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Clifford – a pesky little electronic cricket Meet Clifford – our new little pesky insect friend. A cousin of Horace the Cricket, he has a lot to say – provided it’s dark. He’s easy to look after & doesn’t eat very much – one 9V battery does him for around a month! By DARREN YATES Once upon a time in an old project box, there lived a cricket. Many of you will have seen this cricket before. It was Horace! He was famous a few years ago (SILICON CHIP, August 1990) when he first appeared but now he was looking rather tired and dirty. He hadn’t had a feed of his favourite 9V batteries for a very long time. One day, a young inquisitive and sentimental designer tried to see if he could bring ol’ Horace “back to life”. Having spent five minutes rummaging around for a 9V battery, the designer slipped it into place. Nothing happened. The designer looked and looked but there was no sign of life and so tossed him back in the box – so much for sentiment! But as Horace landed on his head, he let out a bleat. The designer had forgotten that like all well brought-up crickets, he only speaks when spoken to – literally! So after a confusing conversation over the next few minutes, with both of them talking at the same time, Horace told the designer of his little cousin, Clifford. Now Clifford was a different type of cricket, much smaller but just as potentially annoying. Having lived in this dark corner of the store room for some time, he wasn’t short of a word. He basically said that Horace had it all wrong! Crickets aren’t supposed to talk when you make a noise – they’re only supposed to talk when it’s dark. And so Horace was given the boot and the designer took Clifford upstairs and gave him pride of place on the workbench. He sat him on the bench with a nice fresh 9V battery and turned out the lights. One second ... two seconds ... nothing. But a couple of seconds later, the office echoed with the cacophony of cricket chorales. This little bloke really makes a racket. The designer turned the lights on and almost instantly Clifford was as quiet as a church cricket. The designer tried this for the next two days by which time the rest of the office staff were looking for a suitable piece of rope and a rickety chair. The designer knew he was onto a winner and was so happy with his new charge that he took him home and they lived happily ever after. The circuit diagram Clifford is based around a single CMOS 4069 hex inverter IC, a handful December 1994  29 47k 470k 100 16VW A 4069 IC1a IC1b 2 3 4 11 K A D1 D1 1N914 1N914 13 IC1c 12 10k 2.2 25VW 10k 11 IC1d 10 7 100k  LED2 Q1 BC548 10k B 1k IC1e 14 .047 6 5  LED1 LED1 LDR1  100 16VW D2 1N914 1k K 8 4.7k Q2 BC558 B E C 100K 100k PIEZO BUZZER 3.3k C E 9 IC1f B1 9V B A K E C E B C VIEWED FROM BELOW CLIFFORD - HORACE'S COUSIN Fig.1: Clifford starts chirping when the light level falls & the resistance of LDR1 rises. When that happens, pin 4 of IC1b snaps high & this enables the two main oscillators based on IC1c & IC1d and on IC1e & IC1f. Transistor Q1 flashes Clifford’s eyes (LED1 & LED2), while Q2 drives the piezo buzzer to produce the chirping sound. of resistors, a few other components and that’s about it. So that he can fit into the smallest of spaces for maximum annoyance, he is built onto a tiny circuit board measur­ing only 40 x 35mm. Looking at his internals in Fig.1, his light sensor is a light-dependent resistor or LDR. When light falls on an LDR, its resistance falls and when it’s dark, its resistance increases. This LDR is connected to the input of IC1a which along with IC1b forms a Schmitt trigger. The 470kΩ feedback resistor between pins 1 & 4 provides the necessary positive feedback for this to work. The Schmitt trigger has two functions. First, it ensures that when Clifford speaks, he starts and stops instantly rather than slowly building up. However, we don’t want Clifford to start talking as soon as the lights go out and we don’t want him to stop instantly either (Oh, yes we do! Editor). So, we’ve added a 100µF capacitor to the input of IC1a. This slows the rise and fall of the input as the LDR changes its resistance to give this delay. Secondly, the Schmitt trigger controls the two main oscil­lators which produce the chirping sound. IC1c/d and IC1e/f form two square-wave oscillators and these are enabled or disabled by diodes D1 and D2, respectively. With the output of IC1b 30  Silicon Chip PARTS LIST 1 PC board, code 08112941, 41 x 36mm 1 9V battery snap connector 1 9V battery 1 piezo buzzer Semiconductors 1 4069 CMOS hex inverter (IC1) 1 BC548 NPN transistor (Q1) 1 BC558 PNP transistor (Q2) 2 5mm green LEDs (LED1,2) 2 1N914 signal diodes (D1, D2) 1 light dependent resistor (LDR1) Capacitors 2 100µF 16VW electrolytic 1 2.2µF 25VW electrolytic 1 .047µF MKT polyester Resistors (0.25W, 1%) 1 470kΩ 1 4.7kΩ 2 100kΩ 1 3.3kΩ 1 47kΩ 2 1kΩ 3 10kΩ Miscellaneous 1 x 100mm length of light-duty figure-8 cable (to connect buzzer), solder, PC stakes to terminate external wiring connections to batt­ery & buzzer (optional). normally low (that is in the presence of light), diodes D1 and D2 are forward biased and so hold the inputs to IC1d and IC1f at 0.6V. This prevents either oscillator from starting up. Because pin 9 of IC1f is held low, the output at pin 8 is high, which ensures that the following PNP transistor Q2 (which we’ll get to shortly) is turned off. Similarly, because pin 11 is held low by D1, pins 10 & 13 are high and pin 12 is low. This ensures that Clifford’s “eyes” or LEDs 1 and 2, which are con­trolled by NPN transistor Q1, remain off. When the light level drops, the LDR’s resistance increases to the point where the upper threshold of the Schmitt trigger is surpassed and the output of IC1b snaps high. Diodes D1 and D2 are now reversed biased and the two oscillators are allowed to run free. IC1e and IC1f oscillate at a frequency of about 160Hz with the output driving output transistor Q2. This BC558 transistor drives a low-current piezo buzzer. Now since this buzzer produces a 2kHz tone of its own, the job of this circuit is to simply modulate it to make it sound more like a cricket. The oscillator based on IC1c and IC1d has two jobs. First­ly, it drives Clifford’s green eyes, flashing them on and off at a frequency of around 25Hz. Secondly, the output is mixed togeth­ er with the output of IC1f. The result is that the output of IC1f is frequen- cy-modulated by the signal from Q1 to produce the “shrill” in Clifford’s chirp. Feeding requirements Clifford lives off a 9V battery but he certainly doesn’t waste his food. While sitting quietly, he consumes around 1mA which rises to 8mA when he’s talking. However, the good thing is that Clifford will operate from a battery voltage of just 4.5V, so you can wring every last bit of power out of the battery. If you have an old 9V battery from your multimeter, it should work for quite a while to keep Clifford happy. The 100µF capacitor provides the circuit with a reservoir which lowers the supply’s impedance when the battery is going flat. Construction Clifford is created on a small PC board, measuring 40 x 35mm and coded 08112941. To help keep his size down, all of the resistors and diodes are mounted end-on and close together so you’ll need to have a fine-tipped soldering iron to do the job. Before you begin any soldering, check the board thoroughly for any shorts or breaks in the copper tracks. These should be repaired with a small artwork knife or a touch of the soldering iron where appropriate. Once you’re happy that everything appears to be OK, you can begin construction by installing the IC – see Fig.2. This is the lowest-profile component and is more easily installed first. After that, continue by installing the resistors, diodes, transistors, LEDs and capacitors. The resistors are installed vertically with the leads bent over at right angles, as shown in the photo. When installing the diodes, make sure that you follow the over- Fig.2: install the parts on the board as shown here, taking care to ensure that all polarised parts are correctly oriented. Check each resistor on your multimeter before installing it on the board & note that the resistors are all mounted end-on to save space. Fig.3 at right shows the full-size PC pattern. lay wiring diagram and insert them correctly. The LEDs are also installed with their legs bent at right angles and then gently twisted away from each other to give that cute insect look. When you have completed this, check each com­ponent against the wiring diagram (Fig.2) to ensure that it is correctly positioned. In particular, check that all polarised parts are correctly oriented and be careful not to confuse the two transistors. Q1 is a BC548 NPN type while Q2 is a BC558 PNP type, so don’t get them mixed up. The LDR is a non-polarised device and may be installed either way around. Once you are satisfied that everything is correct, connect the piezo buzzer via a 100mm length of figure-8 cable and install the 9V battery snap connector. PC stakes can be used at the external wiring points on the PC board if you wish but these are entirely optional. His first meal Now install the 9V battery in series with your multimeter and set the DMM to a low milliampere range. The current consump­tion should be slightly over 1mA. Now cover the LDR with your finger to block out all light. The current should start to rise slowly and, after a few seconds, Clifford should burst into life. The current consumption should initially be around 9mA and should drop down to around 8mA. If the LEDs don’t light up, check the connection to the base of Q1 and check that the LEDs are correctly installed. If the piezo buzzer doesn’t sound, check that you have its polarity correct. The negative pin should go to ground. Uses Clifford is best used for maximum effect in a well lit area but somewhere inconspicuous. The area of my workbench was pretty good – there’s lots of junk on it which made it hard for anybody to find anything. While the light level is high enough, he won’t make a noise. When the light goes out, there should be enough of a delay to convince someone that there is a real cricket some­where in the room. When the light goes back on, he should also turn off fast enough to make it difficult for the person to locate the offending source. If you’re looking to really drive people batty, remove the two 5mm LEDs so that they can’t see him in the SC dark at all! RESISTOR COLOUR CODES ❏ No. Value 4-Band Code (1%) 5-Band Code (1%) ❏ 1 470kΩ yellow violet yellow brown yellow violet black orange brown ❏ 2 100kΩ brown black yellow brown brown black black orange brown ❏ 1 47kΩ yellow violet orange brown yellow violet black red brown ❏ 3 10kΩ brown black orange brown brown black black red brown ❏ 1 4.7kΩ yellow violet red brown yellow violet black brown brown ❏ 1 3.3kΩ orange orange red brown orange orange black brown brown ❏ 2 1kΩ brown black red brown brown black black brown brown December 1994  31