial contents of SILICON CHIP and the advertising content is all compiled as computer files and sent to the printer on a CD-ROM. Any corrections to the proofs are either emailed or sent to our FTP (file transfer protocol) site and they are then zotted into the computer at Dubbo in the blink of an eye. And that’s how the error occurred. Some small changes needed to be made to the proofs on pages 84 & 85 and these were quickly made to the original file and zapped up to the printer. Trouble was, the pages were laid out using the previous month’s pages as a template. That’s a pretty standard procedure but the previous month’s equivalent pages were 42 & 43. This was not noted, the changes were made and sent and no-one realised that pages 42 & 43 had just been changed — for the worse.

This story has been a little simplified but that is the gist of it. Could it happen again? I sincerely hope not but you never know. That is one of the drawbacks of computer technology. You can make changes so easily and without careful checking (which did not happen) things can go awTy. As soon as we discovered the error we placed the missing pages on our website and they are reproduced this month, starting on page 72. We

apologise to all those readers who were inconvenienced.

Having said all the above, we don’t shrink from continuing to adopt the new technology. It has given us considerable production benefits, better print quality and helped to control costs. We could not and would - not wish to go back to the old days when everything was in the hands of the printers. But it would have been nice to use the excuse “because of a printer’s error”. Since most of you will be reading this at the end of December, I wish all of you a very prosperous New Year. We are very much looking forward to the new millennium.

Leo Simpson

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they can’t be bothered playing them because of the surface noise, clicks and pops. The truth is, we’ve all been spoilt by the pristine, noisefree sound of CDs. In fact, if you have not listened to some of your old LPs for a while, you might be shocked at just how bad they sound... There used to be two solutions to the problem of clicks, pops and noise on LP (long-play) records. You could (try to!) ignore them — not particularly satisfactory. Or you could download your LP to your computer via your sound card and use one of the many software programs which allow you to edit audio files before you dub them to CDs. You can then play the new disk on your home CD or DVD player, in your car or portable disk player, and never have to play the old vinyl record again. To find out more about this approach, go to our feature article on this subject starting on page 4. For many people, the above approach is the complete answer. They get a CD which sounds better and they can play it any time they like. But as always, there are drawbacks to the computer/software/CD approach. First, you do need a reasonably fast Pentium or equivalent computer with 22

SILICON CHIP

a late model sound card, a CD Writer and the necessary software, all of which can add up to a pretty big investment.

Second, as good as these sound editing programs are, only the more expensive let you listen to the disk as the processing takes place. In other words, they don’t all do this magic processing “on the fly”. Instead, you _have to play the record into your computer via an RIAA preamplifier and sound card and store the various tracks

your old vinyl records, you will not have the satisfaction of looking at the sleeve notes while you are listening or deciding which track to play next. That’s one of the nice features of the old 12-inch disks. Sigh... Finally, even if the process of CD dubbing is entirely satisfactory, who is going to go to the trouble of dubbing their entire LP collection to CD, just for the benefit of being able to listen to any or all of them occasionally? So if you a large collection of records, you need an alternative to CD dubbing, one where you can listen to any of them at any time, substantially free of clicks and pops. You can do this by playing them through the LP Doctor. LP Doctor + CD Writer Maybe you don’t have computer but you may want to consider a CD Recorder such as the

as .WAV files on your hard disk. Then you set the software to work to process the .WAV files and finally, they can be dubbed onto a CD with your CD Writer. By the way, you are going to need a pretty big hard disk (say with at least 1 or 2 Gigabytes to spare) to store and

process these .WAV files. Third, and this is more of a subjective problem than a technical one, even if you do create CDs to replace

Marantz DR6000 pictured elsewhere in this article. These make dubbing all sorts of audio material to a CD a real doddle but they do not include any means of removing noise, clicks and pops from your LP records.

This is where the LP Doctor can also play a part. You CD Recorder via the a very worthwhile ment, especially on music records; ie,

dub your LPs toa LP Doctor and get sound improvejazz and classical those with quiet

SILICG CHip ™

~P DOCTOR

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The LP Doctorcanbe _— teamed with any turntable having a magnetic cartridge so you can listen to _ your LPs without clicks and pops or you can feed its :Agee into a PC sound card for processing and dubbing to CD.

passages which can be really plagued with clicks and pops. Or maybe you do have a computer, CD Writer and so on but you are without a sound editing program. Here again, you can dub your records to CD on your computer via the LP Doctor.

LP Doctor — what it does OK, we’ve talked about where and why the LP Doctor might be used but we have not been specific about what it does, apart from the general theme

of click and pop removal. Let’s describe specifically what it does. The LP Doctor has two independent channels which monitor the left and right signals. When a click or pop is sensed, the signal is muted for a brief interval to greatly reduce its amplitude or completely remove it. At the same time, it can apply a slight degree of treble cut filtering, to reduce surface noise and also make the click attenuation more effective.

While it does work well at greatly

preamplifier is fed via

a delay so that the click detector has sufficient time to detect and fully mute signal transients.

Where the LP Doc-

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diagram shows the left channel only. The signal from the RIAA |

speakers (or wear headphones) then

the LP Doctor will also have trouble. On the other hand, while many pop and rock records might have a fairly serious amount of clicks and pops, the general signal level is usually so consistently high that it does not matter — the music masks the noise. So really it is records that have a wide dynamic range, such as classical music and jazz, with quiet passages among the louder ones, that are

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reducing bad clicks and pops, it does not do well on low level clicks which are difficult to discern from the general program signal. In other words, if you have trouble hearing clicks and pops unless you listen up close to the

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But the LP Doctor does not fix bad surface noise which may be caused by lots of dirt being ingrained into the record grooves or may be the result of fungus attack on records which have been stored for long periods in fairly humid conditions. In fact, if you have a record which has been sub-

JANUARY 2001

23

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Fig.3: the overall performance of the magnetic cartridge preamplifier, measured by applying an inverse RIAA signal to the input. Our preamplifier is not ideal but it is pretty close —the curve deviates by no more than +0.3dB over the whole frequency range from 20Hz to 20kHz.

ject to a bad fungus attack, there is nothing you can do because the damage is permanent. And even if you do use the LP Doctor on a regular basis, there is still no substitute for keeping your records as clean as possible and also making sure that your stylus is free of any gunk that may be picked up from the record grooves. In fact, you really need to examine the stylus after every record side has been played, to make sure that it is clean. By the way, if you do lots of critical listening to CDs via headphones, you probably still won’t enjoy vinyl records after they have been processed by the LP Doctor. Let’s be honest: there

is still going to be a huge difference in sound quality between an old LP and a pristine CD.

Operating features | The LP Doctor is housed in rackmounting case measuring 426mm (W) x 44.5mm (H) x 277mm (D), not in-

cluding the side-mounting flanges. Of course, you don’t have to mount a rack-mounting case in a rack! There are three knobs on the front panel: an output level control, a click sensitivity control and a selector switch with three positions: Bypass, Process (click suppression) and Filter. The lat-

ter setting adds treble cut to the click suppression, as noted above.

There are also three indicators on the front panel. Two indicate whenever a Click is detected in either channel while the third is a clipping indicator. The output level control needs to be adjusted so that there is normally no clipping. As well as providing click suppression, the LP Doctor includes a high performance RIAA preamplifier, making it compatible with virtually any amplifier, PC sound card or free-standing CD Recorder. The preamplifier is designed to suit the majority of moving magnet cartridges but may not have enough gain to suit moving coil cartridges. On the rear panel, there are two pairs of RCA sockets. One pair is for the magnetic cartridge signals while the other pair is for the line level output signals to a stereo amplifier, sound card or CD Recorder.

Operating principles So how does the LP Doctor eliminate record clicks and pops? Essentially, each channel has a comparator which looks for the sudden large signal excursions which produce a click Or pop. When the click signal is detected, the audio signal is muted (switched off) to prevent the click from passing through to the output. The signal is shut off for a short period (about seven milliseconds) so that it is more or less

unnoticed by the listener. The problem is, by the time the comparator has detected the signal and has muted it, some of the click will have already passed through to the output. So only part of the click or pop will have been muted, resulting in a shorter click but still just as loud and annoying.

Specifications Frequency response of RIAA phono res Signal-to-Noise

‘560mV; -88dB A-weighted (-840B and -89dB in Bypass mode, respectively) 0.3% at 1kHz and 1V, 3% @ 10kHz @-200BV (0.008% @ ikHz and 01% @

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. 190mV RMS atkHz.

Maximum input signal ..

Signal delay time ......------++-+-+ Click muting time

26

SILICON CHIP

within 40.3dB from 20Hz to 20kHz (seeFig 3) to 10mV input at 1kHz and respect with 20kHz) to (20Hz -83dB unweighted

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Inside the LP Doctor. It uses a digital delay in each channel for effective muting of clicks and pops.

Therefore, we need to incorporate a delay circuit. This allows for the response time of the comparator and muting circuit such that the delayed signal has the full muting applied to it. In this way, the comparator/muting circuit can attenuate the whole of the click or pop instead of just the latter part of it.

Block diagram Fig.1 shows the block diagram of the LP Doctor. For simplicity, only the left channel is depicted; both channels are completely separate and identical. First, the signal from the magnetic cartridge is fed to the RIAA (Recording Industry Association of America) preamplifier. From there, the amplifier signal is fed via four separate paths: (1) straight

through to the output via the bypass position on switch S82; (2) to the onemillisecond digital delay unit (IC3) which converts the analog signal to a digital signal, feeds it into memory, shuffles it out again with the required delay and converts it back to analog; (3) to the click detector which oper-

ates switch IC4 where click muting takes place and (4) to the clipping detector which drives a front panel

LED to indicate that the signal level is too high.

Circuit description The full circuit of the LP Doctor, apart from its power supply, is shown in Fig.2. As with the block diagram, we are only showing the left channel. The equivalent IC and other device numbers for the right channel are shown in brackets. The phono (magnetic cartridge) signal is fed directly from the input socket via inductor L1, a 150Q resistor and a 47uF bipolar capacitor to the noninverting input, pin 3, of op amp IC1a. The inductor, series resistor and shunt 100pF capacitor form a filter to attenuate RF signals which may be picked up by the phono leads. The RIAA equalisation is provided by the feedback network comprising

pacitor to the 10kQ potentiometer VR1. There is also rolloff in following stages to ensure that the signal below 20Hz is attenuated. This rolloff prevents amplification of record warp and rumble which occur at sub-audible frequencies but could possibly overload an amplifier and loudspeakers. The gain of IC1a is 22.45 (27dB) at

1kHz with a boost of +13.1dB at 100Hz and a cut of -13.75dB at 10kHz. Witha typical magnetic cartridge output, the gain provides us with a nominal 100mV of signal. This is further boosted by op amp IC2a, by a factor of 11, to produce a nominal signal level of 1.1V. The frequency response graph of Fig.3 shows the overall performance of the RIAA preamplifier. It was measured by applying an inverse RIAA

The circuit also includes the IEC recommendation for rolloff below 20Hz. This is provided by the 1kQ

signal to the preamplifier. The response should be a dead-flat line if the RIAA preamplifier is ideal. Our preamplifier is not ideal but it is pretty close — the curve deviates by no more than +0.3dB over the whole frequency range from 20Hz to 20kHz. IC6a & IC6b are comparators which form the clipping detector and they monitor the signals from IC2a (left

resistor in series with the 10pF ca-

channel) and IC2b (right channel) via

pacitor and by the 10uF coupling ca-

10kQ resistors and a 10yF coupling

16kQ and 200kQ resistors and the .0047uF and .015uF capacitors, connected between pins 1 & 2. This network provides the standard equalisation time constants of 3180s (50Hz), 318us (500Hz) and 75us (2122Hz).

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capacitor. The non-inverting input of IC6a is held at +1.14V while the inverting input of IC6b is held at -1.14V. _ Thus, IC6a monitors positive swings of the signal while IC6b monitors negative swings. So if the audio signal to the comparators exceeds 2.3V peak-to-peak

16 provides the demodulation of the digital back to analog after the delay within the IC. The 22kQ and 11kQ resistors plus the 560pF and 150pF. capacitors at pins 13 & 14 form an-

other low-pass filter which removes the 500kHz digital switching artifacts from the now-delayed analog signal.

(800mV RMS), LED1 will light.

Muting switch

Digital delay |

The output signal from IC3 passes through a final filter comprising a 270Q resistor and .001pF capacitor and is then fed through a 10uF DC blocking capacitor to the analog switch, IC4. This is an optically controlled FET which has a low impedance when the internal LED is lit and a very high impedance when the LED is off. IC4 is controlled by the click

IC3 provides the digital delay and this is set to provide a time delay of one millisecond by data fed into its pins 4, 5 & 6 at the moment of switchon. This initialising data is provided by ICs 17, 18, 19 & 20. We'll briefly describe their operation later in this article. For the moment, though, all we need to know is that IC3 (IC7 in the right channel) are set to provide a

one-millisecond delay. The input signal from IC2a is coupled into IC3 via some low-pass filter components comprising the 22kQ and 11kQ resistors at pins 23 & 22 of IC3. This filter rolls off at 12dB/octave above 36kHz to prevent quantisation errors in the analog-to-digital conversion process. The .068uF capacitor series 27Q resistor between pins 20 and 21 form the integrator used in the analog to digital (delta-sigma type) conversion. The .068uF capacitor at pins 15 and 28

SILICON CHIP

detection

circuitry involving 1[C11,

C12 & IC13. When switch IC4 isclosed, the signal passes through unaffected to pin 3 of op amp IC5a which is connected as a unity gain buffer. However,

there is a wrinkle here

because the 10kQ resistor and .01pF capacitor following IC4 form a sample-and-hold circuit and the capacitor is constantly following the audio level as it is charged and discharged

via the 10kQ resistor. When

|

switch IC4 is turned off, in

response to a Click, the voltage at pin

3 of IC5a remains roughly where it was until the switch closes again. In this way, the signal is not muted down to zero but to its average level. This results in a less-audible muting effect and it duplicates the muting action of the best software packages in removing clicks. The signal voltage from the sampleand-hold circuit is applied to pin 3 of IC5a via a 10pF bipolar capacitor. This is included to avoid having the input bias voltage for IC5a from being applied to the output side of IC4. If it was, then an audible click would be produced each time IC4 switched on or off. | IC5a’s output is applied direct to switch S2a and becomes the “Proc-

essed” output. It is also fed to op amp IC5b which provides the “Filter” output to switch S2a. IC5a provides a gentle treble cut at 12dB/octave above 10kHz.

Click detection circuitry Apart from the delay circuit just described, the click detection circuit is really the heart of the LP Doctor. It takes the signal from IC2a and further amplifies by 4.7 in IC10a. It is then ACcoupled viaa 1uF bipolar capacitor toa precision full-wave rectifier comprising op amps IC11a and IC11b, diodes D6 & D7 and associated resistors.

Sample

Tek Run: 50kS/s

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Fig.6: the loading process for the delay codes which are’ fed into IC3 (and IC7) at switch-on. Serial data (lower

_trace) is transferred on the negative edge of SCK (centre trace). The REQ line (top trace) must be low before the following 12 clock pulses. The positive edge of REQ signals the end of the serial data stream. On the first clock pulse, the sleep data is input and this must be a low. The following six pulses are the delay codes while next are the low mute and ID1 and ID2 (identification codes). When the input signal goes positive, the output of IC11a goes low, biasing on D7 so that the gain is set by the 10kQ input resistor R1 and the 10kQ feedback resistor R2; thus gain

Fig.7: this is the effect of the delay through IC3 and IC7. The top trace is the input signal and the lower

trace is the delayed signal.

gains gives us +1. For negative input signals the output of ICi1a is clamped high, due to conduction of diode D6 and the cathode of D6 is held at ground, effectively switching the output of IC11a out of circuit. Signal then passes via the 10kQ resistor R3 to pin 6 of IC11b. IC11b inverts the signal and provides gain at -1. Since the input signal is negative, the signal at pin 7 of IC11b is positive. Thus pin 7 of IC11b always goes positive, for both positive and negative swings of the input signal and so we have a precision full-wave rectifier. Trimpots VR2 and VR3 provide offset trimming for IC11a and IC11b respectively. These are set so that pin 1

is -1, This signal is coupled to the inverting input of IC11 via the 5.1kQ resistor. Gain for IC11b is set by the 5.1kQ input resistor and 10kQ feedback re-

sistor between pins 6 and 7 and is therefore -2. Overall gain of the input signal for positive signals is therefore -1x-2=+4+2.

However, there is another path for the input signal via the 10kQ resistor R3 to pin 6 of IC11b. This signal gives a negative signal at the output of IC11b with a gain of -1. Adding the two

and pin 7 of IC11 are at ground (0V)

when no signal is applied.

Comparator has floating threshold The rectified signal from IC11b is applied via the 27kQ resistor and potentiometer VR4a to pin 6, the non-

inverting input of comparator IC12a. It is also applied to pin 5 via a filter network comprising a 4.7kQ resistor and 1pF bipolar capacitor, before being applied to the inverting input of IC12a. So IC12a has a slowly varying DC level at pin 5 and the rapidly moving signal level at pin 6 and it is looking for a sudden transient, ie, a click or pop, which will cause its output at pin 7 to go low.

The Marantz CD Recorder is an attractive alternative to

dubbing your LPs to CD via a

computer. You don’t need a

computer or to learn about new software and the result is much the

sie

a.

ae

cae —

eas

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same, when you process your LPs via the LP Doctor.

JANUARY 2001

29

The oscilloscope waveforms of Fig.4 show how the click detection works. The top trace is the averaged signal at pin 5 of IC12a, while the lower trace is the rectified signal at pin 6. When a transient occurs, pin 6 goes above pin 5 and the comparator output goes low to trigger the monostable timer IC13. IC13 is a CMOS 555 connected as a monostable. It works in two ways. Normally, when pin 7 of IC12a is high IC13 is untriggered (as pin 2 is high) and the

.0068pF at pins 6 & 7 of IC13 will be discharged and the output at pin 3 will - be low. When pin 7 of IC12a goes low, it turns on transistor Q1 to maintain dis-

charge the .0068yF capacitor and low at pin 2 of IC13 triggers the beginning the timing period. The result is a seven-millisecond positive pulse from pin 3 of IC13 and this turns on transistor Q2 which turns

high, to let IC4 unmute via IC13 and Q2.

The scope waveforms of Fig.7 show the effect of the delay through IC3 and IC7. The top trace is the input signal and the lower trace is the delayed signal.

Delay Control

Power

IC3 (and IC7 in the right channel) are set to the 1ms delay time via their REQ, SCK and DATA inputs at pins 4, 5 & 6 at switch-on. IC17 to IC20 are used to provide these clock and data codes and after they have done this, they have no more function in the circuit, until it is next switched on. They function in exactly the same way as in the Digital Reverberation circuit described in last month’s issue so we won’t go into the detail here, except to say that IC20, the 74HC165 serial shift register, is responsible (can we say that about a lowly IC?) for loading in the eight bits of data at switch-on. It is clocked by IC17, the 4060 oscillator/ counter while IC18 & IC19 perform related house-keeping tasks. | The scope waveforms of Fig.6 show the loading process for the delay codes.

The power supply circuit is shown separately on Fig.8. It uses a transformer with two 9V windings connected in series to give an 18V centre-tapped supply. This feeds a full-wave rectifier (diodes D1-D4) and the two 470uF 25VW capacitors. The resulting DC voltage is around +12V. This feeds adjustable 3terminal regulators set to give +7.5V supply rails. The +12V supply is also applied to a 5V regulator, REG3, via diode D5. The diode isolates the supply to the input capacitors of REG3 when power is switched off. The idea is to maintain the +5V supply to the delay ICs (IC3 & IC7) to avoid switch-off thumps. The mains power switch is bypassed with a 250VAC-rated .01pF capacitor. This prevents arcing across the switch when it is switched off. The mains earth connects to the circuit earth via a 0.47uF capacitor to prevent hum in the signal where there is no connection to mains earth in any accompanying amplifier. Should the power amplifier be earthed, then the capacitor will minimise any resulting hum loop. Next month we will complete the presentation of the LP Doctor with all the constructional information and the parts list. SC

low. When the 10pF capacitor charges via the 100kQ resistor, its voltage goes above pin 2 and so pin 1 of IC12b goes

off IC4. 1C13 also drives LED2 which gives a visible indication of the muting action. This is shown in the scope waveforms of Fig.5. This shows only a short click being detected and muted but they can last a lot longer than this so we The serial data (lower trace) is transhave settled for a compromise muting ferred on the negative edge of SCK time of 7ms. (centre trace). The REQ line (top trace) If 1C12a detects a longer transient, it must be low before the following 12 will hold Q1 on and keep the .0068uF clock pulses. The positive edge of REQ capacitor discharged for longer and this signals the end of the serial data stream. will extend the muting period beyond On the first clock pulse, the sleep data the nominal seven-millisecond period. is input and this must be a low. The Comparator IC1i2b is there to provide power-on muting via IC13 and . following six pulses are the delay codes IC4. Initially, the 10yF capacitor at pin - while next are the low mute and ID1 and ID2 (identification codes). 3 of IC12b is discharged and pin 1 is Fig.8: the power supply of the LP Doctor provides +5V and +7.5V rails. The 5V rail powers to the two delay chips. 0.01 250VAC

- 2200uF ¢

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ireless networking

LOOK MUM NO CABLES By GREG SWAIN

Here’s a wireless computer network that’s a cinch to install and get going. It works just like a regular network but without messy cables. IRELESS NETWORKS have two big advantages: convenience and ease of set up. They’re convenient because there are

no cables to install and they’re easy to set up for exactly the same reason. Usually, a wired network (LAN) is

the most cost-effective method but it does tie the PCs to set locations. By contrast, a wireless LAN means that PCs can be moved from one location to another and still have network access — something that can be very useful for mobile users with laptops. A wireless LAN also has advantages in situations where it’s difficult to install cables or if you don’t want to drill holes through walls. And it can save you from digging up concrete paths if you want to “connect” two buildings together. In short, if you can’t go through it, over it or around it, a wireless network is the answer. Wireless network cards Although still relatively expensive, the cost of wireless networking is now on the way down. Diamond Multimedia’s

32

HomeF ree system has been popu-

SILICON CHIP

lar at the bottom end of the market, although hampered by its rather leisurely 1Mb/s transmission speed. By contrast, this new system from Eumitcom Technology (and sold by MicroGram Computers) runs at a brisk 11Mb/s, which is far more useful for business applications. It’s based on PCMCIA wireless networking cards which plug straight into the PCMCIA slots fitted to laptop computers. However, by using a “PCMCIA-to-PCI Adapter”, they can be fitted to conventional PCs as well. Each PCMCIA card is actually a network card and radio transceiver rolled into one. The omnidirectional antenna circuitry for the card is located at one end and protrudes from the laptop (or PC) by about 4cm when the card is pushed into place. The transceiver operates in the 2.4GHz band using spread spectrum wireless technology, to ensure security and reliability. What’s more, the cards don’t really need a separate base station (or access point) to operate. All you have to do is set up several computers with these cards and you have a working network (this is known

as an “Adhoc” network).

Of course, you still have to install the relevant networking protocols, set up workgroups and computer names, and share resources, just as you would for a conventional wired network. The setting up procedure for the PCMCIA card is straightforward — just insert the card into its slot, reboot the computer and install the driver when prompted. You then install the configuration utility software, which is supplied on a separate disk. Transmission mode Among other things, the configuration utility lets you set the channel number and the mode — see Fig.3. The mode is set to “Adhoc” for a small stand-alone wireless network or to “Infrastructure” if you intend using an Access Point. There are also options that allow you to encrypt the transmissions. The configuration utility also shows the

current

transmission

rate

(or

throughput), as well as the link quality and signal strength from an Access Point. The effective range for an Adhoc

_

Fig.2: two or more Access Points can be using to create overlapping “cells” and to link networks together (eg, between buildings). An Access Point is also needed to interface a wireless network to. a conventional wired LAN. : :

FILESERVER

DESKTOP PC

NOTEBOOK WITH WIRELESS PCCARD

ACCESS POINT

:

~S._

PC WITH WIRELESS ~~_” PCI ADAPTER a

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JANUARY 2001

33

eae eG ES ewe

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The PCMCIA cards can either be plugged directly into a notebook computer or used with a PC by installing a PCMCIA -to-PCI adapter or a PC Card Drive.

duced transmission rates. In operation, the system can automatically switch down to either 5.5Mb/s, 2Mb/s or 1Mb/s for line-of-sight ranges of

cutout in the backplane connector — you just slide the card in until it “clicks” home, just as you would with a laptop computer. Installing this card is straightforward enough, although your PC will need to have a couple of spare inter-

50m, 100m and 120m respectively.

rupt request lines (IRQs) — one for the

PCMCIA-to-PCI adapter

adapter card itself and another for the PCMCIA card. The supplied setup dis-

network running at 11Mb/s is about 30 metres

(line of sight), although

ereater distances are possible at re-

Here’s a device that you probably haven’t seen before — it’s called a “Wireless LAN PCMCIA-to-PCI Adapt-

er” and it functions as a PCMCIA card to PCI bridge. In a nutshell, this card plugs into a spare PCI slot on your PC’s motherboard and accepts the wireless PCMCIA network card. The card slot is accessed through a

kette includes

drivers for Windows

95 OSR2, Windows 98, Windows NT and Windows 2000. The setup program automatically identifies the operating system and installs the correct driver. You then slide the PCMCIA wireless LAN card into place and install the driver for this device. In case you’re wondering, the

PCMCIA-to-PCI Adapter works only with the wireless PCMCIA LAN cards. If you want to use other PCMCIA cards in a PC, take a look at MicroGram’s “PC Card Drive”. This device looks very similar to the other unit but has two slots and works with Type 1, 2&3 PCMCIA cards. In fact, it’s the PC Card Drive that’s pictured in this article. Naturally, it’s more expensive than the other adapter card (see below).

Access Point An “Access Point” is required if you

want

to connect

your wireless

LAN to a conventional LAN. In addition, an Access Point effectively doubles the range between wireless PCs on the network, since it acts as a base station. You can also set up multiple Access Points with overlapping coverage areas so that mobile users can freely roam from one “cell” to the next.

Similarly, two Access Points can be

Fig.3: the Wireless LAN Configuration Utility, lets you set the channel number and mode. It also shows the transmission rate and indicates the link quality and signal strength from an Access Point. 34

SILICON CHIP

Fig.4: the Wireless LAN Configuration Utility is automatically launched at start-up and is minimised to the PC’s System Tray.

configuration utility which can be installed on any machine on the net-

Configuration

work — either wireless or wired. Clicking the Scan button in this utility detects the Access Point and displays it as shown in Fig.5. You then assign a temporary IP address to the Access Point (see Fig.6), after which you use the Access Point’s built in web server to assign a permanent address and to make other configuration changes (Fig.7).

Alternatively, you can configure the Access Point to obtain its IP address from a DHCP server on the network. 0

IP Address

The bottom line We tested the system by fitting wireless PCMCIA cards to two computers — one a laptop, the other a conventional PC (via the PC Card Drive). To

complete the setup, we plugged an Access Point into the hub on our wired network.

192.168.0.5

Initially, we set the mode to Adhoc

| |

|

used to link two building together, using Point-to-Point mode. Disarmingly simple in appearance,

the Access Point is housed in a grey plastic box and is fitted with an omnidirectional antenna at the back.

Also on the back panel are an RJ-45 socket for connection to a standard network hub, plus an RS-232 socket which provides alternative connectivity direct to a PC. The Access Point also comes with a

so that we could test the wireless network between the two computers fitted with the PCMCIA cards. The network came straight up — no problems. We then set the mode to Infrastructure” and scanned for the Access Point. It too came straight up and after running the configuration software, we had access between the wireless machines and the wired segment of the LAN. In short, it all worked — what more can we say?

oint - Micro:

Internet

Explorer

Cost and availability Unfortunately, you’re still paying for the R&D for this type of gear, so it’s not exactly as “cheap as chips”. At the

tp./7192.16 i

time of writing, the Access Point (Cat.

11339) was priced at $1155; the Wireless LAN PCMCIA card (Cat.11340) at

$490; and the Wireless LAN PCMCIAto-PCI Station Adapter (Cat.11344) at $79 (prices include GST). _ The PC Card Drive (Cat.6523) re-

tails for $259 but note that you don’t ERY,ely

need this card unless you want to fit

other types of PCMCIA cards to a PC. Normally, you would use the much cheaper PCMCIA-to-PCI Station Adapter instead. Further information is available from MicroGram Computers, Unit 1, 14 Bon Mace Close, Berkeley Vale 2261.

|

|

Fig.7: the Access Point comes with an inbuilt web server. This lets you change the configuration settings, including assigning a permanent IP address, subnet mask and default gateway. The IP can also be obtained from a DHCP server.

Phone

(02) 4389

8444.

Their

website is at www.mgram.com.au or you can email [email protected] for up-to-date pricing. sc

JANUARY 2001

= 355

Need a weird signal waveform for testing a new circuit or to _ produce an unusual sound effect? Here’s a very low cost waveform generator which hooks up to the printer port of a PC and makes it easy to generate oddball waveforms at low frequencies. The software also lets you generate standard waveforms, just like a function generator - and even programmable DC voltage levels.

We )

By David Sible

The Wavemaker (or software-controlled arbitary waveform generator) fits into a small utility box and is connected to the PC’s parallel (printer) port via the multi-way cable coming from the rear. Power is supplied by a 12V DC plugpack. 36

SILICON CHIP

f you’ve ever played with audio circuits (designing, servicing or whatever) you'll know just how essential a signal generator (or function generator) is. But what happens if you want to generate a real “oddball” waveform — not your usual sine or square wave, not even a triangular or sawtooth. Perhaps it’s because an amplifier only misbehaves with certain types of signals. Perhaps it’s simply because you want areally unusual sound effect (eg for a theatrical production). Perhaps it’s for a host of other reasons. How do you go about it? The usual way to generate an ‘arbitrary’ waveform in R&D labs is by using an arbitrary waveform generator. Now that makes sense, doesn’t it! But these are usually big and complex instruments costing big bucks — and they can be very complicated to use, too. They’re a bit like a Formula 1 racecar: even if you could afford one, you probably couldn’t drive it. One or two home-brew arbitrary waveform generator designs have been published but they’ve generally used special components and these too have been pretty expensive. Sometimes you can get away with a low-cost function generator but (usually) these can only create standard waveforms — sine, square, triangle, sawtooth, etc. So if we can’t generate

the waveform we want using a standard function generator, we’re forced to find another way of tackling the problem. When we need an unusual waveform it’s usually at a fairly low frequency — a few hundred hertz or so and often even less. In view of this, it seemed to me that you should be able to produce these waveforms using a really low-cost approach, based on using software running in a PC to send a ‘stream of digital samples’ out to a digital to analog converter (DAC). So I tried it. . . and it worked. That’s how this project came about. The hardware side is really just a ‘DAC in a box’, which hooks up to the PC’s

printer port. The software in the PC does all the tricky part, preparing the waveform samples and sending them to the DAC. The project is called, for fairly obvious reasons, the “Wavemaker”. Speaking of software, I’ve written four separate programs to go with the

Looking inside the case from the front. Note the cutout on the rear panel to allow room for the ribbon cable to exit. Wavemaker. One is a testing program,

static — when the PC is disconnected.)

so you can quickly confirm which printer port the unit is connected to and check that the two are ‘talking to each other’. Another is a simple program which lets you use the Wavemaker as a programmable DC voltage SOUICE. A third program lets you ‘draw’ your arbitrary waveforms on the PC’s screen, and then save them as disk files. And finally there’s the Wavemaker program itself, which drives the generator box and gets it to ‘play’ either arbitrary wave files loaded from disk, or one ofa range of standard ‘function generator’ type waveforms: sine, square, triangle, sawtooth falling or sawtooth rising. More about the software later. Let’s look first at what’s inside the little hardware box.

The latch’s load enable input is also driven from the port’s strobe line, via inverter IC3a. So when the software running in the PC sends a data byte out to the port, the strobe pulse causes

the eight data bits to be latched into IC1 and they accordingly appear at its outputs.

|

Because IC1 is operating from a regulated +5V rail, the voltages at all of these outputs will therefore swing between +5V (for a digital ‘1’) and 0V (for a ‘0’).

But the effect of the binary weighted ladder of resistors is to combine these into a single output DC voltage which automatically ‘scales’ the contributions of each output, according to its position along the ladder. Each position down the ladder contributes half that of the position above it, giving exactly the right proportions we need

Circuit description

to produce the analog equivalent of

As you can see from the circuit, there isn’t much to it: the software does most of the work.

the digital input. For example, when the top-most output (pin 12) goes high, this contributes exactly 2.50V to the output. But when the next output down (pin 9) goes high, it contributes only 1.25V. Similarly when pin 15 goes high, it

Since we’re only working at frequencies up to 2kHz, I decided to use an ‘el-cheapo’ DAC rather than a fancy (expensive!) dedicated DAC chip. So in this case the DAC consists of just a low-cost CMOS octal latch (IC1) and

the network of 20kQ and 10kQ resistors connected to its outputs. These form what’s usually called a ‘binary weighted ladder network’. The inputs of the latch chip are connected to the 8-bit lines of the PC printer port via 100Q suppressor resistors, as you can see. (There are also 1.5kQ pulldown resistors, to prevent the inputs being damaged - eg, by

contributes only 625mV; and so on,

right down to pin 2 which contributes only a whisker under 20mV. If you work them all out you’ll discover this gives quite an accurate digital to analog conversion. The DC output voltage at the top of the ladder varies between a maximum of 5.00V (for a digital input of FF hex, or 255 decimal) and a minimum of 0V

(for 00 hex input), in steps that are very Close to 20mV.

JANUARY 2001

—:37

|

———— — _

To make the output from the generator a little more useful and also to minimise loading on the ladder network, its output is fed to IC2b, half of a TLO72 dual op amp, connected here as a non-inverting buffer with a gain of two. So the output voltage at pin 7 now varies over twice the range from the DAC ladder: from 0 to 10V. This output is then fed through a simple low-pass filter network formed by the 100Q resistor and 0.1pF capacitor, which filter out any sample clock components and ‘glitches’ in the DAC output.

The smoothed output appears across

the 20kQ pot, which allows you to control the maximum output from the generator. From here the signal simply passes through IC2a, the other half of the TL0O72, which is used here as a voltage follower and output buffer. The 680Q resistor in series with the output protects the output of the op amp against damage from accidental shorts. The rest of the circuit is to support this basic DAC and buffer amplifier system. IC3b re-inverts the PC port’s strobe pulse and drives the LED, to indicate when the generator is being driven with data. The same signal is then fed back via IC3c to the port as

the BUSY/READY-bar signal, with the

100Q resistor and .01)F capacitor providing a small amount of delay. This delay gives the DAC time to “digest” the information coming to it before more data is received. While this might marginally slow the DAC operation, it is essential when used with fast computers. The TLO72 dual op amp is connected to the unregulated 12V DC input for its positive supply but needs a negative supply rail as well so that it can cope with output voltage swings right down to OV. To provide this negative rail, I’ve used the other three in-

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38

T

WAVEMAKER SILICON CHIP

CHAR GE PUMP VOLTAGE INVERTER

Fig.1: a cheap CMOS octal latch forms the basis of the digital-to-analog converter. This is a much cheaper approach than using a dedicated DAC chip.

verters of IC3 as the heart of a simple negative voltage generator.

IC3d operates as a relaxation oscillator, running at

about 1.8kHz and driving the other two inverters in parallel. The resulting 5V peakto-peak square wave is then fed to a simple chargepump rectifier using D2, D3 and the two 33pF capacitors, to produce a negative

rail of about -3.3V when loaded with the TLO72. Diode D1 provides reverse-polarity protection for the main +12V power input, while the 7805 regulator (REG1)

provides

the

Ee ee et ER BR WE et

regulated +5V needed by the DAC, hex inverter and

LED. The complete generator

:i

Rees A

Soe

runs from a nominal 12V DCG, which can come from either a battery or a mains power supply. As the cur- This shot inside the case is reproduced with the PC board same size to make assembly easy. rent drain and dissipation Use this in conjunction with the component overlay below. in REG1 are both quite low there should be no problem about us04101011. As you can see from the corner holes are drilled 3mm diaing an unregulated 12V plugpack supphotos the board, together with the meter to take mounting screws, and ply. remaining parts, fits in a readily availthat there’s also a fifth 3mm hole ready Construction able small low profile instrument box, for the screw used to hold down the measuring 140 x 111 x 35mm. tab of regulator IC4. Apart from the pot, LED and conBefore fitting anyof the parts on the nectors for DC power and signal outThe board layout diagram shows PC board, check it carefully for track put, all the components used in the where all the board-mounted parts go, breaks or shorts between tracks. It’s generator mount on a PC board meastogether with their orientation. Where uring 105 x 76mm, and coded also a good idea to check that the there’s any doubt the internal photo

00

av

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4g

26 Px

Fig.2: there aren’t too many components to

solder to the PC board, as this overlay shows.

JANUARY 2001

—_ 39

should help, as well as showing the off-board parts and wiring. I suggest you fit the header strip for IDC ribbon cable first, followed by the PC board pins used to simplify the other off-board connections. There are two of these for the LED, two for the output, two for the 12V DC input and three for the pot connections. Next, I’d fit the resistors, bending

their leads carefully so they mount down against the board without straining the components. Then do the capacitors, taking care with the polarity of the polarised electrolytics (includ-

ing the tantalums). The correct polarities are shown on the layout diagram. : The three diodes can be fitted next. Note that D1 mounts with its.cathode band end towards IC2, while D2 and D3 both have their cathode ends towards the edge of the board. Finally, fit the four ICs, again taking care with their orientation. Also be careful when you're bending the leads

We’ve unplugged the ribbon cable from its on-board connector to make this rearpanel shot much clearer. The socket at left is for DC power.

effort. There are just the three holes in the front panel, a hole and slot in the rear panel and four holes to drill and

of regulator (IC4) down at 90°, so that

countersink in the bottom of the case

there’s no strain on them when the IC is mounted down against the board. I usually fit the leads through their holes and bolt the regulator down with an M3 screw and nut before soldering the leads to their pads. Your board should now be finished and can be put aside while you prepare the case. This doesn’t involve a great deal of

for the PC board mounting screws. A photocopy of the front panel artwork can be used as a template to drill the holes for the LED, pot and output connector.

.

Similarly you can use a photocopy of the PC board artwork as a template for the board mounting holes. The location of the hole and slot in the rear panel are not critical, and you

Parts List ~ Wavemaker Arbit

should be able to use the photos as a guide. In view of the low profile of the _ plastic case, I elected not to use mounting pillars for the PC board. Instead it was mounted lower in the case using four 10mm-long M3 countersunk-head screws, coming up from underneath. Each screw has a star lockwasher and nut fitted first to fasten it inside the case, then a second nut to act as a spacer. The PC board sits on these second nuts, with a further lockwasher and nut on the top to hold it in place. If you have access to a photocopier you may be able to make your own front dress panel from the artwork, on adhesive backed aluminium or matt white plastic sheet. This can be stuck carefully on the front panel after the holes have been drilled, and before fitting the pot and other parts. At this stage I cemented the LED into its hole in the front panel using a dob of Araldite at the back, leaving it aside overnight to harden, before fitting the pot and connector to the panel. Once the LED is firmly cemented in place and everything else is fitted into the case you’re ready for the final step: the off-board wiring. This can all be done in light-duty ribbon cable wire,

although I used resistor pigtail offcuts to extend the LED wires so they reached their PC board pins. These were insulated with sleeving to prevent shorts. Once the off-board wiring is done, the generator itself should be complete. All that should remain is mak40

SILICON CHIP

Tek

20kS/s |.

Tek

122 Acqs

SOkS/s |

267 Acqs

Ch1 Freq 206.6 Hz Low signal amplitude

Chi Pk-Pk 10.08 V

Here’s a triangular wave at about 200Hz. It’s quite linear, and still quite clean.

A sinewave at about 50Hz from Wavemaker; as

you can see it’s pretty clean.

Chi Freq 20.72 H2

Unstable histogram

Ch1 Pk—Pk 10V

ing up a suitable cable to connect it to your PC’s printer port. This is easy if you use IDC connectors and 26-way IDC ribbon cable. All you’ll need are a 26-way IDC socket, an IDC type DB25 plug and a suitable length of cable — say 2m or so. Just be careful that you fit both connectors so their ‘pin 1’ ends are at the side of the cable marked with the red stripe; then the connections will be right. You should now be ready to power your generator up and connect it to the PC, to try it out with the software.

The software

A ramping-down sawtooth wave at close to 20Hz; again it’s very linear and good for testing amplifier linearity.

Tek

20kS/s 355 Acqs [EY

As mentioned earlier, I’ve written four programs to go with the Wavemaker. They’re all written in Visual Basic for DOS and will therefore run happily on most IBM-compatible PCs. This means that you can use almost any PC to drive the generator, including those elderly desktops and laptops that many of us have gathering dust in our cupboards. Although you probably won’t want to run the software on a modern machine running Windows 98 or NT, it should run quite happily on these too, in a DOS window. You'll probably get a ‘device conflict’ warning from time to time Tek

300kS/s -— 44-80

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This is a true arbitrary waveform, made using MAKEWAVE.EXE. The narrow negative spikes were programmed in, for scope triggering.

This is at 1kHz, showing how the waveform gets a little

‘segmented’ at higher frequencies.

JANUARY 2001

41

Here’s the ‘control window’ for SOFTTEST.EXE, the

program you use to check the printer port and save the generator’s config file.

When SOFTARBG.EXE is running, it gives you this window. You can select a waveform and frequency, and also control the generator.

when the programs try to send data to the generator, but once you hit the ‘Y’ key to confirm that you want the DOS program to have access to the port, Windows usually backs away and lets them run. Zipped, free-running EXE versions of all four programs will be available on the SILICON CHIP website, for you to

where to find the generator. SOFTTEST.EXE also lets you set the value of a ‘delay constant’, which the main generator program SOFTARBG.

download, unzip and use ‘as is’.

on PCs with different processor chips and clock speeds, making it difficult to control absolute timing. To get around this problem I decided to have SOFTARBG.EXE use a timing loop to set the basic time-per-sample of the output waveforms, with this time set by a loop delay variable which is saved in the config file along with the port address. This makes it fairly easy to get the generator frequencies right, simply by adjusting the delay variable by trial and error using SOFTTEST.EXE. For example, on a 486DX2 running

However for those who would like to see how they work, zipped text files with the VBDOS source code will also be on the website for you to download and inspect with a text editor/viewer. Here’s a quick rundown on each of the four programs, so you'll know what each one does and how it’s used. There are a few screen shots to show what their ‘user interfaces’ are like, and also a couple of output waveforms captured via a digital storage oscilloscope. SOFTTEST.EXE: Normally SOFTTEST.EXE is the first program you'll need to use, because it’s the one that lets you check the I/O address of the printer port the Wavemaker is connected to, and confirm that they’re talking to each other. It also lets you save the port address in a ‘config’ file (SOFTAGEN.CFG),

which the other programs can read when they’re started up, so they’ll know

EXE uses to set a software timing loop which controls the frequency of its out-

put waveforms. This is necessary because the pro-

gram will tend torun at different speeds

at 33MHz, a delay value of ‘3’ turned

out to give generator frequencies that were within about 3% over most of the range, which is quite acceptable. On machines with faster processors and higher clock speeds you'll need a larger value to achieve the correct frequencies. SOFTTEST.EXE gives you a screen

window with five large control buttons, and a small ‘display panel’ which shows the current I/O port address you're trying. To change this address you simply click on the top button, which brings up a dialog box to let you select one of the other common printer port addresses. To check whether the generator is at that address, you simply click on the next control button. If you’ve found the correct address, this will cause the generator’s LED to blink on and off five times, at arate determined by the delay constant value. So finding the correct I/O port is simply a matter of trying the various addresses until the LED blinks when the second button is clicked. The third button down is the one which lets you set the software delay constant. This is explained in a message dialog which appears when you click on the button. You can then set the value via a second dialog box. The fourth button then lets you save the port address and current value of the delay constant on disk, in the config

file ‘SOFTAGEN.CFG’ expected by the main generator program.

Finally the fifth control button lets you quit SOFTTEST.EXE, and return to DOS — ready to try the main proArbitrary

The third program is SOFTVOLT.EXE, which lets you use the generator as a programmable voltage source. This is very handy when you’re troubleshooting projects! 42

SILICON CHIP

Waveform

This is the opening window for MAKEWAVE.EXE, the program you use to design your own waveforms and save them as disk files.

Silicon

Chip

Software

Driven

Arb

Generator

ARE REAR

When you enter MAKEWAVE.EXE’s edit mode, you get this screen to design your waveform graphically.

gram and check its output frequencies, perhaps. SOFTARBGEN.EXE: This is the main generator program, which gives you a screen window with four control buttons and two ‘display panels’ — one to show the currently selected waveform and the other to show the waveform’s frequency. The two uppermost buttons let you set the waveform and frequency, re-

spectively. Click on the Select Waveform button at top left and you get a small dialog box with six options to choose from: Sine, Square, Triangular, Sawtooth Down, Sawtooth Up or

Custom. The last of these is to select an arbitrary waveform file on disk, and if you select this option you get another dialog asking for the name of the waveform file you want. These files have the extension ‘.SWF’, and some sample files will be available on the SILICON CHIP website to get you going.

The top right Set Frequency button calls up a dialog button which, as you’d expect, lets you set the frequency of your waveform, in hertz. However, note that this button doesn’t work if you’ve selected a custom waveform, as the frequency of these is set by the length of the waveform in the loaded file. (If you want a similar waveform of a different frequency, you'll need to make it using

MAKEWAVE.EXE.)

With any of the five ‘standard’ waveforms you can select a frequency between 1Hz and 2500Hz, although

RUNNING

the frequency resolution and accuracy are not wonderful above 1kHz. Note that when you select a waveform and frequency, these are displayed on the ‘panels’ above the buttons — a bit like a hardware generator.

The third button at lower left lets you start and stop the generator, running whatever waveform and frequency you ve selected. Note, though, that because the selected waveform is either calculated or loaded in from disk only when you click on the button, there can be a short delay before the generator starts producing the waveform — especially for very low frequency waveforms, which have a lot of samples to calculate or load. The final button is again Quit Program, which is self-explanatory. SOFTVOLT.EXE: The third program is SOFTVOLT.EXE, which is designed to make it easy to use the Wavemaker as a programmable DC voltage supply. This one gives you a screen window with three small ‘display panels’ and three control buttons. The display panel at far right simply shows the I/O port that the program has loaded in from the config file, as a reminder. The other two show the current DC output voltage and the current ‘maximum’ (i.e., full

digital scale) voltage respectively, and each of these figures can be set by clicking on the buttons beneath them.

The idea of this ‘dual control’ system is that you can use the Wavemaker as a DC voltage source programmable

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The Operatic Mignon used a vibrator to derive the HT from the 32V DC input. As with all vibrator sets, there is a trap in that if the 32V supply is reversed, all the electrolytics on the HT rail will be damaged. In fact, a series silicon diode (rated at 3A) would be good insurance.

where a much higher square wave AC voltage is developed. A second set of contacts on the vibrator “rectify” the secondary voltage to produce the HT (high tension) for

the plates and screens of the valves. This sort of vibrator with two sets of contacts is said to be “synchronous” because the contacts work in unison. Asynchronous vibrators have one set of contacts, to switch the primary current, and the secondary voltage is rectified by a valve rectifier. One trap with vibrator supplies is that if the 32V supply is reversed, the electrolytic capacitors get charged up with the wrong polarity and the set won't work. It doesn’t do the electrolytics much good either!

,

The transformer must be “tuned” using “buffer” capacitors so that there is minimal sparking at the vibrator points, otherwise the vibrator will have a very short life. Even with buffer capacitors, there is still some sparking at the vibrator contacts (also called “points”) and radio interference is produced. This interference would wipe out all radio reception if it were not dealt with.

Typically, the whole vibrator power supply is shielded, as can be seen in the Operatic. Some sets had doubleshielded supplies and also single point earthing was commonly used to prevent interference currents circulating around the receiver chassis. Some areas are very remote from radios stations which means that daytime radio reception on the medium wave broadcast band is virtually nonexistent. At night, many stations are heard but suffer from selective fading and often there is more than one station on the same frequency. So that day-time reception could be achieved, at least one shortwave band was installed. This allowed the domestic shortwave stations to make up for the lack of medium-frequency reception during daylight hours. These receivers also had to work off a very variable power supply, which could be as low as 28V and as high as 40V. Sometimes extra cells were added to the 32V bank of batteries to make up for voltage drop in the cables and the voltage could reach 45V. Some sets had a 3-position power switch marked “Off”, “Charge” and “On”. In

the “Charge” position, a resistor was

placed in series with the supply to reduce it to around 32V when the batteries were being charged. As can be understood, remote country listeners really had: it tough in regard to getting reasonable radio reception. The radio set designers had quite a task to design suitable receivers for these remote locations. That they succeeded can be seen in the Operatic Mignon and sets produced by other manufacturers.

The Operatic Mignon RF A12 Bland Radio of Adelaide may not be a manufacturer known to many but the Operatic brand name was well known in South Australia and Westerm Victoria for many years. In coun-

try areas, their 32V radios gained a reputation, over several decades, as reliable and sensitive receivers that were well-suited to rural conditions. The Mignon was quite a standard set with nothing unusual in its appearance. It was a good solid brown Bakelite set of 1951 vintage. It is of

average size, has the usual slide type dial, and four controls to operate the JANUARY 2001

87

the chassis is just slid out with the dial and all the works attached. I wish all receivers were as convenient as this to disassemble. The chassis can be tipped onto its end where the vibrator box is located or even tipped upside down with no damage to components.

A view underneath the chassis shows that it is not unduly cluttered, despite being dual wave and having an RF stage.

Vibrator supply & series heater wiring

The metal box on the righthand side of the chassis is the shielded vibrator supply. This shielding was crucial in minimising interference to fringe area reception.

set. Yes, the wrong knobs are on this set, as it was purchased without them. I’m on the lookout for the right knobs. A view of the back of the receiver shows a metal box on the right which is the shielded vibrator power supply. To the far left is the 3-gang tuning capacitor. The set uses a 6N8 RF stage,

6AN7 converter, 6N8 455kHz IF stage, a 6BD7 detector/AGC and first audio

stage, followed by a 6AQ5 audio output.

The Mignon is very easy to remove from its case. The four knobs are removed, then two screws, one at either end of the cabinet are removed and

~ How to Power A 32V Radio

Power supplies that put out 32V at an amp or so are quite scarce. However, it is possible to build quite a simple supply that will easily power this and other sets. The following parts are needed: a transformer with a 24V @ 2A secondary, a bridge rectifier rated at 100 PIV or higher and a current rating of greater than 2A, a 4700yuF 50VW electrolytic capacitor and two .01pF 200V greencap or polyester capacitors, plus any necessary mounting hardware and cabling. This supply will comfortably provide up to 1.5A at around 32V DC. 88

SILICON CHIP

The vibrator power supply has been found to be quite reliable. Some vibrator supplies are extremely reliable, rarely, if ever, needing a replacement vibrator while others need a new one relatively frequently. If you do replace a vibrator, it is a wise policy to replace the buffer capacitors as a matter of course. In this vibrator power supply, the buffer components are the .004p)F capacitor and 10kQ resistor in series and the 0.5pF capacitor — all these being connected to the vibrator transformer in the lower right of the circuit diagram. The voltage ratings of these capacitors must be strictly adhered to as must their capacitance values. The voltage rating on the .004yF capacitor may be as high as 2kV working. WES Components in Ashfield, NSW have suitable capacitors, which are normally used in TV receivers. A value of .0039yF is near enough to .004pF but a 0.47pF capacitor should have a .027pF capacitor placed in parallel with it to nearly equal 0.5pF. This is a 32V DC receiver but the vibrator is rated at 24V. However, this only applies to the reed drive of the vibrator and a 1000 resistor is used to drop the voltage down from 32V to 24V. Throughout the receiver it can be seen that cathode bias is used, instead of the more popular “back bias”. With most vibrator sets, it is not possible to separate the low tension and the high tension circuits and they have a common negative which goes to chassis or earth. As a result back bias cannot be used. However, it does mean that it is quite practical to measure the current drain of each valve by checking the cathode to earth/chassis voltage. Note that the valve heaters are in series across the 32V supply and there are also resistors across some heaters. The resistors are there to balance the

& Tube Company (EVATCO) stocks a large range of valves for vintage radio, amateur radio, industrial and small transmitting use. Major current brands such as SOVTEK and SVETLANA are always stocked and we can supply some rare NOS (New - Old stock) brands such as Mullard, Telefunken, RCA and Philips. Hard to get high-voltage electrolytic capacitors and valve sockets are also available together with a wide range of books covering valve specifications, design and/or modification of valve audio amplifiers.

PO Box 487 Drysdale, Victoria 3222. Tel: (03) 5257 2297; Fax: (03) 5257 1773 Mob: 0417 143 167;

on to make sure that the negative line of the supply goes to the chassis. If it is positive to chassis the HT voltage will be reversed. The set was then tried out — it was rather sick, with the high tension (HT)

aan

The under-chassis wiring is relatively uncluttered. Quite a few of the old paper capacitors were replaced with polyester or metallised polyester types.

relatively low. No shorts were found on the HT line so the vibrator was thought to be the culprit for the lack of voltage. The vibrator was removed from the power supply and the mechanism itself removed from its case. To do this, it was necessary to desolder the small lug on the side of the base. The next step was to remove the circlip inside the bottom of the base using a screwdriver and then slip the vibrator out of the case.

voltages across each valve. The 6BD7 and the 6AN7 only draw 0.23A of heater current and this is padded out to 0.3A by the 175Q resistor. If a GAN7A was to be used as a replacement, the heater equalising resistor would need to be changed so that it was only across the 6BD7 and be reduced to 90Q. Likewise the 150Q resistor bleeds off 0.15A so that the 6AQ5 gets the right current through it

(0.45A) and the other valves get 0.3A

through their supply line. The dial lamps are fed off their own series 62Q resistor.

Getting it up & running As has been said, the Mignon is a quite conventional receiver designed for use on 32V DC. All of the usual critical capacitors were replaced. It is quite important before turning the set

Vibrators aren’t easy to come by so I decided to clean up the points. This was done by running a points file between each set of points until they appeared reasonably smooth. Fine wet and dry paper was then used to polish the contacts. During this process, the points were closed together under slight pressure to help the polishing action. There are a total of five gaps to

JANUARY 2001

89

brators. If you have any vibrator set, I recommend that you always replace the buffer capacitors, except where they are mica and test OK.

A general check-up

The vibrator was removed from its metal case so that its contacts could be cleaned up with an automotive points file.

clean in these synchronous vibrators. I checked that the points were reasonably smooth and shiny, by using a magnifying headset. If the points are very pitted, it will not be possible to get them into first class condition. Be careful not to bend the points out of position if you decide to overhaul a vibrator. This vibrator had obviously had a long and hard life, as there was quite a bit of black around the insides, and still is. The foam rubber buffers and the insulated rubber sleeves on the leads to the vibrator plug had all disintegrated. I used contact adhesive to glue some thin rubber strap to either side of the top of the vibrator to act as a buffer so that it wouldn’t bang against

the side of the mounting can and make a noise when it was operating. It was not practical to re-sleeve the braid wires coming into the vibrator so 10mm plastic tubing was cut and placed so that all the flexible braid leads were kept apart. This work can be seen in the photograph.

A vibrator in poor condition will not provide as much output voltage as a new one but since the receiver is unlikely to be used much, a slightly dodgy vibrator is not worth replacing. 90

~—. SILICON CHIP

The vibrator in this set isn’t 100% but is still quite adequate. Another problem that sometimes occurs with vibrators is that they vibrate well but there is no output from the supply. I have found some that haven’t been used for years develop an insulating film on the contact points, hence the contacts never make electrical contact with one another. The exception is the reed drive circuit, so a thorough clean even of new vibrators is needed, if a fault like this shows up. As stated earlier, the buffer capacitors are critical to the long life of vi-

Now that the supply was producing a voltage somewhere near the 160 volts expected the receiver started to perform. The IF stages were aligned and no problems were found, with all the adjustments being close. This was done by forcing a strong signal through the set with the signal generator on 455kHz attached to the aerial terminal, the receiver on the broadcast band and the gangs closed. By doing it this way, test instruments do not interfere with the tuning of the IF channel. The tuning peak can be located by placing a digital multimeter (with 10MQ input resistance) across the volume control and tuning for maximum voltage. By the way, there is an error in the circuit diagram around the pick-up terminals; it won’t work without radio programs also coming through loud and clear along with the record you are playing. The correct portion of this circuit is shown separately. It just goes to show that draughtsmen and proof readers didn’t always get this correct. A liberty was also taken in the way that the IF transformers were drawn in that the resonating capacitors for each winding were omitted. It was always assumed that anyone reading the circuit would know this. | The tuning of the RF sections is a relatively complex task which we don’t have space to cover here. Suffice to say that the stages all peaked nicely and the set performed well.

Summary A receiver with an RF stage is always desirable; the extra stage of radio frequency amplification really does make a difference on the broadcast band as well as on shortwave. The Operatic Mignon is no exception. It is sensitive, has a good delayed AGC system and a moderate audio output level. Operatic receivers do not have any whiz bang circuitry or anything that appears exotic but they work well and just keep on going. They are part of our rural radio heritage. I’m pleased to have it in my collection. SC

Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097.

12-to-24V ied

inverter wanted I am inquiring about a 12V to 24V inverter. I have several Hella 24V revolving orange lights that I would like to operate on 12V. (M. L., via email).

@ We have not produced a suitable circuit for your application. It might _. be easier to change the lamps to run on 12V — the 24V motors would prob-

ably still run at 12V.

Amplifier for

deliver around 9W into two woofers and 1.5W into two tweeters and also included an electronic crossover. It was designed onto a PC card and was internally powered by the PC itself. Sounds ideal, eh?

Jumbo clock

sound card - Iam looking to build an amplifier to drive a pair of 4Q loudspeakers to reasonable levels from my sound card. I want more level than the existing multimedia speakers but I don’t need hifi levels. And I would also like to

build the amplifier into the PC itself and power it from the PC as well. I was thinking of using your “Mini Amplifier For Personal Stereos” which was published in the October 1992 issue. Would this be a good approach? (G. C., via email).

is still a current design, its output is quite low and probably not enough to suit your needs. Have a look at the MultiMedia Sound System amplifier featured in the October 1996 issue. This used three TDA1519A bridge amplifier ICs to

|

@ The Mini Stereo Amplifier featured in October 1992 used the TDA2822M dual 1W 8-pin IC amplifier. While it

:

is very slow I’ve built the Jumbo Clock described

in the March 1997 issue of SILICON CHIP. The clock is running very slow or it doesn’t seems to be running at all. All the LED segments are working, as well as the hour and minute switches. Do you have any suggestions? I think

PC tracks. If the colon does not flash or is very slow, check that the pin 14 output of IC1 does go high and low at a 2Hz rate. You can use a LED connected in series with a 2.2kQ resistor to check this. Alternatively, use an oscilloscope, logic probe or even an analog multimeter. | If IC1 is not producing the correct rate, then the problem could be with the crystal or components connecting

to it. Note that it is unlikely that any of the ICs are faulty. Simply check for other problems either with the PC board or soldering. Alternatively, the resistors for the oscillator could be incorrect or the capacitors the wrong value.

RC speed controller is temperature sensitive I built the RC Speed Controller described in the May 2000 issue of SILICON CHIP. I had a problem in that the

it might be related to the crystal or

ZN409’s 1.5ms reference oscillator

4060 chip. (A. W., via email)

varied significantly from day to day and a hair dryer showed it to be a temperature thing. The nominal 1.5ms reference, adjusted at “room temperature” (about 18°C in my garage), varied between 0.5ms and almost 2ms when the PC

@ The first thing to check is that the colon flashes at a one-second rate. If so, then the problem will be in IC2 or IC3. Check for dry joints, solder between IC pins or hairline cracks in the

board was heated or cooled (between

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maybe 40°C and 15°C). Even simply placing a finger on the capacitors caused the rise time to begin to drift. There’s not much in the way of external parts for the oscillator, so I replaced the two 0.1p/F capacitors with tantalum types — now it’s rock steady, even at elevated temperatures. Measuring the two old capacitors out of circuit showed they had a room temperature capacitance of 0.1yF, which dropped down to under .05uyF once warmed. | All components have some kind of temperature coefficient, though I’ve never seen such a sensitivity before! (The supplied units were those small blue, non-polarised types, polyester I think?) PerhapsI received part of a bad batch? JANUARY 2001

91

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$30.00 fix that may be unnecessary.

i

Can you help? (K. M., via email).

@ This project was published in the July 1995 issue of SILICON CHIP. Two errata on this project have been published. First, we recommended changing the 6.8Q resistor to 1.2Q to increase the output to 10kV. If you short out the resistor, the coil will deliver

full output. © Second, use a 500mA fuse as 250mA fuses can have high resistance.

Shifting a sawtooth oscillator I’ve made up the Waveform Generator from Jaycar’s Short Circuits Volume 3 handbook and it works fine. But can I have it go down to say 20Hz (rather than the present 100Hz) so I could use it to test subwoofers? (G. K., via email).

@ You can change the minimum frequency to 20Hz by using a capacitor

Also, I have a suggestion for people who want to use the controller with relatively light loads — rather than cut a hole in the case to fit the loaded PC board, cut off the MOSFET tags. This way, you don’t need to make any holes in the little case. The tags don’t add much in the way of heatsinking and with 20A loads or less, the MOSFETs don’t even get warm. Finally, this mod makes it possible to make the case water-resistant via a silicone seal around the case lid. (B. L., via email).

@ The blue capacitors are monolithic (stacked ceramic) types intended for supply bypassing on computer and logic boards. They are OK for that task but as with any high-K ceramic capacitor, they do have quite a large temperature coefficient and should not be used in any circuit involving critical time constants.

Improving the Class A amplifier I have read with interest your recent project on the Ultra-LD Amplifier (March, May & August 2000) and as I already have a very good class AB amplifier I am interested in your July 1998 15W class-A design. Would it be possible to amend this 92

SILICON CHIP

to use the later generation of transistors used in the output stages of the 100W class AB amplifier? Is it possible to raise the voltage with these new transistors to increase the output to about 25W? What changes would be required to increase the output of the 15W project by using paralleled output transistors? (C. M., via email).

@ Since the distortion is already extremely low, there is little point in going to the more expensive transistors. It would be possible to increase the supply rails to get more power but this would greatly increase the overall power dissipation. We have not done any work along these lines and therefore we are reluctant to recommend that it be done.

Electric fence

needs extra zap Some years ago I purchased an Electric Fence kit from Jaycar that I believe came from your magazine. It used a car ignition coil to produce the charge. I am having some difficulty gaining any real noticeable “boot” at the fence and need some pointers regarding what to look for to fix the problem. It may be as simple as replacing the coil (which

was

old) but that is a

at pin 2 which is at least five times bigger; try .056uF or larger. However that will mean that the maximum frequency will also be five times lower, at around 4kHz. The sawtooth waveform is really not suitable for testing any loudspeaker since it has quite a high harmonic content. You need a low distortion sinewave oscillator for testing speakers.

VU meter needs auto level control I have constructed a LED VU level meter for my car stereo, which is purely for aesthetic purposes; ie, regardless of volume, the display should work over most of its displayable range. The only problem is that the stereo does not have a constant volume output and therefore the input sensitivity of the VU meter must be varied each time the volume of the stereo is varied. Is there a way to obtain a constant volume level from the stereo, so that I won’t have to turn two dials each time I want to change the volume? (J. P., via email). @ Short of building our CD Compressor described in the July 2000 issue, the only way to avoid the need to change the LED VU setting is to take the signal from across the volume control; ie, you have to access the signal inside the car stereo.

Spark won’t climb Jacob’s Ladder

|

I have built a kit for the Jacob’s

Ladder described in the September 1995 issue of SILICON CHIP. I want to know why the spark only travels half the way up the wires? The length of my wires are about 220mm with a gap no bigger than 20mm. How can I make the spark go farther up the wire? (L. C., Albany, WA). @ The spark will travel all the way up the wire if the following requirements are met. First, there can

be no kinks in the wire as these will

Mailbag: from page 65 The strong work ethic and individualism brought by the NZ pioneers from the United Kingdom were reinforced in the isolation of NZ hinterland, increasing the concepts of selfworth, egalitarianism and the importance of the individual to be self-directing. Thus many civil liberties are protected by the “culture”. For these reasons, NZ “pollies” and regulators are reluctant to write and enforce restrictive regulations. Australians have allowed AS/NZS 3000-2000 Wiring Rules to be interpreted in a legalistic manner. Little imagination is needed with the Australian interpretation to see the work of insurance underwriters in league with powerful legal lobbies and with industry associations and employee unions for the stated purpose of “safety”. But the bottom line is dollars: income from electrical trade employment, consultants’ fees and legal argument and minimising the insur-

cause the spark to stop at this point. Second, the flare or angle at which the wires slope outward towards the top must be very gradual. Also make sure the two wires are exactly vertical since the rising spark relies on convection of the heated air (above the spark). Finally the length of spark is ultimately dependent on the coil used. You may be able to obtain slightly more spark if you change the 0.33uF capacitor at pins 2 & 6 of IC1 to a slightly larger value. This will increase the dwell or charge time for the coil. Try a 0.47pF instead. ance companies’ liabilities and payouts for injury and property loss. Quite rightly your “Letter” comments that New Zealanders are not dying like flies from electrocution and I add that if many NZ houses were burnt down due to electrical wiring faults, then restrictive regulations would be enacted. It seems to me that if the governments of Australia were really concerned about stopping “illegal” elec-

Notes & Errata Pink Noise Source, January 1997 &

Electronics TestBench: the 22kQ resistor shown connected between pins 1 & 2 of IC1 on the PC board overlay diagram on page 42 (January 1997) should be 220kQ. The circuit diagram is correct. 2-Channel Guitar Preamplifier November 2000: the circuit diagram incorrectly shows S1 as a 2-pole (DPDT) power switch. It should be a SPDT

trical work then every hardware shop in the country would be banned from selling all electrical cable for fixed electrical wiring, and all switches, socket outlets, batten holders and junction boxes. These items are exclusively made for fixed wiring installation. As everyone knows, these are available in “bubble packs” for retail sale to any person regardless of age, gender or qualification. Perhaps supermarkets should be banned from selling replacement light globes. The Australian community would not tolerate such an outrageous and

preposterous erosion of our liberty to “have a go” to make our own repairs. Your “Letter” mentions the need to lobby our politicians to get restrictive electrical worker regulations scrapped but our “pollies” only act on the advice of technical specialists when they see it is to their electoral advantage. | I suggest that until we change Australian culture to be more like the New Zealand model, we will keep on

the legalistic path (as USA copycats) of more control by bureaucrats who believe more control is good for its own sake. I. Morrison, Marleston, SA. type and it switches the Active mains lead only. In addition, the parts list for the main PC board contains some errors. First, there should be 6 x 2.2uF NP PC electrolytic capacitors (not five) and a 1 x 1pF NP PC electrolytic capacitor should be added to the list. Second, there should be 15 x 10kQ resistors and 4 x 150Q (not 14 & 3).

Finally, the 4.7kQ resistor connecting to pin 3 of IC3 on the overlay (Fig.4) should be 27kQ.

JANUARY 2001

93

MARKET CENTRE FOR SALE

Place your classified advertisement in

SILICON CHIP Market Centre and your advert will also appear FREE in the Classifieds-on-the-Web page of the SILICON CHIP website, www.siliconchip.com.au. And if you include an email address or your website URL in you classified advert, the links will be LIVE in your classified-on-the-web!

VCR Controller use a standard home VCR for Surveillance Event Record-

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Advertising rates for this page: Classified ads: $11.00 (incl. GST) for up to 12 words plus 55 cents for each additional word. Display ads: $27.50 (incl. GST) per column centimetre (max. 10cm). Closing date: five weeks prior to month of sale.

| | | |

To run your classified ad, print it clearly in the space below or on a separate sheet of paper, fill out the form & send it with your cheque or credit card details to: Silicon Chip Classifieds, PO Box 139, Collaroy, NSW 2097. Or fax the details to (02) 9979 6503.

| | | |

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Satellite TV nocspene International satellite TV reception in your home is now affordable. Send for your free info pack containing equipment catalog, satellite lists, etc or call for appointment to view. We can display all satellites from 76.5° to 180°. AV-COMM P/L, 24/9 Powells Rd, Brookvale, NSW 2100. Tel: 02 9939 4377 or 9939 4378. Fax: 9939 4376; www.avcomm.com.au

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Positions At Jaycar We are often looking for enthusiastic staff for positions in our retail stores and head office at Rhodes in Sydney. A genuine interest in electronics is a necessity. Phone 02 9743 5222 for current vacancies.

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KIT ASSEMBLY ANY KITS assembled/repaired: professional, speedy service. Phone continued next page JANUARY 2001

95

DONT MISS THE “BUS

Advertising Index Acorn Icon (Akhurst Calendar) .. 60 Altronics

Av-Comm Pty Ltd Dick Smith Electronics

Do you feel left behind by the latest advances in computer technology? Don't miss the bus: get the ‘bus! Includes articles on troubleshooting your PC. installing and setting up computer networks, hard disk drive upgrades, Clean installing Windows 98, CPU upgrades. a basic introduction to Linux plus much more.

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Price: $12.50 {incl. GST] Order now by using the handy order form in thisissue i or Call (02) 9979 5644, 8.30-0.30 Mon-Fri with your credit card details. J ol-YoiF-1 10]o}-1erg lo) dre) amevai=)ar-NVZ-1it-1e)(- melahai all (=m) celel¢mT- ie Mass Technology

Microgram Computers

ae Each binder holds up to 143issues *« Heavy board covers with 2-tone green vinyl covering % SILICON CHIP logo printed in gold-coloured

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Price: $A12.95 plus $A5 p&p each (Australia only; not available elsewhere). Buy five and get them postage free. Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number.

Neville Walker (07) 3857 2752 or email flashdog @ optusnet.com.au

NEG. Tel John@AER (03) 9482 4958 0415 305 470.

WANTED

WE PAY UP TO $60 for contributions to

PERSON WITH EXPERIENCE / APTITUDE able to fault find & repair PCBs — without diagrams. GENEROUS PKG

Circuit Notebook. Send your circuit with a brief description to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097.

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Quad Non-[solated Inport Module Four general purpose inputs for use with external voltage levels.

Each input channel has a reverse-biased diode to block positive voltages and may safely be used with input voltages to +24VDC. When a positive voltage is applied the input pullup resistance holds the AVR input at 5V or logic ‘1’. Ifa OV levelisapplied to the input,

the AVR input is pulled to ground or logic ‘0’

Now FOR YOUR NEARE: T RESELLER ©

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