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A game to improve your hand/eye coordination Build a laser pistol & electronic target Most people can’t legally own guns any more so if you have a yen for target shooting, this project will fit the bill. It uses a visible LED laser in the pistol & a bullseye target which responds visibly & audibly when you score a direct hit. By RICK WALTERS Most people are attracted to the idea of target shooting even if they have no wish to own a gun. With this project, you can indulge that whim in a completely harmless way and have a lot of fun in the process. The game consists of a laser pistol and an electronic tar­get. The pistol is a readily available plastic toy which has been modified to hold a battery, a switch for the trigger and a 5mW red laser. Each time it is fired, the laser 58  Silicon Chip emits a brief pulse. Holding down the trigger does nothing; you must pull the trigger fully each time to fire it. The target is quite different from anything you might have experienced in the past. It is active rather than passive and it gives you immediate feedback, if you hit the bullseye or if you miss. While it uses the “Official 100-yard small bore rifle target”, as produced by the National Rifle Association of the USA, we have mod- ified it with quite a bit of electronic circui­try. Around the outer ring of the target are 24 evenly spaced LEDs which chase around the circle for a random period while a siren sounds. Then all LEDs go out and you must fire the pistol within one second and hit the bullseye. If you miss, you get the sound of a machine gun which means you have been SHOT. If you hit the target, you get one of three different sound effects which can be a police siren, ambulance or fire-engine. These are selected randomly by the circuit as your reward for hitting the bullseye. At the bullseye is a PIN diode which is matched to the gun’s laser. If the laser beam hits this diode at the appropriate time, the police siren or one of the other reward sounds will indicate that you hit your target. PARTS LIST (TARGET BOARD) Fig.1: the circuit of the laser pistol. Each time switch S1 is closed, the laser circuit dis­ charg­es the 100µF capacitor to give a brief pulse. The target’s electronics is powered by a 9V DC plugpack. How it works Let’s start with the pistol circuit shown in Fig.1. The 100µF capacitor is always charged by the battery to 3V via the 1.5kΩ resistor. When the trigger is pulled, the switch closes, rapidly discharging the capacitor through the laser diode and associated circuit. The current drawn is such that the laser only emits one pulse of light before the voltage drop across the 1.5kΩ resistor causes it to turn off. Therefore you can’t cheat by holding the trigger down and pointing the barrel at the bullseye. The laser diode assembly consists of a near infrared emit­ter optically cou- pled to a detector diode. This is used to moni­tor the light output from the IR emitter and keep it constant, even while the battery voltage is falling. Q2 monitors the voltage across the 330kΩ resistor, this voltage being proportional to the light output from the diode. The voltage across LED1 is used as a reference for Q2’s emitter. The difference between its base and emitter voltages cause just enough collector current to flow through the 10kΩ resistor to turn Q1 on to give the required light output. The 4.7µF and 0.47µF capacitors slow the rate of rise of the current to ensure that there is no overshoot, which could damage the IR diode. WARNING: the pulse of light from the pistol, while of short duration, is dangerous. It should never be point- PARTS LIST (PISTOL) 1 toy pistol, Toys-R-Us Power Ranger Dart 099236 or equiv­. 1 PC board, code 08112961, 43mm x 16mm 1 momentary contact toggle switch C&K 7109 or equivalent (S1) 2 AAA 1.5V batteries 1 AAA or AA battery holder 1 BC338 NPN transistor (Q1) 1 BC328 PNP transistor (Q2) 1 3mm red LED (LED1) Semiconductors 1 660nm 5mW laser diode and lens assembly, Oatley Electron­ ics 660-5I or equivalent Resistors (0.25W, 1%) 1 330kΩ 1 470Ω 1 10kΩ 1 1.5Ω 1 1.5kΩ Capacitors 1 100µF 16WV electrolytic 1 4.7µF 16WV electrolytic 1 0.47µF MKT 63VW or monolithic ceramic 1 PC board, code 08112962, 140mm x 80mm 1 38mm 8-ohm loudspeaker 1 National Target Co target, TQ-4(T) or equivalent 1 sheet of white perspex to suit target, 355 x 355mm 1 9V DC plugpack 2 10mm x 3mm tapped spacers 2 3mm x 5mm countersunk screws 2 3mm x 5mm screws 24 5mm LED bezels 11 PC stakes Semiconductors 1 40106 hex Schmitt trigger (IC1) 1 555 timer (IC2) 2 4017 counter (IC3, IC5) 1 4093 quad 2-input NAND Schmitt trigger (IC4) 1 4016 or 4066 quad bilateral switch (IC6) 1 UM3561A sound effects generator DSE Z-6203 (IC7) 1 LM311 comparator (IC8) 1 BC338 NPN transistor (Q1) 8 BC328 PNP transistor (Q2-Q9) 1 PIN diode (PD1) Oatley Electronics 04PC2 or equivalent 1 3.3V 500mW zener diode (ZD1) 8 1N914 signal diodes (D1-D8) 1 1N4004 rectifier diode (D9) 24 5mm red LEDs (LED1-LED24) Capacitors 1 100µF 16WV electrolytic 2 10µF 16WV electrolytic 4 4.7µF 16WV electrolytic 3 1µF 16WV electrolytic 3 0.1µF MKT polyester 1 .022µF MKT 1 .01µF MKT Resistors (0.25W 1%) 1 4.7MΩ 1 220kΩ 1 3.9MΩ 1 150kΩ 1 2.7MΩ 4 100kΩ 1 1.8MΩ 5 10kΩ 1 1.5MΩ 1 1.8kΩ 1 1.2MΩ 1 1.5kΩ 3 1MΩ 6 820Ω 1 470kΩ December 1996  59 60  Silicon Chip Fig.2: the target circuit has a LED chaser driven by IC3, a sound effects circuit based on IC5 & IC7, and a random timer based on IC1a, IC1b & IC1c. ed at anyone’s eyes as damage could result. Target circuit Now let’s have a look at the target circuit in Fig.2. We’ll start with the chaser circuit which is based on IC3, a 4017 decade counter. We are using just six of its outputs. As it counts, each of the six outputs will go high (+V) while the rest are low (0V). IC3 is clocked by a Schmitt trigger oscillator based on IC1e together with the 1.5MΩ resistor and the 0.1µF capacitor. Since five of the outputs of IC3 will always be low, five of the six groups of four LEDs will always be turned on by the emitter followers Q4-Q9. Each time the oscillator clocks the counter, the “off” group will step, giving the appearance of rotation. The oscilla­tor resistor and capacitor values are selected to make the target LEDs appear to rotate at a suitable speed. Pin 5 of IC3 (the seventh output) is connected to the reset terminal, so each time the 4017 steps to this output it will reset itself and start over again. Switching another resistor in parallel with the 1.5MΩ resistor using IC6a increases the speed of rotation, as we will see later on. Random timer Schmitt triggers IC1a, IC1b and IC1c, together with their resistors and capacitors, are three oscillators running at slightly different frequencies. Fig.3: dimensions of the bracket for mounting the trigger switch. Their outputs are fed to an AND gate formed by diodes D1-D3. When all three oscillator outputs are low, the voltage across the associated 100kΩ resistor will also go low. The diode AND gate is connected to the input of IC1f via a .01µF capacitor and so when the AND gate output goes low, IC1f’s input will be pulled momentarily low. This causes pin 12 of IC1f to go high and this will rapidly charge the 10µF capacitor at pins 2 & 6 of IC2, via diode D6. IC2 is a 555 timer but the way in which it is connected is not conventional. In effect, it is a monostable and when pins 2 & 6 are taken high via diode D6, the output at pin 3 goes low for a period set by the 100kΩ resistor and 10µF capacitor on pins 2 & 6; ie, around one second. IC2’s output is normally high and when it goes low it af­fects four functions. First, the oscillator formed by IC1a will stop as D5 will hold its input pin near 0V. Second, the monostable formed by IC4a and IC4b will be triggered, taking pin 4 of IC4 high. This will hold the voltage across the 100kΩ resistor high through D4, preventing any further pulses being applied to IC1f for around four seconds. The third consequence will be for all the chaser LEDs to extinguish, as the output pin of IC2 is the supply voltage for them. The LEDs going out is the signal to shoot at the target. The fourth effect is that pin 3 of IC4 will go low and will take pin 1 of IC8 low, thereby grounding the emitter of an inter­ nal transistor which allows its output (pin 7) to go low. If the PIN diode (PD1) is now illuminated by the laser pistol, its current will increase, pulling pin 2 of comparator IC8 below its pin 3. This causes pin 7 of IC8 to go RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 1 3 1 1 1 1 4 6 1 2 6 1 1 Value 4.7MΩ 3.9MΩ 2.7MΩ 1.8MΩ 1.5MΩ 1.2MΩ 1MΩ 470kΩ 330kΩ 220kΩ 150kΩ 100kΩ 10kΩ 1.8kΩ 1.5kΩ 820Ω 470Ω 1.5Ω 4-Band Code (1%) yellow violet green brown orange white green brown red violet green brown brown grey green brown brown green green brown brown red green brown brown black green brown yellow violet yellow brown orange orange yellow brown red red yellow brown brown green yellow brown brown black yellow brown brown black orange brown brown grey red brown brown green red brown grey red brown brown yellow violet brown brown brown green gold brown 5-Band Code (1%) yellow violet black yellow brown orange white black yellow brown red violet black yellow brown brown grey black yellow brown brown green black yellow brown brown red black yellow brown brown black black yellow brown yellow violet black orange brown orange orange black orange brown red red black orange brown brown green black orange brown brown black black orange brown brown black black red brown brown grey black brown brown brown green black brown brown grey red black black brown yellow violet black black brown brown green black silver brown December 1996  61 This is what the pistol looks like after being disassembled and having the electronics installed. The spring-loaded switch is operated by the existing pistol trigger. low, discharging the 1µF capacitor on pin 13 of IC4 via D7. Phew! But we’re not finished yet, as this convoluted circuit has more tricks up its sleeve. We will now talk about the functions of gates IC1d, IC4c & IC4d, counter chip IC5, quad analog switch IC6 and IC7, the sound effects chip. If PD1 is illuminated while pin 1 of IC8 is high, the output will not change. This prevents anyone cheating by continu­ously shooting at the target, hoping to hit the bull just before the LEDs go out. When pin 3 of IC2 goes high after its 1s period, it pulls pins 8, 9 & 12 of IC4 high via the 1µF capacitor. Pin 10 will always go low, turning on Q2, thus applying power to the sound effects chip IC7 and to pin 13 of IC6a via a 10kΩ resistor. This causes the LED flasher to speed up. There will be two different outcomes from the sound effects chip, depending on whether the bullseye was hit or not. IC7 is a low-cost sound effects chip. If pin 1 is taken to the chip’s supply voltage (+3.3V), a machine gun sound is generated, regardless of the voltage on pin 6. If pin 1 is left floating (ie, open circuit), three additional sounds can be generated. If pin 6 is high, a fire engine sound is generated; if low, an ambulance sound; and if left floating, a police siren sound will be heard from the speaker. If the bullseye is missed, both inputs of IC4d go high and its output goes low to turn on Q3. Q3’s collector going high will take pin 12 of switch IC6d high, to connect pin 11 to pin 10. This generates the machine gun sound and YOU ARE DEAD! At the same time, pin 13 of IC6a will be pulled low via D8, switching out the additional 1MΩ feedback resistor for IC1a, thus returning the chaser speed to normal. Conversely, if the photodiode is illuminated while the LEDs are off (IC2, pin 3 low), then D7 will discharge the 1µF capacitor. When pin 3 of IC2 goes high again after one second, Q2 will apply power to the sound chip as previously. As both inputs of IC4d are not high, its output will stay high and Q3 will stay off. This leaves pin 1 of IC7 floating; (ie, IC6d open-circuit). IC1d with its associated resistor and capacitor form an oscillator which clocks IC5, another 4017 decade counter. This time we only use three outputs, resetting it on the fourth. When pin 3 of IC5 is high, the SEL1 input of IC7 is connected to ground and the output sound will be an ambulance. If pin 4 of IC5 is high, the SEL1 input will be high and the sound will be a fire engine. When pin 2 of IC5 (the Q1 output) is high, both IC6b and IC6c are open circuit and therefore the SEL1 input will be float­ing and the sound will be a police siren. Note that pin 2 of IC5 is not connected in the circuit and therefore is not shown. When Q2 turns on it takes the Clock Enable (pin 13) of IC5 high, effectively freezing the selected output. This prevents the selected siren sound from changing halfway through. Thus, IC1d and IC5 together randomly select the siren “reward” sound heard each time the bullseye has been hit. In both the above cases (ie, bullseye or no bullseye), after the 2.7MΩ resistor on IC4 pin 12 has discharged the 1µF capaci­tor, the outputs of IC4c and IC4d will go high again, turning off the siren and returning the chaser to normal speed. The 4-second inhibit monostable (IC4a & IC4b) operating via diode D4 Fig.4: this diagram shows the wiring details of the laser pistol. Make sure the IR1 (the laser diode) is wired correctly and take care to ensure that Q1 (BC338) and Q2 (BC328) are the correct type numbers. 62  Silicon Chip Fig.5: the parts layout for the target PC board. Note that IC3 & IC5 face in the opposite direction to the other ICs. prevents the random timer (IC1a, 1b & 1c) from almost immediately starting the “fire” sequence again, which could be the case from time to time. Pistol assembly Our prototype pistol was purchased from Toys R Us. The red plastic pieces on the handle were prised apart with a knife blade, giving access to three small Phillips head screws which hold the main body together. Once the pistol is apart the black pillar on each half near the trigger must be cut off to make room for the spring-loaded switch. The type of switch we specified springs back to the off position and is actuated by the existing plastic trigger of the pistol. We made a small metal bracket (see Fig.3) to mount the switch and positioned it so that it operated smoothly with the plastic trigger. When the trigger is released the switch pushes it back to the rest position. The elastic bands which previously restored the trigger can be discarded. Most of the laser circuit of Fig.1 was supplied assembled and tested by Oatley Electronics, as a laser pointer. While it could have been used like this, we still required the 1.5kΩ resistor and the 100µF electrolytic to be mounted somewhere. As the parts can be supplied in kit form, we elected to make another small PC board which would accept all the components and be a better fit inside the pistol barrel. Its component layout is shown in Fig.4. Having so few parts it should only take a few minutes to build. Just ensure that the electrolytic capacitors are inserted with the correct polarity and make doubly sure that the wires to the laser diode are con­nected to the correct pins. The pistol wiring is straightforward and should cause no problems. An AAA battery holder is not readily available so we used an AA holder. If the batteries are loose, stretch the springs a little until they are held firmly. The pistol barrel comes with a white plastic tubular insert which was used to hold the dart. The dart is discarded and the tube trimmed 10mm from the end. This piece is used to hold the laser diode in the end of the barrel. Test your work before re-assembling the pistol by pointing the laser at a wall and pulling the trigger. A brief pulse of red light should be seen. The lens will also need to be focused before final assembly. Stand at about the distance you intend to be from the target and, with the 1.5kΩ resistor shorted out, hold the trigger down and rotate the end of the lens until the spot of light is as small as you can get it. Remove the short and assemble the pis­ tol. The red barrel needs to be superglued to the black butt on both pieces before assembly. Target PC board The component layout for the target This close-up view shows the assembled target PC board. Note how the infrared diode sits directly behind a small hole which is drilled through the bullseye. December 1996  63 Fig.6: this diagram shows how the LEDs are wired around the target. PC board is shown in Fig.5. After checking the PC board for open or shorted tracks and undrilled holes, the first step is to fit and solder the 13 links and 11 PC stakes. Fit the resistors, diodes, ICs and other low-profile components first, then move on to the taller components. Be sure to double-check the diode and electrolytic capaci­tor polarities. You should also carefully check the orientation of the various ICs and that the single BC338 transistor (Q1) is in 64  Silicon Chip the cor­rect place. Testing This board can be tested now, before you wire up the target LEDs. Solder the cathodes of each of six LEDs onto the six PC stakes near the 820Ω resistors, with all six anodes connected in parallel to the LED common pin. Connect the speaker to its terminals and connect the DC plugpack or a power supply to the +9V and 0V pins. Apply power and five of the six LEDs should light. After a short time each of the six LEDs should have turned off, but not in sequence, then they should all go out and a second later a burst of machine gun fire should be heard from the speak­er. If all is OK so far, ground pin 7 of IC8. Over a period you should hear the three different sirens. If the board is working, continue with the target wiring, as shown in Fig.6. Fault finding If the LEDs don’t light, check that their polarity is cor­rect by reversing one of them. Pin 3 of IC2 should measure around 11V or thereabouts, depending on the actual output voltage of the 9V DC plugpack. If pin 3 of IC2 is at 0V, look for shorts or a faulty chip. If the LEDs don’t step, check around IC3 or IC1e for faulty or incorrect components or perhaps a blob of solder shorting two pins. If the three siren sounds are not produced (after a number of tries) suspect IC5 or IC1d and its components or a solder bridge. To force a burst of machine gun fire, use a jumper lead from pins 8 & 9 of IC4 to Vcc. If pin 13 is then grounded the other sirens should be heard. Target wiring The cardboard target specified is available from most gun shops. We mounted ours on a piece of white perspex. Before drilling the 24 LED holes, we drilled a hole at the bullseye and two holes to mount the pillars which support the target PC board. These were positioned so that the PIN diode sat behind the bull­seye. The LEDs are wired in series in groups of four, one in each quadrant as shown in Fig.6. If you use a different coloured wire for each group it will help you to keep track of them. All the anodes in the first quadrant are commoned and con­nected to the LED common PC stake. The last LED We used a sheet of white perspex to hold the target. The loudspeaker and PC board mount on the back, with the PIN diode behind the bullseye hole. in the first group should be connected to Q4’s 820Ω resistor, the last LED in the second group to Q5’s 820Ω resistor and so on, until the sixth group LED is connected to Q9’s resistor. You will have to follow the PC overlay of Fig.5 carefully, as the transistors are not in sequence. Now power up the target and check that all functions are working. You will need to carefully “sight” the pistol so that it shoots straight and then you will find that you need a fair amount of practice to hit the target consistently. Have fun. SC Fig.7: here are the fullsize etching patterns for the laser pistol (above) and the target board (right). December 1996  65