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The incredible This novel unit will produce the sounds of two birds singing together in a way which will intrigue you. They start slowly and then sing rapidly increasing trills as they compete with each other for virtuoso supremacy. By JOHN CLARKE Do you love the song of birds around your home but hate the idea of caging birds? Perhaps you can't stand the idea of cleaning out the cage - birds are messy little critters. Or perhaps you can't afford bird seed. 50 SILICON CHIP Whatever the reason, you can now have a pair of electronic birds to entertain you and your friends and, most important, you can turn them off when you don't want to listen to them. The idea of producing artificial birdsong isn't new. There have been clockwork instruments going back centuries and in the last decade or so a number of electronic birdies have come out of the emporiums of Asia. But it's been a while since we saw or heard any and so we thought, ''Why not build a new circuit?" We could have been really clever and built the unit into a fancy gilt cage complete with ornamental bird. Instead, we took an easier approach and built the circuit into a standard zippy box and then sat a couple of ornamental birds on top of it. Using two garden variety ICs and 47k +9V +9V VR1 50k 100k .,. +9V +9V + 01 1N4148 100 470k 100k 33k + 68k +9V 1000..r- +9V 470k .,. CHIRP CONTROL OSCILLATOR (a) + 471 470k .,. Bn SPEAKER CHIRP OSCILLATOR (a) .33k 330pFI 100k .,. 02 1N4148 TONE OSCILLATOR (a) 47k .001 MIXED OUTPUT +9V +9V 100k VR2 50k +9V .,. :r:<o +9V 100k POWER -'I' 9V 1 I .:;.&.. i +9V + 471" B + 47k 470+ EOc VIEWED FROM BELOW .,. CHIRP CONTROL OSCILLATOR (b) + 47.I: .,. .,. CHIRP OSCILLATOR (b) 270pFI HOT CANARIES .,. TONE OSCILLATOR (b) Fig.I: the circuit uses seven Schmitt trigger oscillators based on LM324 op amps. ICla is the on/off oscillator while the other six oscillators generate the sounds of the two canaries. a handful of resistors and capacitors, our "Hot Canaries" generate the sounds of two canaries happily chirping and trilling away. The period of trilling, chirping and pitch of each bird is different, creating a random effect as the birds come in and out of chorus. One of the problems with trying to reproduce a birdsong circuit is that so many song parameters have to be controlled - the pitch. rate of chirps and trills, and the length for which trills last. Such a circuit either tends to be very complicated or very incestuous, as certain circuit sections have to perform more than one function. Our approach was to try and come up with a good compromise produce a circuit which was not too complicated but which would also be reasonably easy to build and troubleshoot (perish the thought), if necessary. PARTS LIST 1 plastic case, 130 x 67 x 43mm 1 PCB, code SC0811 2891 , 107 x 60mm 1 front panel, 126 x 64mm 1 57mm 80 loudspeaker 1 SPDT toggle switch 1 9V on-board battery holder (DSE Cat. P-6200) 1 9V 216 battery Semiconductors 2 LM324 quad op amps (IC1,IC2) 1 BC328 PNP transistor (01) 2 1 N914, 1 N4148 signal diodes (D1 ,D2) Capacitors 1 1 000µ,F 1 6VW PC electrolytic 1 4 70µ,F 16VW PC electrolytic 1 100µ,F 16VW PC electrolytic 3 4 7 µ,F 1 6VW PC electrolytic 2 .001 µ,F metallised polyester 1 330pF ceramic or polystyrene 1 270pF ceramic or polystyrene Resistors (0.25W, 5%) 5 470k0 2 180k0 5 100k0 7 68k0 4 47k0 5 33kfl 2 15k0 2 10k0 2 3.3k0 1 330 2 50k0 miniature vertical trimpots Miscellaneous Solder, hookup wire, 4 PC stakes F EBR UARY1990 51 The completed PCB assembly clips neatly into a standard plastic zippy case. Check to ensure that none of the on-board components are fouled when the lid is screwed down. Circuitry What you need for an electronic canary circuit is oscillators - quite a few of them. And you also need to use CMOS circuitry to keep the battery drain as low as possible and so the CMOS 74C14 hex Schmitt trigger suggests itself as a device which will do the job. This is because you can make a very simple oscillator with a Schmitt trigger and just two other components: a resistor and a capacitor. A circuit using the 74C14 for an electronic canary was published quite a few years ago in another magazine but this device does have one big drawback. In any Schmitt trigger oscillator, the operating frequency is very dependent upon the high and low switching thresholds of the Schmitt trigger device. This would not be serious if the 74C14 had closely defined high and low thesholds but it doesn't. Consequently , any oscillator designed around the Schmitt triggers in the 74C14 will have an operating frequency which can vary by more than 3:1. For some circuits, the large variation can be acceptable but for a chirping canary circuit, it ain't. Clearly then, although we would 52 SILICON CHIP have liked to use the 74C14, it was not the ideal device. What we needed was a cheap device with low battery drain which could act as a Schmitt trigger device with precisely defined high and low thresholds. Well, we have accomplished that aim by using the LM324, a quad op amp package. It has low current drain and will operate from a single supply rail. Each op amp in the package can be used as a comparator and with the addition of a resistor connected between the output and the noninverting input ( + ), it can have precisely defined hysteresis which is the difference between the upper and lower thresholds. Thus, it can be used as an oscillator with a fairly precisely defined operating frequency. So let's now have a look at the complete circuit for the Hot Canaries which uses two LM324 quad op amp ICs. Essentially, the circuit consists of 7 oscillators which are connected to obtain the sounds of two canaries singing. First, there is an on/off control oscillator which switches the canary sounds off for a short while after a minute or so of continuous chirping - it's like a rest break for live musicians. The remaining six oscillators are used to generate two almost identical canaries which are then mixed together and amplified by a single transistor which drives a small speaker. IC1d, IC1c and IC2c are the three oscillators for the first canary while IC1 b, IC2a and IC2b are the three oscillators for the second canary. Note that the canary circuits are almost identical apart from changes to two capacitor values. As noted above, each of the LM324 op amps is connected as a Schmitt trigger, by virtue of the resistor between its output (pins 14, 8, 7 or 1) and its non-inverting input (pins 12, 10, 5 or 3, respectively). Also necessary to set the upper and lower threshold of each Schmitt trigger section is a voltage divider, consisting of two resistors, with the centre-point connected to the noninverting input. To make each of these Schmitt trigger sections operate as an oscillator it is necessary to connect a resistor/capacitor network from the output to the inverting ( - ) input. Each oscillator then works as follows. When power is first applied, the capacitor at the inverting input (eg, at pin 9) will have no voltage across it and the op amp output will be high. The capacitor will then start to charge up via its associated resistor, until it reaches the threshold set by the reference voltage at the non-inverting input. When that happens, the op amp output switches low and the capacitor then starts to discharge. It will continue to do so until it reaches the lower threshold voltage, again as set by the voltage at the non-inverting input. This causes the op amp output to switch high and the cycle begins again. The result is an oscillator with an approximate square wave at the output and a sawtooth waveform at the inverting input; ie, across the capacitor. So that describes the general operation of each of the 7 oscillators in the circuit. Oscillator interaction To understand how the oscillators work together to produce the sounds of canaries, let's go to one of the oscillators which is last in its TO S1 TO SPEAKER 0 ~ -- ~ '- I Fig.2: check the resistor values with a digital multimeter before installing them on the board and take care with the polarised components. You can use sockets for the two ICs if you wish. chain, IC2b. This is labelled as a "tone oscillator" and its basic frequency is set at around 2-3kHz. In fact, if the 180k0 and 470k0 resistors connecting it to other parts of the circuit were removed, it would just oscillate continuously at around 3kHz or so. Well, that would be all very nice but it wouldn't sound much like a canary. They chirp and warble. To get IC2b to chirp, we modulate it at a rate which starts at about 1Hz and then rises until ultimately IC2b is running continuously. This "chirp" frequency is generated by IC2a. To get · the chirp frequency to rise, as just mentioned, we control it with a lower frequency oscillator, !Cl b. As the voltage across its 470µ.F capacitor increases, the chirp frequency rises. So we call ICl b the "chirp control" oscillator. When the chirp oscillator rises fo its highest value, which effectively lets IC2b run continuously, the series RC network consisting_ of a 180k0 resistor and .001µ.F capacitor between pin 7 and pin 2 causes the two oscillators to modulate each other and so the tone oscillator "warbles" just like a canary. You will see that ICld, IClc and IC2c are virtually identical to ICl b, IC2a and IC2b. They produce the second canary. The outputs of IC2c and IC2b are mixed together via two 10k0 resistors to drive transistor Ql and the miniature 80 loudspeaker. This brings us, finally, to the 7th oscillator, ICla which turns the duet on and off. It initially has a low output for about 60 seconds since the 100µ.F capacitor has to charge from the full + 9V down to + 2.3V. From then on, its output goes high for about 20 seonds, low for 20 seconds and so on. When its output is low, the canaries sing. The output of ICla enables the chirp control oscillators (ICld and IClb} via diodes Dl and D2. Construction We built our Hot Canaries circuit onto a printed circuit board (PCB) measuring 107 x 60mm and coded SC 08112891. This was housed in a plastic case measuring 130 x 67 x 43mm. No particular order of assembly needs to be followed when putting the components on the board. However, we suggest you put in the PC stakes and resistors first. You can then install the two diodes, the ICs and the transistor, followed by the capacitors and trimpots. The battery holder was obtained from Dick Smith Electronics (Cat. P-6200). It is a lot dearer than the usual battery snap connector but it neatly solves the problem of mounting the battery securely. The case lid can now be drilled for the speaker holes and switch mounting hole. Secure the switch, glue the speaker to the rear of the lid and complete the wiring to the switch and speaker. Testing Now the circuit is ready for testing. Insert the battery and switch on. Test that each op amp has power and listen for the chirping sounds. Adjustment of VRl and VR2 is best done by temporarily desoldering each of the 10k0 resistors {Rl & R2} in turn. Lift Rl first and adjust VR2 so that chirping starts at a slow pace and continues up to a faster rate and then stops before starting again. If the trimmer is too far anticlockwise, the canary will hardly chirp at all and if too far clockwise, it will not stop chirping. Once this has been done, reinsert Rl, lift R2 and adjust VRl. Finally, the PCB can be clipped into position by pushing it down into the plastic case. I§;] ,0) 00 ~ ,- 0 ~ Fig.3: here is an actual size artwork for the PC pattern. FEBRUARY1990 I 53