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ER VOICE Are you a fan of Star Wars? Come on now, almost everyone is. Now you can sound like Darth Vader or a number of other diabolical characters from the nether regions of the galaxy. By DARTH JUNIOR Possibly, you just might not see the reason for building such a handy space-war accessory. If this is the case we will not waste valuable magazine space explaining why such a unit is desirable. It should be self-evident to anyone who has ever seen the Star War movies. Apart from its obvious everyday utility, the Vader Voice is cheap and easy to build. It is housed in a small plastic box with an electret microphone on one side and a tiny speaker on the other. There is a press-to-talk switch which you press when you want to talk - truly amazing. And, should you ever want to be heard with your own everyday boring voice, there is a toggle to switch CH PPER MICROPHONE from Vader to Normal. We doubt whether anyone would want to use it but we put it in to make it more complicated to use. After all, you don't want everyone figuring out how to use it more after than one sitting. The circuit Fig.1 shows the block diagram for the Vader Voice. A microphone feeds an amplifier which drives the following chopper circuit. The chopper switches between the amplified microphone signal and ground at a rate set by the 3.5kHz oscillator. The output signal from the chopper is then fed to a low pass filter. This removes the residual 3.5kHz signals from the LOW-PASS FILTER LOUDSPEAKER A r"XJ r"XJ AMPLIAER x15 'v 0 POWER AMPLIAER 3.5kHz OSCILLATOR Fig.1: how it works. The signal from the microphone is amplified and drives a chopper circuit which is switched by a 3.5lcHz oscillator. The output from the chopper is then filtered, amplified and fed to a loudspeaker. 60 SILICON CHIP resultant waveform before it is fed to the power amplifier and loudspeaker. Fig.2 illustrates how the circuit works. Waveform A is the signal from the microphone after passing through the first amplifier while waveform B is from the 3.5kHz oscillator. Waveform C shows how the audio signal is chopped at 3.5kHz while waveform Dis the output from the low pass filter. Note tliat waveform D is quite different to the original waveform A. Fig.3 is the full circuit diagram. It employs just two integrated circuits: a 4051 CMOS multiplexer which provides the switching function and an LM324 quad op amp. The rest of the circuit comprises two transistors plus the resistors and capacitors. The microphone is an electret type which is supplied via a 10k0 resistor. Signal from the electret microphone is fed via a .0033µF capacitor to the input of ICla which is connected as a non-inverting amplifier with a gain of 15 (set by the 470k0 and 33k0 feedback resistors). A 220pF capacitor across the 470k0 feedback resistor rolls the signal off above about 1.5kHz. Because of the gradual rolloff rate of this filtering, the signal is only reduced by half at 3kHz. ICla is biased at half the 9V supply, via the 470k0 resistor connected to pin 10 and the two 180k0 voltage divider resistors. This "half supply" voltage is decoupled with a lOµF capacitor and is used to bias pin 3 of IClc and pin 13 of IC2. IClb is connected as a Schmitt trigger oscillator. It has positive feedback via a 470k0 resistor from the output at pin 7 to the noninverting input at pin 5. Pin 5 also has a 470k0 resistor to ground and one to the positive supply. This means that when the output of the amplifier is high, the voltage at the non-inverting input is about 6V and when low the input is at about 3V. This establishes the upper and lower thresholds for the Schmitt trigger. The oscillator works as follows. The .0022µF capacitor at the inverting input is charged via the 100k0 resistor when the op amp output is high (ie, + 9V) and discharged when the op amp output is low (OV). When the capacitor voltage rises above 6V the output of the op amp A TIME -TIME Fig.2: these waveforms illustrate the circuit operation. Waveform A is the signal from the microphone, waveform B the signal from the 3.5kHz oscillator and waveform C is the audio signal chopped at 3.5kHz. Waveform Dis the signal from the low pass filter. suddenly flicks low and the capacitor then begins to discharge via the 100k0 resistor. When the capacitor is discharged to 3V, the op amp then flicks high and the capacitor starts to charge up again. The result of this continuing cycle is a square wave at the output of op amp ICl b at a frequency of about 3.5kHz. This waveform is us- 1k .----..-'WV.-.....- - - - - - - - - - - - - - - - - - - + - - - - - - . - - + - - - - + 9 V .01 3kHz LOW-PASS Al TER 1 2 5 .015 15 16 22k 14 1 +4.5V IC2 4051 13 O ELECTRET MICROPHONE 3 C 470k 11 A 220pF 6 7 0 12 .,. +9V POWER +9V + 220 16VW+ 01 BC337 S1~ T ...._ 9V : -i B ELJc VIEWED FROM BELOW 100k .,. .,. .,. VADER VOICE ·1 220 16VW B sn LOUDSPEAKER .00221 SCOB-1-0888-1 Fig.3: the circuit is basically a conventional amplifier with the addition of a chopper which operates at 3.5kHz. IC2 is the chopper and is controlled by IClb which works as a 3.5kHz oscillator. SEPTEMBER 1988 61 I° 0 ~ VADER VOICE i NORMAU PRESS VADER ON L:. 0 0 ..:J Fig.5: the PCB pattern and front panel artwork are shown here full scale. The metallised polyester capacitors should lie flat across the PCB so that there is room to squeeze the battery inside the case. The board sits upside down in the case, on top of the integral slide pillars. ed to toggle IC2, at pin 11. IC2 is connected as a single-pole switch which is toggled by the square wave signal fed to pin 11. The output of the switch, pin 3, is thus connected alternately to the audio signal, at pin 14, or the "half supply" point, at pin 13. This effectively chops up the incoming waveform. Switch S2 disables ICl b as an oscillator and forces its output high (when closed). This causes IC2 to 62 SILICON CHIP provide a normal straight through signal path for the audio signal. The output from IC2 feeds a third order 3kHz low pass filter formed by IClc and the ancillary components. This provides a steep rolloff of signals above 3kHz at a rate of 18dB/octave. The filtered signal is fed directly to ICla which drives a pair of complementary symmetry transistors, Ql and QZ, to form a minimum component amplifier. 100% feedback around the amplifier helps minimise the inevitable crossover distortion. The output transistors drive an sn loudspeaker via a 220µ,F capacitor. Overall, the circuit has been devised to give very low current drain and so a standard 9V battery (Eveready 216 or equivalent) should last quite well. To further conserve the battery, power switch Sl is a momentary contact pushbutton the press-to-talk switch mentioned earlier. Construction The Vader Voice is housed in a plastic case measuring 83 x 54 x PARTS LIST The 32mm loudspeaker is mounted on the back panel. The speaker shown is from IRH but virtually any miniature 80 loudspeaker could be used. 31mm. Most of the components are mounted on a small printed board measuring 50 x 63mm and coded SCOB-1-0888-1. Begin construction with the PCB. Insert and solder in the low profile components such as the resistors, ICs and links. Make sure that the ICs are correctly oriented as shown on the overlay diagram of Fig.4. Now the transistors can be soldered in as well as the capacitors. The electrolytic capacitors must be oriented correctly as shown and the filter capacitors (.015µF, .01µF and .001µF) plus the .0022µF oscillator capacitor should lie flat across the PCB so that there is room to squeeze the battery into the case. You can now run the wire connections to the switches, microphone, loudspeaker and battery. Holes need to be drilled in the lid of the case for the switches and microphone. For the loudspeaker, two 1mm holes need to be drilled for its terminals. Our prototype used a 32mm 80 loudspeaker supplied by IRH Components. (Cat. No. KSS-3108). It mounts directly onto the case and is retained by the two terminals at the rear. Finally, connect the battery and check that the circuit works. If a feedback howl occurs, try swapping the connections to the loudspeaker. The PCB is designed to mount upside down and sit on top of the integral slide pillars. 1 plastic case, 83 x 54 x 31mm 1 PCB, SC08-1 -0888-1 , 50 x 63mm 1 Scotchcal front panel, 50 x 79mm 1 32mm 80 loudspeaker (IRH KSS-3108) 1 electret microphone 1 9V 216 battery and clip lead 1 SPOT toggle switch 1 SPOT momentary action pushbutton switch Semiconductors 1 LM324 quad op amp 1 4051 1 to 8 analog multiplexer/demultiplexer 1 BC327 PNP transistor 1 BC337 NPN transistor Capacitors 2 220µF 16VW PC electrolytic When speaking into the microphone, the volume level can be adjusted by talking closer or further from the microphone. Alternatively, if a larger speaker is used, it may be necessary to provide a volume control in the form of a 1k0 potentiometer connected in series with the output. Alternatively, you could dispense with the loudspeaker altogether and couple the output from the 1 4 7 µF 1 6VW PC electrolytic 1 1 OµF 16VW PC electrolytic 1 O. 1µF metallised polyester (greencap) 1 0.015µF metallised polyester 1 0.01-µF metallised polyester 1 0.0033µF metallised polyester 1 0.0022µF metallised polyester 1 0.001 µF metallised polyester 1 220pF ceramic or polystyrene Resistors (0.25W, 5%) 5 X 470k0, 2 X 180k0, 1 X 1 OOkO, 1 x 47k0, 1 x 33k0, 1 x 22k0, 4 x 1 OkO, 1 x 1 kO Miscellaneous Solder, tinned copper wire, 250mm twin hookup wire. filter, IC1c, to the line input of a public address amplifier or cassette deck, if you wanted to record your Vader Voice. We'll leave it to you to think up the possibilities. Troubleshooting If your Vader Voice doesn't work at switch-on, you can start checking it out by meauring the supply continued on page 98 SEPTEMBER 1988 63 close tolerance is paramount. Where these qualities are desirable, polystyrene capacitors are the answer. Allied Capacitors of Brookvale manufacture polystyrene capacitors in virtually any value over a wide range of voltage ratings. Minimum order quantity is 25, to make it worthwhile to set the machines up. For further information, contact Allied Capacitors, PO Box 740, Brookvale, NSW 2100. Phone (02) 938 4690. Vacuum power solder sucker At one time or another all of us have come up against the problem of having to desolder and remove components from a printed board. Most of us manage with a variety of solder suckers or solder wick. For servicing or manufacturing operations which involve a lot of IC replacements, these methods are not practical. A soldering iron with a hollow tip and a separate vacuum INTERESTED IN ELECTRONICS? Right now we have quite a number of vacancies for staff throughout Australia. We need: • • • Sales staff - ambitious self-motivated people who are interested in a retail career. Junior Technicians - we will train you in the audio, digital. communications, security and video fields. Senior Technicians - experienced in the RF and digital fields. (Technical vacancies particularly in Sydney and Melbourne and from time to time in other capitals.) We offer: _ _ _ _ _ Security (our compan.,,_ is 20 years old) Job satisfaction (over "5000 interesting i:1roducts) Opportunity for advancement An attractive salary package Generous staff discounts Give us a calll Sydney: R Johnson (02) 888 3200 Perth: P Evans (09) 227 8243 Melbourne/Adelaide: A McEwin (03) 592 2366. Brisbane: N Wickson (07) 391 6490 98 SILICON CHIP pump is the usual set up but this is quite expensive. Now there is the SC-5000 desoldering tool. It is a 240VAC 60 watt handheld tool with its own built-in vacuum pump. It is easy to use. Just touch the iron tip to the joint to melt the solder and then press the button to suck it off. We found it very quick and effective and there was no problem with lifting off tracks or IC pads. It is a touch noisy as the vacuum pump motor is actuated but that is not a real problem. A tubular reservoir holds the accumulated solder and this is easily removed and emptied. A cleaning rod for the iron tip is stored in the pistol grip handle where it is always readily available. There is also a temperature control, in the base of the pistol grip, adjacent to the power cord entry. As a final bonus, the unit can be converted to a miniature hot air blower which could be handy when testing small components for their temperature sensitivity. When not in use, the unit stores in a neat vinyl carrying case. Available from Geoff Wood Electronics, the SC-5000 desoldering tool sells for $399 plus sales tax where applicable. For further information, phone (02) 427 1676 or call in at 229 Burns Bay Rd, Lane Cove. Vader Voice - ctd from page 63 voltage at pin 14 of IC1 and pins 1, 2, 4, 5, 15 and 16 of IC2. Second, check that about + 4.5V is present at pins 1, 2, 3, 8, 9 and 10 of ICl and pins 3, 13 and 14 of IC2. When S2 is closed, pin 7 of IC1 b should be high (ie, close to + 9V), pin 6 should be low and pin 5 should be about + 6V. If these checks aren't positive, check around the associated circuit closely for faulty soldering, reversed electrolytic capacitors or wrong components . Remember that the circuit should work as a normal mini public address system when S2 is closed, for Normal operation. Operation of the oscillator, ICl b, can be checked by increasing the .0022µF capacitor to 4.7µF. This will cause it to operate at below 201-iz. When an analog multimeter (switched to a 10VDC range) is used to measure the output at pin, the meter's pointer should jitter rapidly. ~