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APESKY INSECT TO ANNOY YOUR FRIENDS Horace the cricket always chirps back whenever he hears a noise. If you keep quiet, so does Horace. Make a noise and he joins in and flashes his LED eyes at the same time. He can also be concealed to play his favourite game of hide and seek. By JOHN CLARKE PARTS LIST You know how crickets can be quite annoying at night. You can hear the little beggars chirping away somewhere under the lawn but when you go creeping out to find them, they shut up until you go away. And so you should, you intolerant humanoid! But why should crickets be confined to the great outdoors? Why not have your own pet cricket who can pester people when you want him to? So here's Horace, the electronic cricket. He's a 6-legged beastie, fully house-trained and under your command. Horace incessantly chirps away while there is any noise or talking going on and he only stops when the noise ceases. We are sure that our readers will be very resourceful in finding uses for Horace. And Horace is inexpensive. He only uses a single cheep (sorry cheap) IC and a few other bits and pieces all built onto a PC board. The board itself becomes Horace's body and the battery his belly. He also has a couple of LEDs for eyes, a miniature microphone mouth, a slider switch tail and six resistors for legs. He has a certain jaunty style, aided by the springyness of his resistor legs. 66 SILICON CHIP Best of all, Horace is easy to build. So let's see how he works. Circuitry The circuit for Horace uses just one low power quad op amp IC, an LM324. It is ideal for this task since it will operate happily from a single low voltage supply and has a low current drain of 3 milliamps or less. Horace has two modes of operation. The first is the "listen" mode, whereby Horace listens with its electret microphone for any noises. Having detected a noise above the threshold level, the circuit switches into its "chirping" mode. Only one mode is possible at any one time, so that the circuit can only listen or chirp - it can't do both at the same time. In the listen mode, IClc functions as a non-inverting amplifier with a gain of 455, as set by the lMQ and 2.ZkQ resistors connected to pin 9. The non-inverting (+)input of IClc, pin 10, is biased to about + 4.5V by the lO0kQ resistor connected to pin 13 of ICld. During listen mode, the pin 14 output of ICld normally sits high (ie, + 9V). This is because of the way in which the inputs of ICld are biased. The lMQ pullup resistor at pin 12 1 PC board, code SC08106901, 91 x 61 mm 1 electret microphone insert 1 piezo transducer 1 9V battery holder (DSE Cat. S-61 50 or equivalent) 1 9V 216 battery 1 subminiature DPDT slider switch 2 PC stakes Semiconductors 1 LM324 quad op amp (IC1) 1 1 N4148 diode (D1) 2 red LEDs (LED 1, LED 2) Capacitors 1 1 0µF 16VW PC electrolytic 2 0.22µF RBLL or tantalum electrolytics (or monolithics) 2 .039µF metallised polyester Resistors (0.25W, 5%) 3 1MQ 1 4.7kQ 9 1 00kQ 1 2 .2kQ 1 22kQ 1 1 kQ 6 1 0MQ 1W (for legs) Miscellaneous Solder, tinned copper wire , 5 screws and nuts for battery holder and piezo transducer. and diode Dl holds this input at one diode voltage drop above the output voltage of IClc. And the pin 13 inverting input is effectively at the +9V 10 100k ~, + 16VW.,._ 100 k s+11, J + J 100k .039 T 9V : ...I... ELECTRET MICROPHONE l 100k PIEZO TRANSOUCER 100k 2.2k 22k .I,;" C3 ... 0.22I 100 k RBLL - .,. . 0391 1M 10Mi 1W LEG1 10Mf 1W LEG 2 10Mf 1W LEG3 I I 10M 1Wf LEG4 10Mf 1W LEGS 10M' 1W LEG6 ""'~ 100k .,. LE02 ~ i HORACE same voltage as the output of IClc since IClc itself is biased from pin 13. This incestuous arrangement means that pin 12 of ICld is normally above pin 13 and therefore pin 14 is high. Chirper and LED driver IClb provides the chirp part of the circuit. It is an oscillator which drives the piezo transducer but while ever pin 14 of ICld is high, !Cl b can't function; similarly with ICla, which is the LED driver for Horace's eyes. We'll come back to these two op amps later. Now let's go back to the front of the circuit to the electret microphone. This is biased from the 9V supply via a 4. 7kQ resistor while the input signal is fed to pin 10 of IClc via Cl, a .039µF capacitor. With the circuit in the listen mode, the electret generates a signal voltage which is amplified by IClc. For louder sounds, the output of IClc swings strongly up to the positive rail and down to the 0V rail. The negative swings of the signal pull pin 12 of ICld low and so pin 14 flicks low too. Once that happens, the whole circuit is in the chirp mode. With pin 14 of ICld low, pin 12 is pulled down low too, via the associated lO0kQ resistor and this means that ICld is " latched". It will stay that way until the capacitor at pin 13 can discharge sufficiently for pin 13 to get below pin 12 (which is at about + 0.BV). Until that happens, ICl b is "enabled" as a Schmitt trigger oscillator with its frequency set by the 22kQ . resistor and .039µF capacitor connected to pin 6. It functions as follows: initially, pin 6 is high and the output at pin 7 is low. The .039µF capacitor begins to discharge via the 22kQ resistor until it reaches the lower threshold of 3V (set by the lO0kQ resistors connected to pin 5). Then the output of IClb [pin 7) goes high and begins to charge the .039µF capacitor via the 22kn resistor. This continues until the upper threshold of 6V is reached when the output of ICl b again goes low. This cycle repeats itself and so ICl b oscillates at around 1700Hz to drive the piezo transducer. While IClb is oscillating, ICla turns on the two LEDs at its output. This is because ICla is connected as an Schmitt inverter. When its pin 2 input is low, its output at pin 1 Fig.I: the circuit is based on a single quad op amp IC. IClc is the microphone amplifier and this controls Schmitt trigger oscillator ICld. ICld in turn controls chirp oscillator IClb, while ICla drives the LEDs. is high and this drives the LEDs via a lkQ resistor. So while ever ICld's output at pin 14 is low, the LEDs are alight and the piezo transducer is sounding. None of this sound and light la sts for long though, since the circuit conditions around IClc and ICld don't stay constant. Tone modulation Recall now that pin 10 of IClc, the audio amplifier stage, is normally biased to about + 4.5V via the lO0kn resistor from pin 13 of ICld. When ICld's output flicks low, this bias voltage is removed and so IClc can't function as an audio stage. Instead, the 0.22µF capacitor (C2) on pin 9 begins to discharge from 4.5V via the 2.2kQ resistor and the lMQ resistor to the output of IClc. Some discharge current also flows via the pin 9 inverting input but this is small enough to ignore. The discharge time for CZ is about 200 milliseconds and during this time, the 0.22µF capacitor (C3) AUGUST 1990 67 9V 4.5V "\ IC1c, PIN B Fig.2: this diagram shows the waveforms at various points on the circuit. Notice how the output of IC1d switches high & low to modulate the output of IC1b. at pin 13 is being rapidly charged and discharged. C3 charges and discharges because ICld is now operating as an oscillator by virtue of the lOOkO resistor connected between pin 13 and pin 14. So ICld's output doesn't stay low as we implied. OK, so we led you astray but you'll get the whole picture bye and bye. Thus, ICld's output actually flicks low and high at about 70 times a second (70Hz) and this frequency switches on and off (modulates) the tone produced by ICl b and thus makes it sound richer. The voltage at pin 10 of IClc is essentially the same as that across C3 except that it is filtered by the associated lOOkO bias resistor and Cl. When the voltage at C2 finally discharges below that at pin 10, IClc's output goes briefly high which allows ICld to revert high again too. This puts the circuit back into listen mode and the output of IClc settles back to around + 4.5V. The waveforms of Fig.2 show the circuit functions graphically. The first waveform shows pin 8 of IClc. It starts off sitting at about + 4.5V but with noise superimposed. Then it flicks negative as a strong noise signal is picked up. This causes Cl, C2 & C3 to discharge at different rates, as shown in the second waveform. The third waveform shows the output state of ICld which may be thought of as controlling the whole circuit. Finally, the 4th diagram shows the high frequency waveform which drives the piezo transducer. Construction As noted above, Horace is basically a PC board with a 9V battery slung underneath and the whole lot suspended on six resistor legs. The PCB is coded SC 08106901 and measures 91 x 61mm. Check the board carefully for un- This view shows how the battery holder is mounted. You will have to bend the two leads so that they go through the holes in the board. drilled holes or shorted or open circuit tracks. Fix these first before going further. Assembly can begin by inserting the two PC stakes used for terminating the piezo transducer. Now insert the low profile components such as the links, resistors, diode and IC. Be careful with the orientation of the diode and IC. The piezo transducer is mounted using two small screws and nuts directly onto the PCB. If these have not been supplied you could use a Fig.3: this is the full-size PC pattern for Horace. RESISTORS D D D D D D D 68 No 3 9 1 1 1 SILICON CHIP Value 1MO 100k0 22k0 4.7k0 2.2k0 1k0 4-Band Code (5%) brown black green gold brown black yellow gold red red orange gold yellow violet red gold red red red gold brown black red gold 5-Band Code (1%) brown black black yellow brown brown black black orange brown red red black red brown yellow violet black brown brown red red black brown brown brown black black brown brown ELECTRET MICROPHONE Fig.4: here's how to install the parts on the PC board. Take care with component orientation and note that the battery holder (shown dotted) mounts on the copper side of the board. You can use any value 1W resistors down to 10k0 for the legs. piece of double-sided tape or a spot of Superglue. The wires from the transducer are cut short to terminate to the PC stakes. Note that although the + sign is shown on this transducer, the polarity is unimportant. Next, the capacitors can be installed. Make sure that the electrolytics are oriented correctly. Cl & C2 must be low leakage aluminium or tantalum electrolytics or monolithics. If you use monolithics, you don't have to worry about the polarity. The two LEDs are mounted with long legs which are then bent over, as shown in the photos. The longer lead of each LED is the anode. The miniature slide switch is mounted directly on the PCB. The electret microphone is mounted on Don't stomp on poor Horace if he fails to work first time. Instead, check your work carefully against the wiring diagram (Fig.4). In particular, check the polarity of the IC, LEDs and microphone. two short lengths of 1mm tinned copper wire so that it stands about 10mm above the PCB. Note that the electret is polarised and the negative terminal is the one which is connected to the microphone body. The legs are simply lOMO lW resistors which are mounted at one end of their leads. The free end is folded over for a rounded foot. Actually, you can use resistor values down to about 10k0 for the legs but don't go below this.figure otherwise you risk excessive supply loading if the unit is placed on a metal surface. Before installing the battery holder, it is best to try out the circuit first since the holder obscures many of the tracks underneath the PCB. Use some short leads to temporarily connect power to the circuit. It should operate when switched on - by chirping and switching on the LEDs whenever you make a sound. Troubleshooting If Horace does not work, don't fling him across the room. Check your work very carefully against the wiring diagram. Are all the component values correct? Is there any sign of shorts because of solder splashes underneath the board. Still no sign of life? Use your multimeter, switched to a DC volts range, to check for + 9V betwe.e n pins 4 & 11 of the IC. If that checks out, briefly short pin 14 to pin 11. This should cause the piezo transducer to sound and the LEDs to light. If so, the fault lies in the circuit associated with IClc & ICld. Check the circuit very carefully for wrong components or components installed the wrong way around. Briefly shorting pin 8 to pin 11 should also cause the LEDs to light and the piezo transducer to sound. Any deliberate short circuits applied in this way should only be of brief duration though, otherwise you run the risk of blowing the IC. Finally, the battery holder can be installed. The two leads from it require bending so that they will fit into the PCB holes. This is done so that the leads can be soldered easily on the underside of the PCB. The battery holder is secured to the PCB using screws and nuts. A UGU ST 1990 69