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Build this fun project: Remote-controlled electronic cockroach This version of the Electronic Cockroach has its steering controlled via an infrared link. You just put it on the ground, switch it on & steer it left or right by pressing one of two buttons on a handheld transmitter. By JOHN CLARKE In February 1993, we published an Electronic Cockroach which automatically steered itself towards a dark corner. This new version - dubbed the Remote Control Cockroach - dispenses with the dark-seeking feature and has infrared remote steering instead. The Remote Control Cockroach consists of a PC board, two small motors, and a handful of cheap components to make the control circuitry and the IR transmitter. Admittedly, it's cheap72  Silicon Chip er to go out and buy a commercial remote-controlled toy but that won't teach you anything. By contrast, this project will test your electronic and mechanical skills. It's just for fun. A real cockroach has six legs but our electronic version has to make do with three wheels – two at the front and one at the back. The two wheels at the front are independently driven by separate motors while the rear wheel, which is mounted on a swivel, trails behind. Steering is accomplished by stopping one of the motors. The simple but effective drive arrangement uses rubber bands to drive the two front wheels directly from the motor spindles. In order to obtain maximum torque, each motor is driven by a pulse width modulated (PWM) control voltage rather than by a varying DC voltage. This technique ensures that the maximum peak voltage is always applied to the motor, regardless of the speed setting, and helps prevent stalling. Another worthwhile feature of the circuit is speed regulation for the motors. Speed regulation helps the vehicle maintain its speed despite changes in load; eg, due to gradient or rough terrain. Fig.1 shows the basic principle of the motor speed regulator circuit. What happens is that the circuit monitors the back-EMF generated by PARTS LIST RECEIVER Fig.1: the motor speed of the vehicle is controlled by comparing the motor’s back-EMF with a triangle waveform to derive a voltage pulse train. If the motor slows, the back-EMF falls & the pulse length increases to bring the motor back up to the correct speed. the motor (the faster the motor spins, the greater the back-EMF). This backEMF is compared against a triangle waveform generated by an oscillator and the resulting pulse waveform then drives the motor. When the motor is running at high speed (with a light load), the back-EMF is high and so the resulting pulses fed to the motor are quite narrow. However, if the motor is heavily loaded, the back-EMF voltage drops because the motor slows down. This then increases the width of the pulses applied to the motor to bring the motor back up to speed. Circuit details Fig.2 shows the circuit details. While it may look complicated at first glance, it can be readily split into two sections: (1) a remote control receiver (IC3 & IC4); and (2) the motor control circuitry (IC1 & IC2). Furthermore, the motor control circuitry can be split into two identical sections. IC1c, IC1b, IC1a and Q1 control the righthand motor, while IC1d, IC2b, IC2c and Q2 control the left motor. IC2a is the triangle waveform generator referred to earlier. This device is wired as a Schmitt trigger and operates as follows: when power is first 1 PC board, code 08307931, 84 x 238mm 2 hobby motors (M1, M2 - Jaycar Cat. YM2707) 2 42mm diameter plastic wheels (Aristo-craft or equivalent) 1 130mm-length of 1/8-inch brass tubing 1 150mm-length of 1/8-inch brass threaded rod 4 brass nuts to suit 1 22mm aluminium knob 2 12mm brass untapped spacers 2 9mm brass untapped spacers 2 6mm brass untapped spacers 4 1/8-inch steel washers 1 4-way AA square battery holder 1 battery clip for holder 4 AA 1.5V alkaline cells 4 6 x 60mm diameter rubber bands 1 SPDT toggle switch (S1) 2 10kW horizontal trimpots (VR1,VR2) 1 200mm-length 1.5mm copper wire 1 250mm-length 0.8mm tinned copper wire 1 80mm-length red hook-up wire 1 80mm-length black hook-up wire Semiconductors 2 LM339 quad comparators (IC1,IC2) 1 4049 hex CMOS inverters (IC3) 1 LM358 dual op amp (IC4) 2 BD646 PNP Darlington transistors (Q1,Q2) 1 BC548 NPN transistor (Q3) 1 3.3V 400mW zener diode (ZD1) 2 1N4004 1A diodes (D1,D2) 3 1N4148 switching diodes (D3,D4,D5) 1 BPW50 infrared photodiode (IRD1) applied, pin 1 is high and the 2.2µF capacitor at pin 6 begins to charge via the 22kW resistor. When the capacitor voltage exceeds the voltage on pin 7, pin 1 goes low and the capacitor now discharges until the voltage at pin 6 drops below the voltage on pin 7 again. Pin 1 then switches high again and so the process is repeated indefinitely while ever power is applied. The resulting triangle waveform at pin 6 is applied to the non-inverting Capacitors 1 1000µF 16VW PC electrolytic 1 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 3 2.2µF 16VW PC electrolytic 3 0.1µF MKT polyester 1 .047µF MKT polyester 5 .01µF MKT polyester 5 100pF ceramic Resistors (0.25W, 1%) 3 470kW 2 4.7kW 12 100kW 1 2.2kW 1 68kW 7 1kW 3 47kW 1 390W 1 22kW 1 180W 1 15kW 1 120W 10 10kW 1 47W TRANSMITTER 1 plastic case, 82 x 54 x 30mm 1 PC board code, 08307932, 47 x 45mm 2 momentary click action PC-mount switches 1 216 9V battery 1 battery clip 8 machined IC pins (from socket) Semiconductors 1 ICM7555, LMC555CN CMOS timer (IC1) 2 CQY89A infrared LEDs (LED1,LED2) 1 BC328 PNP transistor (Q1) 2 1N4004 1A diodes (D1,D2) Capacitors 1 220µF 16VW PC electrolytic 1 0.1µF MKT polyester 1 0.01µF MKT polyester Resistors (0.25W, 1%) 1 4.7MW 1 100kW 1 5.6kW 1 150W 1 5.6W inputs of IC1a, IC1b, IC2b & IC2c. IC1b compares the triangle waveform with the voltage on its pin 4 input, as set by trimpot VR1 and the back EMF developed by motor M1, to produce a pulsed waveform. IC1b's output is inverted by IC1a. Thus, each time the output of IC1b swings low, pin 1 of IC1a is pulled high (via a 10kW pull-up resistor) and Q1 is held off. Conversely, when IC1b's output swings high, IC1a's output goes September 1993  73 74  Silicon Chip IRD1 BPW50 B1 6V  A K 47k 8 1 10 1000 16VW POWER S1 B CE IC3a 4049 9 470 16VW 180  A K .01 B V+ 100 16VW 14 14 IC4b AGC +3.3V 10k 10k 100k +3.3V 5 6 .047 10k 15 2.2 16VW 7 D5 1N4148 IC3c 100k 100pF 6 7 10k 10k IC2a LM339 68k 120  .01 3 1 1k REMOTE CONTROL COCKROACH ZD1 3.3V 400mW 47  +6V 15k .01 470k +3.3V 2.2k 470k VIEWED FROM BELOW Q3 BC548 0.1 10k IC3b 12 11 100k 100pF IC3d 2 22k 100k 100pF 10k 100k 100k +3.3V 100k 100k 100k +3.3V .01 5 8 9 10 10 11 11 IC3e 100k 100pF 4 IC1d 2.2 16VW 14 2.2 16VW M2 SPEED VR2 10k IC1c 10k 13 M1 SPEED VR1 10k .01 47k 7 1k 1k 5 4 5 4 IC1b 6 10 16VW IC2b 10 16VW IC3f 100k 100pF 2 1k 0.1 D4 1N4148 10k D3 1N4148 2 10k 470k 100k 0.1 9 8 1k 7 6 390  3 V+ D2 1N4004 4.7k 8 1 10k 14 10k 4 IC4a LM358 D1 1M4004 4.7k 12 3 V+ IC2c 2 3 47k 12 IC1a LM339 10k M2 LEFT TURN Q2 BD646 M1 RIGHT TURN BACK EMF 1k +6V C E +6V C Q1 BD646 E B BACK EMF 1k 1 +3.3V Fig.2 (left): IC1b, IC1a & Q1 drive motor M1 on one side of the vehicle, while IC2b, IC2c & Q2 drive motor M2 on the other. IC2a is the triangle waveform generator – its output is compared with the back-EMFs generated by the two motors using IC1b & IC2b. Infrared diode IRD1 receives steering pulses from the transmitter. These pulses are processed by IC3a-f, IC4a & IC4b & used to switch the motor drive circuits. low and turns transistor Q1 on via a 1kW current limiting resistor. Because Q1 is a Darlington type (BD646), it requires only a small amount of base current to fully switch on. Diode D1 protects Q1 against any large voltage spikes that are generated by the motor M1 when the transistor turns off. The back EMF developed by the motor is sampled by a voltage divider consisting of a 4.7kW resistor and a 1kW resistor and the sampled voltage then applied to D3. When the motor is off, D3 will be forward biased and so a sample of the back-EMF also appears across the associated 10µF filter capacitor. This voltage is then further filtered by a 1kW resistor and 2.2µF capacitor and applied to pin 4 of IC1b. If the back EMF rises, the voltage on pin 4 also rises. As a result, the pulses from IC1b become narrower and so the motor slows down. Conversely, if the back-EMF falls, the voltage on pin 4 of IC1b also falls and the output pulses become wider to bring the motor back to the set speed. The initial speed of the motor is set by trimpot VR1. When Q1 is switched on, D3 is reverse biased and so the filtered backEMF voltage in unaffected (ie, the back-EMF is monitored only when the drive to the motor turns off). Motor M2 is controlled in exactly the same manner by IC2b, IC2c and Q2. The back-EMF of this motor is monitored via diode D4, while VR2 sets the overall speed of the motor. Infrared receiver The infrared receiver consists of linear amplifier stages IC3a-IC3f and comparators IC4a & IC4b. This section of the circuit is powered from a regulated 3.3V rail so that it will be unaffected by battery voltage fluctuations due to motor operation. Because op amps have very poor frequency response and low gains when powered from 3.3V, CMOS inverters have been used as amplifiers instead. These are biased to operate in a linear mode by connecting a 100kW feedback resistor between each input and output. IR pulses from the transmitter are picked by infrared receiver diode IRD1 which then applies voltage pulses to pin 9 of IC3a. The resulting voltage pulses on IC3a's pin 10 output are then amplified by IC3b-IC3f. Each of these amplifiers operates with a gain of 10, as set by their 100kW and 10kW This “under-the-chassis” view shows the arrangement of the front & rear wheel assemblies. A small piece of black cloth was glued to the rear wheel so that its appearance matched the other wheels. Fig.3: the left & right turn signals consist of 40µs pulses with repetition rates of 33ms & 0.7ms, respectively. The filtered signal on pin 2 of IC4a is about 0.3mV for a left turn signal & about 150mV for a right turn signal. feedback resistors. The .01µF capacitor at the input of each amplifier rolls off the frequency response below 1.6kHz to filter out 50Hz mains signals. As an additional precaution, a 100pF capacitor is connected across each feedback resistor to roll off the response above 16kHz. Note that pin 7 to IC3f is tied to the 3.3V supply rail via a 47kW resistor. This ensures that pin 6 of IC3f remains low when no IR signals are being received. When IR signals are received from the transmitter, pin 6 of IC3f delivers an amplified positive-going pulse train. The output from IC3f is split two ways. First, it drives the inverting input (pin 2) of IC4a via an RC filter circuit. And second, it drives an AGC filter consisting of a 120W resistor, diode D5 and a 0.047µF capacitor. When an IR signal is received, the positive-going pulses from IC3f charge the .047µF AGC capacitor via D5. If the voltage across the capacitor rises above 1.4V, Q3 turns on and shunts the signal at pin 11 of IC3b via a 0.1µF capacitor. This forms a crude form of automatic gain control (AGC) that prevents the amplifier stages from overloading when a strong infrared signal is received. The DC level at pin 2 of IC4a is used to discriminate between a left or right September 1993  75 ► 2x1N4004 D2 D1 LEFT TURN S1 Q1 BC328 RIGHT TURN S2 4.7M 100k 5.6k B1 9V 7 4 150  6 A LED1 CQY89A  K A LED2 CQY89A C A C 5. 6  .01 B VIEWED FROM BELOW 3 1 E B 8 IC1 7555 2 E 220 16VW 0.1 Fig.4: the transmitter circuit uses 7555 timer IC1 to drive two infrared LEDs via switching transistor Q1. The pulse repetition rate depends on whether the 4.7MW or 100kW timing resistor is selected & this in turn depends on whether S1 or S2 is pressed.  K K REMOTE COCKROACH TRANSMITTER turn signal from the infrared transmitter. Fig.3 shows how it works. As shown, both the left and right turn signals consist of a train of 40µs pulses. However, whereas the left turn pulses have a repetition rate of 33ms, the right turn pulses have a repetition rate of just 0.7ms. As a result, the filtered signal on pin 2 of IC4a will be close to 0V (0.3mV to be exact) for a left turn signal and about 150mV for a right turn signal. IC4a compares the filtered signal on it pin 2 input with a 120mV reference voltage on its non-inverting (pin 3) input, as set the 10kW and 390W divider resistors. Its output at pin 1 will thus be high for a left turn signal and low for a right turn signal. The 47kW feedback resistor provides hysteresis so that the op amp switches cleanly at the transition point. If the output from IC4a is low (for a right turn signal), pin 9 (and thus pin 14) of IC1d will also be low. The output of IC2c will thus be pulled high and so Q2 and motor M2 will be off. Motor M1 continues to run however, and so the vehicle turns right. Conversely, if a left turn signal is received, pin 1 of IC4a goes high and so motor M2 runs. Pin 10 of IC1c will now be at ½Vcc (due to the two 100kW divider resistors), while the output of IC4b will be low due to the AGC signal on pin 6. Pin 11 of IC1c will now be lower than pin 10 and so Q1 and motor M1 turn off. Motor M2 is Fig.5: the top two traces on this oscilloscope photograph show the triangle waveform at pin 5 of IC2b superimposed on the back-EMF (pin 4 of IC2b). The lower trace shows the motor drive signal at pin 14 of IC2c. (Note: The vertical sensitivity is 0.2V/div for the top two traces and 1V/div for the bottom trace). 76  Silicon Chip running, however, and so the vehicle now turns left. When no infrared signal is received, the outputs of IC4a and IC4b are both high and both motors are free to run. Power for the circuit is derived from a 6V battery pack comprising four AA cells. S1 switches power on and off and the 6V rail is used to directly power the Darlington transistors (Q1 & Q2). This rail is decoupled using a 1000µF capacitor. IC1 & IC2 are powered via a decoupling circuit consisting of a 180W resistor and 470µF capacitor, while the remainder of the circuit is powered from a regulated 3.3V rail derived using ZD1 and a 100µF capacitor. Transmitter circuit The transmitter circuit uses a 7555 timer (IC1) to drive two infrared LEDs via switching transistor Q1 - see Fig.4. IC1 is wired as an astable oscillator and delivers 40us wide negative-going pulses to transistor Q1 when power is applied. Each time a pulse is received, Q1 turns on and drives the two infrared LEDs (LED1 & LED2) via a 5.6W current limiting resistor. This results in brief 1A current pulses through the LEDs but since the average current is much lower than this, it is well within the LED ratings. The pulse repetition rate depends on which of two timing resistors is selected and this in turn depends on whether S1 or S2 is pressed. If S1 is Fig.6: this oscilloscope photograph shows the right turn signal from the transmitter. The trace shows the voltage developed across the 5.6W currect limiting resistor in series with the infrared LEDs. The 40µs pulses occur once every 0.7ms (scope settings: 1V/div vertical sensitivity & 0.1ms horizontal timebase). 1k 68k 10k IC2 LM339 1 2.2k 10k 1000uF D1 1k 10k 1k 1 10k D3 1k VR1 Fig.7: install the parts on the PC board as shown in the wiring diagram. Make sure that all polarised parts are correctly oriented (see Fig.2 for semiconductor pin-out details) & note that the metal bodies of the motors must be grounded. pressed, the 4.7MW resistor is selected and the pulses occur once every 33ms. If S2 is pressed, the 100kW timing resistor is selected and the pulses occur at 0.7ms intervals. SOLDER Power for the transmitter is derived from a single 9V battery and is applied to the circuit via D1 or D2, depending on which switch is pressed. These two diodes isolate the timing resistors from NUT WASHER 30mm PCB 9mm UNTAPPED BRASS SPACER SOLDERED IN HOLE IN PCB WASHER SOLDER NUT NUT 1/8" THREADED BRASS ROD 22mm DIA ALUMINIUM KNOB 100pF 100k 0.1 470k 47k 2.2uF 0.1 100pF 100k 100pF 47k 10k .01 47k IRD1 A K 120  0.1 1 100k 390  IC4 LM358 MOTOR 1 2.2uF IC1 LM339 100k 10uF 1 10k Q1 4.7k 100pF 10k .01 100k 100pF VR2 100k 1k S1 B1 6V D4 10k 10k 1k D2 .01 10k .01 100k 10k 100k D5 Q3 .047 470k 10k 15k 100k 1k 4.7k .01 470k 2.2k 10k 100k ZD1 IC3 4049 MOTOR 2 470uF 10uF 100k Q2 100uF 47  100k 2.2uF 180  each other. A 220µF capacitor decouples the supply rail and helps supply the peak current to the LEDs, while the 0.1µF capacitor provides supply decoupling for IC1. Construction All the parts for the Remote Control Cockroach are installed on a PC board coded 08307931 – see Fig.7. No particular order need be followed when installing the parts on the PC board but make sure that all polarised parts are correctly oriented. These include the electrolytic capacitors, diodes, transistors and ICs. Take care also with the orientation of the infrared photodiode (IRD1). After mounting, bend its leads at right angles so that its photosensitive area faces upwards (see photo). The circuit diagram (Fig.2) shows the pin details for IRD1 and the transistors. SOLDER NUT 9mm BRASS SPACER NUT 60mm SOLDER DRILL HOLE THROUGH KNOB THIS END Fig.8: the rear wheel assembly is made up using a 22mm-diameter aluminium knob, a 150mm-length of threaded brass rod, two 9mm spacers & several nuts & washers. Make sure that the knob spins freely on its spacer & that the pivot assembly rotates freely before soldering the nuts to the threaded rod. Fig.9: a convex mound of solder must be built up on each motor shaft to prevent the rubber bands from coming adrift while the motors are running. This is done by applying solder to the shaft while the motor is running (wear eye goggles) & then filing the solder to shape. September 1993  77 MOTOR SHAFT Fig.10: this plan view shows how the motor shafts are coupled to the front wheels via the rubber bands. Position the axle so that the rubber bands stretch by about 7mm when they are installed & adjust the spacers so that the wheels clear the PCB by about 2mm. MOTOR SHAFT RUBBER BAND RUBBER BAND UNDERSIDE OF PC BOARD 12mm UNTAPPED BRASS SPACERS SOLDERED TO PC BOARD 6mm UNTAPPED BRASS SPACERS WASHERS WHEEL WHEEL CRIMP END WITH PLIERS 1/8" BRASS TUBING ADJUST FOR RUBBER BAND TENSION 2mm 2mm 130mm The two motors are secured to the PC board using enamelled copper wire straps (1.5mm-thick) – see photo. In each case, one strap is soldered to the motor body to provide shielding for the receiver circuitry. You will have to scrape away some of the enamel on each of the two straps to achieve a good solder joint. Once the motors have been secured, they can be wired to the PC board as LED1 A LED2 K A 5. 6  K 5.6k .01 220uF S1 TO B1 100k Q1 S2 4.7M IC1 7555 D1 150  0.1 1 D2 Fig.11: parts layout for the remote control transmitter. The two switches are mounted on machine IC pins & must be correctly oriented (see text). 78  Silicon Chip shown in Fig.7. Note that the motor terminals are not identified. If either motor subsequently runs backwards, just swap the wiring to the PC board. The 9mm spacer for the rear wheel pivot can now be soldered into place. This spacer is mounted vertically immediately to the left of IC3 and should be installed so that it protrudes about 3mm above the board surface. The circuit can now be checked for correct operation. To do this, wind both trimpots fully clockwise, apply power and check for +5V (approx.) on pin 3 of IC1 and on pin 3 of IC2. ZD1 should have a nominal 3.3V across it and this voltage should appear on pin 1 of IC3 and pin 8 of IC4. If the supply voltages are correct, rotate each trimpot until its corresponding motor runs reliably at slow speed. Check that each motor exhibits a fair amount of torque when you try to stop it by grabbing hold of its shaft. If one or both motors fails to operate, go over the board carefully and check for wiring errors. in Fig.9. This ensures that the rubber bands remain on the shafts and don't wind off when the motors start to run. To form this solder mound, run the motor at a slow speed, apply the iron and allow the solder to slowly build up on the shaft (important: wear eye goggles to avoid getting solder in your eyes). When a sufficient mound REMOTE CONTROLLED COCKROACH + + LEFT RIGHT Mechanical assembly The first step in the mechanical assembly is to apply a convex mound of solder to each motor shaft, as shown Fig.12: this is the full-size artwork for the transmitter front panel. Bend the leads of the photodiode (IRD1) through 90° so that its sensitive area faces upwards as shown in this photograph. This close-up view shows the solder mound on the shaft of one of the motors. The two motors are fastened to the PCB using straps made from 1.5mm-diameter copper wire, with at least one strap soldered to each motor body to provide shielding for the receiver front end. has built up, remove the iron and the solder to cool with the motor still running. Once the solder has cooled, it can be carefully shaped using a small file. Again, this is best done while the motor is running. The front wheel assembly is next. Temporarily fit one of the wheels to the axle, position it on the underside of the vehicle and fit the rubber band as shown in Fig.10. Position the axle so that the rubber band is just stretched by about 5mm and mark the position of the axle on the board with a pencil. The two 12mm spacers can now be soldered to the underside of the PC board (see Fig.10). Position these spacers so that additional 6mm spacers can be fitted as shown. These spacers ensure that the inside edges of the wheel clear the PC board. The wheels can now be fitted and secured by crimping the axle ends with pliers. Note that two small washers are fitted between each wheel and the crimped axle end so that the wheel turns freely. Don't just use one washer here. If you do, it may bind on the crimped end of the axle and stop the wheel from rotating freely. The pivoting rear wheel assembly is shown in Fig.8. We used an aluminium knob for the wheel and 1/8-inch threaded brass rod for the swivel. The normal shaft hole in the knob was drilled right through to accept the brass rod, while a 9mm brass spacer serves as CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.1µF   100n   104 0.047µF   47n   473 0.01µF  10n  103 100pF  100p  101 RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 3 13 1 3 1 1 10 1 2 1 7 1 2 1 1 1 Value 4.7MW 470kW 100kW 68kW 47kW 22kW 15kW 10kW 5.6kW 4.7kW 2.2kW 1kW 180W 150W 120W 47W 5.6W 4-Band Code (1%) yellow purple green brown yellow purple yellow brown brown black yellow brown blue grey orange brown yellow purple orange brown red red orange brown brown green orange brown brown black orange brown green blue red brown yellow purple red brown red red red brown brown black red brown brown grey brown brown brown green brown brown brown red brown brown yellow purple black brown green blue black gold 5-Band Code (1%) yellow purple black yellow brown yellow purple black orange brown brown black black orange brown blue grey black red brown yellow purple black red brown red red black red brown brown green black red brown brown black black red brown green blue black brown brown yellow purple black brown brown red red black brown brown brown black black brown brown brown grey black black brown brown green black black brown brown red black black brown yellow purple black black gold green blue black black silver September 1993  79 Fig.13: full-size etching pattern for the transmitter PCB. The transmitter PCB clips into a small plastic utility case, leaving enough room at one end for the 9V battery. Bend the leads of the two IR LEDs at right angles so that the devices protrude through holes drilled in one end of the case. the wheel bush. This brass spacer fits into the existing 6mm-diameter shaft hole in the knob. The wheel assembly is fitted to one end of the brass rod and secured with a nut on either side. Check that the wheel turns freely before soldering the nuts in position. This done, bend the rod into a U-shape around the wheel, taking care to ensure that it finishes up at right angles to the axle. The end of the rod is then bent upwards through 90° about 60mm from the axle, so that it fits through the vertical spacer on the PC board. Finally, the battery holder can be secured to the PC board using two more rubber bands. Transmitter assembly Fig.11 shows the assembly details for the infrared transmitter. All the parts are installed on a PC board coded 08307932 and this clips neatly into a small plastic case. Before mounting any of the parts, drill out the mounting holes for each of the two switches using a 1/16-inch drill. A machined IC pin (obtained from a machined-pin IC socket) should now be pushed into each mounting hole. Push each pin down to its top flange, so that only about 0.5mm of the pin remains above the board. This done, the two pushbutton switches can be mounted and soldered directly to the tops of the pins (see photo). Be sure to orient the switches exactly as shown in Fig.11 – ie, with the flat side of each switch towards the IR LEDs. Adjust trimpots VR1 & VR2 on the main board so that the two motors run at the same speed. This will ensure that the vehicle tracks in a straight line with no steering input. If one of the motors runs backwards, just swap its lead connections to the PCB. 80  Silicon Chip The two pushbutton switches are mounted by soldering their leads to machined IC pins that sit about 0.5mm above the surface of the PCB. This close-up view shows how the battery clip is modified so that the battery assembly fits inside the case. Part of the plastic moulding around two of the screw holes in the lid must also be cut away to provide clearance for the battery. The remaining parts can now be installed on the PC board. Mount the two infrared LEDs at full lead length and make sure that you orient them correctly (the anode lead is the longer of the two). After mounting, the two LEDs are bent over at right angles so that they protrude through two holes drilled in one end of the case. You will also have to drill two holes in the lid of the case for the pushbutton switches. This can be done by first attaching the self-adhesive label as a drilling template. Note that the battery clip must be modified to allow and the battery assembly to fit inside the case. This simply involves removing the plastic cover from the top of the clip and soldering the two leads to the sides of the eyelets instead of to the top. In addition, you will have to cut away part of the plastic moulding around two of the screw holes in the lid, to provide clearance for the battery. Test the operation of the transmitter by checking that the left and right switches stop the right turn and left turn motors respectively. Warning: do not hold the transmitter too close to the receiver diode, as this will only overload the front end of the receiver and cause incorrect operation. Finally, check the transmitter operation with the car on the ground. By walking directly behind the vehicle, you should be able to steer it left or right at will with the transmitter. Note that the range of the infrared link is limited to about three metres, due to the low supply voltage used SC for the receiver circuit. Fig.14: full-size etching pattern for the main PCB. September 1993  81