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Constructional Project By Phil Prosser & Zak Wallingford Build your own This project is fun and ideal for people learning electronics, especially kids. It introduces some basic skills, such as soldering, and demonstrates what can be achieved with simple circuits. It is perfect for building with young family members or as a teaching aid for students. T he Laser Communicator is for play; it is not a ‘practical’ device, although you might find uses for it beyond fun and learning, in which case, all power to you! As we all know, everything is better with a laser on it – even sharks! So, what is the Laser Communicator? It allows you to transmit voice or music over a laser beam. That might be across the room, down the corridor or even further! The link is far from hifi and requires you to adjust things to make it work, but it isn’t too hard to set up. During testing and trials, my 10-year-old grandson, Zak, was able to talk over this down our corridor over a distance of about 15 metres. Photo 1: the transmitter box is relatively modest; the screw jack is needed to adjust the elevation of the laser beam. Once your elevation is set, it is easy to nudge the beam azimuth. 8 It would be fair to say that keeping things aligned over this distance was a challenge, as the deflection of our floorboards as we walked on them caused the laser to wobble around a lot at the receiver. We have kept the layout very spread out and used beginner-friendly pads to keep construction straightforward. The hardest part of this project is cutting and drilling the enclosure. We have made the transmitter board so you can use it ‘bare’, but we think the boxed version is better if you can deal with making it. Zak enjoyed drilling the mounting holes but left the larger speaker hole to my more experienced hands. When building this with Zak, we split the PCB construction into two sessions of about an hour each, plus one for drilling and preparing the cases and another for assembly and testing. I made a point of building a unit alongside Zak to demonstrate what he needed to do, and with that guidance, he could undertake the majority of tasks alone. Older constructors may go faster and require less assistance. Let’s start with a caution. This project uses a laser (we could have used an IR LED, but that is nowhere near as fun). We have used a 1mW laser diode and designed the driver so that it cannot deliver more power than that, which ensures this remains a Class 2 laser. This is the same power level as your average laser pointer. The Class 2 laser we are using will cause a “blink reflex”, and people will normally look away. This Class 2 laser “Would not harm an eye unless a person deliberately stared into the beam. Laser protective eye wear is normally not necessary. A Class 2 laser is not a skin or materials burn hazard.” As a further caution, we have designed this circuit to operate the laser diode at 60% of its standard operating current. This results in the average laser output power being much lower than 1mW, giving us headroom to apply amplitude modulation to the laser output for transmitting the audio signal. Laser beams have very low divergence, and even a 1mW laser can cause visual interference at well over 100m, so never point this toward people or vehicles. If you build this with a youngster, ensure that they fully understand that this is never to be pointed at a person, and supervise them while using it. How does it work? Many things in our day-to-day lives use wirelessly transmitted signals. TVs, radios and mobile phones all use the RF transmission of electrical signals. These systems use radio-­ frequency signals to transmit the data, with antennas at each end (transmitter and receiver). In this project, we transmit the audio information optically using light (the laser) as the carrier. The actual audio is impressed on the light as an amplitude modulation, which means we are changing the intensity of the laser to carry the audio information we want to send. Practical Electronics | February | 2025 Laser Communicator One way to think about it is that it’s a 430THz (terahertz) radio system, although electromagnetic radiation at that frequency certainly behaves a little differently compared to 430MHz or 5GHz! We can amplitude-modulate a laser by changing the current through it, which is a simple way of implementing AM (that’s basically how it’s done for RF). At the receiver end, we need to sense the laser light and somehow turn the amplitude modulation into an electrical signal we can deliver to a speaker (ie, demodulate it). Our approach is to use a photodiode and ignore the DC part of the intensity received by passing it through a series capacitor. The remaining AC part of the intensity is fed to the amplifier. Both the transmitter and receiver are about as simple as we can make them, as this is a learning project. Much more complex approaches are used in a realworld laser communications system, but the spirit of this project is learning and some play. The Laser Communicator comes in two parts: a transmitter and a receiver. Each fits in a standard Jiffy box: UB3 (130 × 67mm) for the transmitter and UB2 (197 × 112mm) for the receiver. The transmitter block diagram is shown in Fig.1, while the receiver block diagram is in Fig.2. The transmitter is shown in Photo 1 and the lead image. This box includes an electret microphone driver, bias generator, voltage-­to-current converter and the laser itself. We have used a fixed bias for the laser diode that sets the current to about 20mA. This has proven sufficient to drive all the laser diodes we tried and keeps using the transmitter simple. The combined bias and audio signal drive our voltage-to-current converter with five transistors implementing an operational amplifier (op amp) with buffer. We selected a Keyes unit for the laser diode. These are very commonly available as Arduino breakout modules. We have included a screw jack on the base of the box using a 3/16-inch nut and bolt that we found in the shed glued to a PCB offcut (an M5 or M6 nut and bolt/machine screw would also work). This allows fine adjustment of the tilt of the transmitter, which is essential to align it with the receiver over longer distances. Practical Electronics | February | 2025 Fig.1: the modulator in the transmitter uses a differential amplifier set up as a voltage-to-current converter. Fig.2: the receiver uses a phototransistor driving a LM386 IC amplifier, which in turn drives a 100mm loudspeaker. The receiver is housed in a much larger box, as shown in Photo 2 and the lead image. This box includes the PCB with the phototransistor and amplifier as well as a 100mm loudspeaker. The receiver has a sensitivity control that doubles as a volume control. The illumination level on the receiver will vary greatly over different ranges and depending on how well-aimed the laser is. That means the phototransistor must operate over a wide dynamic range of intensities. We achieve this by making the phototransistor’s load resistance adjustable. This also affects the volume, so there is no need for a separate volume control. Even though we are running the laser at a low power, it is quite intense. We can use this fact to make aiming easier by sticking a piece of white paper over the receiver hole in front of the phototransistor. We put a target on this so we had a clear aim point. The benefits of this are twofold: we can see exactly where to aim, and the paper diffuses the laser light into the inside of the receiver box, which spreads it onto the phototransistor even if the aim is not exact. We found that to be the best way to make it work even over pretty long ranges. Transmitter circuit details The transmitter circuit is shown in Fig.3. It uses an electret microphone, which converts sound into an electrical voltage. At normal ‘voice levels’, its output signal is a few hundred millivolts. If you want to use a phone or other line-level input to drive this link, you can omit the leftmost 4.7kW resistor and replace the microphone with a 3.5mm jack socket. We are coupling the electret to the differential amplifier via a 100nF capacitor. This fairly low value was selected as younger users tend to talk right into the microphone, which would cause a lot of popping and saturate the laser link if a higher value were used. Caution: Class 2 Laser — Do not stare into the beam. — This power level is safe for unintended exposure for less than 0.25 seconds (250ms). — Never view the laser using telescopic optics. 9 Constructional Project +9V LASER COMMUNICATOR TRANSMITTER SC Ó2024 S1 POWER IN 1 9V BATTERY +9V +9V 4.7kW 4.7kW 1kW 1kW BC546, BC556 22W 10kW B 220mF 2 0V E ELECTRET MIC 1 Q1 BC556 B CON1 Q3 BC546 100nF C C C 100kW 1 E LASER A 2 K CON3 E 100kW CON2 C B B 2 Q2 BC546 E 1mW l LASER DIODE 22W A D1 1N4148 A D2 1N4148 D3 1N4148 K Q4 BC546 K C B C 10mF A TP1 E B Q5 BC546 E 100mF 56W 1N4148 A 330W K K Fig.3: a handful of discrete components are used to implement an amplitude-modulated laser with direct modulation of the drive current. We made a simple handheld microphone using an empty ballpoint pen case. While basic, this works well, and Zak really enjoyed gluing and shrinking it all together. He also learned that super glue on your fingers is very sticky! More on how we did that later. Photo 2: the receiver box doubles as the speaker baffle. The Post-it note with a target drawn on it is important, as it gives you something to aim at and spreads the laser light, making the link easier to set up (masking tape also works). 10 We want to modulate the laser diode amplitude with the audio voltage. Laser diodes need to be driven by a current source, rather like LEDs, which means that we cannot simply connect the microphone to the laser. Furthermore, as shown in Fig.4, laser diodes have a threshold current below which they do not lase, so we need reasonable control over this. We convert the microphone voltage to a laser current using a differential amplifier. The non-inverting input is fed with the microphone voltage imposed on a bias voltage, while the inverting (feedback) input is a voltage derived from the current through the laser diode. The laser current is converted to a feedback voltage by a resistance in series with the laser diode. The five transistors form a differential amplifier as follows. NPN transistors Q4 and Q5 act as a constant current sink, pulling a fixed current from the junction of the emitters of NPN transistors Q2 and Q3. Those two transistors act as the voltage comparator; as their total emitter current is fixed, whenever one conducts less current, the other must conduct more. The one with the higher base voltage of the two will pass more current than the other, as it will have the higher base-emitter voltage (because the emitters are joined). PNP transistor Q1 is the output buffer that drives the laser. Note how the col- lectors of Q2 and Q3 both connect to the same +9V rail via 1kW resistors. That means any extra current needed for Q3 (when Q2 is conducting less) will tend to come from the base of Q1, so its base current is related to the difference in the two input voltages. When Q3 conducts more, Q1 switches on harder, and when Q3 conducts less, Q1 starts to cut off. The active current sink comprising Q4 and Q5 is probably unnecessary. Still, this current controls the maximum laser current, and we want to ensure it is consistent as the battery discharges and its terminal voltage drops. The 330W resistor sets the tail current for the differential pair to 1.8mA, so about 0.9mA through each of the two 1kW collector resistors for Q3 & Q4 (although Q3 normally conducts a little more than Q2). The DC bias point for the laser diode is set by the three 1N4148 diodes, which will have a combined forward voltage drop of 1.8V. In the absence of a signal, and as the average of an AC signal, the DC voltage on the base of Q3 is set by this via the 100kW resistor. There is a DC base current for Q3 of about 40μA, so the bases of Q2 and Q3 sit at about 1.4V. A feedback loop is created around Q2 and Q3, with the base of Q2 driven through the 100kW resistor that senses the cathode voltage of the laser diode. The cathode current goes to ground Practical Electronics | February | 2025 Laser Communicator Fig.4: the laser optical output as a function of input current. Laser diodes do not operate as a laser until they have sufficient current flowing through them, so we need to set a minimum bias current when modulating the power to the laser. through 22W and 56W resistors. The feedback loop keeps the base voltages of Q2 and Q3 the same, so the average voltage across these two resistors is 1.4V. Thus, the DC bias current for the laser diode is 18mA (1.4V ÷ [22W + 56W]). All the laser diodes we tested had a threshold current much less than that, so they operated without adjustment in this circuit. If, for some reason, your laser diode is way too dim and everything else in the circuit is correct, the laser bias point can be altered by reducing the value of the 56W resistor. Be very careful doing that, though, as you could create laser intensities that exceed Class 2, which is unacceptable without eye protection. The keen-eyed will note a 100μF capacitor in parallel with the 56W resistor. It increases the system’s AC gain, allowing us to get double service from the voltage-to-current amplifier. It provides about 11dB of extra gain for audio signals. The AC laser current is 45mA/V. The maximum input voltage before clipping is about 500mV peak. Receiver circuit details As shown in Fig.5, the receiver uses a simple phototransistor with a resistive load to detect the incident laser radiation. Because we are amplitude modulating the laser, the output of this detector contains both the DC bias on the laser and the AC content that we have modulated on top. Because the phototransistor acts like a diode that responds only to the intensity of incident light, ignoring the carrier frequency, it also demodulates the signal. The current through the phototransistor develops a voltage across potentiometer VR1. This voltage includes a DC component (the average intensity of the laser signal) and an AC component (the modulated audio waveform). If the phototransistor’s load resistance (VR1) is too high, the laser DC bias from the transmitter will saturate it. This will be seen as the voltage on the phototransistor collector increasing until clipping occurs. At high intensities, VR1’s resistance can be reduced to avoid saturation of the photodetector. This allows us to set the receiver’s sensitivity to the intensity of incoming laser light while also acting as a volume control. The 330W resistor is in the circuit so that if VR1 is set to zero, the phototransistor still has a 330W load rather than being shorted across the battery. We have AC-coupled the signal to the input of a venerable LM386 power amplifier, IC1. This is used in pretty much a textbook configuration. We have minimal bypassing on pin 7 as we have battery power, so there should be little rail noise. We have used the gain setting pins (pins 1 and 8) to set a reasonably high Fig.5: the receiver is straightforward, utilising an old-school LM386 power amplifier driven by a phototransistor. Practical Electronics | February | 2025 11 Constructional Project ◀ Photo 3: we got some user feedback on the prototype build, resulting in some tweaks to the design and layout to make it more approachable for all builders. I built the two units simultaneously with Zak so he could watch how I did it, but I let him build his own. Photo 4: this shows how the shielded cable is soldered to the electret microphone insert. The screen braid goes to the pad connecting to the mic case. gain. If you need to reduce the receiver’s gain, you can increase the value of the 1kW resistor. We have specified a 100mm speaker for this project and recommend that it be mounted in a UB3 Jiffy box. Doing so is necessary to achieve decent efficiency and sound output from the receiver. One of our initial prototypes used a much larger hifi speaker, which worked a treat. So if you wish to build a ‘bare’ version of this project, wiring the receiver’s output to a large speaker is a good option. We found that using a tiny 57mm speaker without a box was pretty disappointing, so avoid that. Construction The wide layout and large pads make this an ideal starter project (see Photo 3). The intention was to make it approachable to people of all experiences with a little guidance. We won’t reiterate how to solder as we expect that most readers will have at least some experience doing so. The process for the two boards is similar. Fig.6 is the transmitter’s overlay diagram, which shows where each component goes, while Fig.7 is a similar diagram for the receiver. In each case, start with the resistors. Check the values as you go; if you are unsure, use a multimeter to check their values. We used this as a Fig.6: here’s where to solder the components on the transmitter board. For the electrolytic (can-type) capacitors, ensure the longer leads go into the holes marked with + symbols. The transistors have flat faces that are orientated as shown here. 12 chance to show our youngster how to decode resistors. The transmitter has eleven resistors, while the receiver has only three. Either way, check them against the marked values on the PCB. We start with these as they are the ‘flattest’ parts. Next, install the three diodes on the transmitter board. Make sure they are the right way around, or the transmitter won’t work. We have specified the 1N4148 (a common type, similar to the 1N914 but with lower leakage), but you could use just about any silicon diode. Still, it’s better to stick with the parts that we’ve tested. Next, mount the capacitors. We have Fig.7: similarly, fit the components for the receiver like this. The IC will have a dot or other indicator for pin 1, which has to go at upper left. Like with the transmitter, be careful with the orientation of the electrolytic capacitors and also the phototransistor sensor, Q6. Practical Electronics | February | 2025 Laser Communicator Fig.8: the transmitter lid drilling is straightforward, with just four 3mm holes to drill in a rectangular pattern for mounting the PCB. The transmitter base needs just two holes drilled, with the larger one sized to suit the laser diode, plus a further three holes in the side. ensured that all the electrolytic capacitors face in the same direction, but double-check them as, if they are the wrong way around, bad things will happen. Follow by soldering in the transistors on the transmitter. Q1 is the PNP type (eg, BC55x), while the remainder are NPN types (BC54x). You can happily use BC556/7/8/9 for the PNP and BC546/7/8/9 for the NPN. The main thing to watch for is that you do not get the two types mixed up. Now mount the LM386. You might need to squeeze the pins in a bit to get it to fit. This is a tough old chip, so don’t be afraid of giving it a squish to get it in. Finally, mount the potentiometer on the receiver PCB, along with all of the screw terminals. Use a logarithmic potentiometer here; a linear pot will work but will be more fiddly to adjust. Now install the laser diode. We bent the middle leg of our laser diode module out straight; the remaining legs slot straight into the screw terminal. The “S” marked on the module indicates the anode or positive lead, while “−” indicates the cathode or negative. If you have a different laser diode, you can check which is the anode and which is the cathode using a Practical Electronics | February | 2025 9V battery with a 4.7kW resistor connected in series (you have one of these for your power LED). The laser will light up when the anode is wired to the positive battery terminal. Don’t forget the resistor, or you could burn it out! Wiring advice We have wiring diagrams over the page, so refer to them once we get to that stage. But first, here is some advice. The flying leads of the battery clips will form a fair bit of your wiring. Any other power wiring can be done with light-duty hookup wire. The power LED for the receiver comprises a red LED and a 4.7kW current-­limiting resistor. Make sure that the anode of the diode (longer lead) is wired to the switched 9V 13 Constructional Project This gave us a simple microphone at minimal cost. To make the microphone look neat and for some fun, we put 10mm heatshrink tubing over the whole microphone, down to the cable. If you don’t have a hot air gun to shrink it, many hairdryers are hot enough to work. Housing the boards Fig.9: gluing a nut to the base and threading a screw into it allows you to easily adjust the angle of the case relative to the ground in small increments so you can aim the laser precisely. input, while the resistor goes from the anode of the LED to the ground pin on the power input. The microphone input should be made using shielded cable; we used about a metre of it. Connect the braided screen to the electret ground. This pin connects to the case of the electret, which is visible on the back of the microphone capsule (see Photo 4). Solder the cables’s inner conduc- tor to the electret’s output (the other pad). The screen of this cable goes to the GROUND terminal of the microphone connector on the transmitter board, while the inner core goes to the MIC terminal. We used an old ballpoint pen case as a handle for the microphone by running the coaxial cable through the middle of it, then soldering the electret on top. We eventually ended up gluing it in place with super glue. Assembly into the cases is optional, but we really recommend it. We are providing drilling diagrams that will allow you to assemble the transmitter and receivers into tidy boxes. For the transmitter, mark and drill the holes in the case lid, as shown in Fig.8. Check the location of your marks by placing the PCB on them before drilling. Next, mark and drill the holes in the base, also shown in Fig.8. That includes holes for the power switch, microphone lead and a zip tie (cable tie) to hold the battery still. Now mark and drill the laser output hole. The laser hole can be anything large enough to ensure you can get the laser out. Run the microphone cable through its hole, mount the switches and terminate the leads on the transmitter board. Use 10mm M3 standoffs, 6mm screws and shakeproof washers to mount the PCB to the top of the case. Make a screw jack base for the transmitter, as shown in Fig.9. Our baseplate was 100mm long and 40mm wide, though anything will do that allows you to adjust the tilt of the transmitter. We glued a nut to our Fig.10: there isn’t much to the transmitter wiring, but watch the polarity of both the battery leads and the electret microphone. The laser needs to be screwed to the LASER header. You will need to bend the middle leg out of the way or snip it off with a pair of side cutters. 14 Practical Electronics | February | 2025 Laser Communicator stand so a screw or bolt could be used as a screw jack. Glue the screw jack base to the base of the transmitter box. Sand the ABS plastic so that your glue sticks well. We roughened the base of the case with sandpaper and used Araldite to glue the nut onto the screw jack. Sanding the base gives the Araldite a good surface to adhere to. Make sure you have good ventilation while it cures. Finally, stick rubber feet to the front of the screw jack. Transmitter wiring There is just a little bit of wiring to do, as per Fig.10. You can use any colour wires you choose, but we recommend red and black for the battery and switch wiring. For the microphone, we ran the coax through the case and tied a knot inside as very low-cost strain relief, ensuring that any enthusiasm from young users does not tear the microphone cable from the terminal on the transmitter board. Receiver case assembly & wiring Mark and drill the holes in the lid, as shown in Fig.12. There are two holes in the main case for the zip tie to secure the battery; see Fig.13. The speaker hole might be fiddly to cut. We used a circle saw for ours. ABS plastic is very soft, so a handsaw will do this job easily. This is one task that is best undertaken by an adult if working with young constructors. More tips for kids from a kid! What was important when assembling the boards? » Working out which part needs to go on the board. » Searching for the numbers on the board and working with an adult to make sure I had the right parts. » Learning to ‘decode’ the resistor codes, to check that an adult had given me the right bits. Some soldering tips, how to do it and tricks for people to know: » Keep the iron’s tip away from people! » Go slow; remember not to rush soldering each joint. » Remember where to put the tip of the iron. Put the soldering iron on one side of the joint and put the solder on the other side. » Also, it’s fun just to melt the solder! Do you have any tips for putting heatshrink tubing on wires? » Don’t point the hot air gun at people or their fingers (and watch your fingers when helping!) » Take your time while doing it so that you shrink the tube fully. » Don’t put the tip of the hot air gun right on top of the heatshrink. There needs to be a gap. Tips on drilling the box » Wear safety glasses for protection, and never turn a drill on with your fingers near the bit. » Put tape where you will drill and mark it with a pen. » Hold the parts tight when you drill them. Keep your hand tightly on the box when drilling small holes in the box. Putting stuff in the box: » Make sure the box is drilled properly with the holes where they belong. Phil helped with this. » Put stuff in spots it can fit, and get some help. » Keep stuff steady when you put a zip tie or nuts and bolts on. Using the communicator: » Don’t put the boxes too far away from each other because it’s harder to line up (it was pretty tricky at 15m apart). » Don’t put the microphone right in your face when talking or put it too far away. » You do not need to shout. » To play music over the link, you play a song of your choice and put the small speaker of the phone or whatever you use against the microphone. This works really well. Are there any other cool things? » Waving your hands in the beam makes some really interesting sounds. » Waving a strainer through the beam makes even crazier sounds. » Putting your hand in the beam totally stops the sound. How to get it all lined up: » First, turn both boxes on. » Look for the dot from the laser. It is bright and you won’t miss it. » Turn the screw to get the laser dot to go up and down until it is at the right height. » Then move the box left and right until the dot is on the paper. You are all set to go! Fig.11: when wiring up the receiver, the speaker’s polarity doesn’t matter, but the battery polarity does, so check it. If you wire the LED incorrectly, it won’t light up. Practical Electronics | February | 2025 15 Constructional Project Parts List – Laser Communicator (Transmitter) 1 single- or double-sided PCB coded 16102241, 81.5 × 55.5mm 1 UB3 Jiffy box, 130 × 67mm 1 9V battery 1 9V battery clip with flying leads 1 1mW red laser diode module [eg, AliExpress 1005006404528787] 1 electret microphone capsule (MIC1) 1 solder tag mini toggle switch (S1) 3 2-way mini terminal blocks (CON1-CON3) 1 ballpoint pen case (to use as a microphone case) Semiconductors 1 BC556/7/8/9 100mA PNP transistor (Q1) 4 BC546/7/8/9 100mA NPN transistors (Q2-Q5) 3 1N4148 or similar signal diodes (D1-D3) Capacitors 1 220μF 16V radial electrolytic 1 100μF 16V radial electrolytic 1 10μF 16V radial electrolytic 1 100nF 63V MKT Resistors (all 1/4W 1%) 1 10kW 2 4.7kW 2 1kW 2 100kW 1 56W 2 22W 1 330W Hardware 1 M5 or M6 × 40mm panhead machine screw and hex nut 8 M3 × 6mm panhead machine screw 4 M3 × 10mm tapped spacers 8 M3 star washers (toothed type) 2 6mm-tall rubber feet 1 150mm cable tie 1 1m length of single-core screened cable 2 200mm lengths of light-duty hookup wire (red & black) 1 150mm length of 10mm diameter heatshrink tubing 1 100mm length of 3mm diameter heatshrink tubing 1 100 × 40mm PCB offcut Note how the laser diode is mounted into the screw terminal block, with its third middle lead bent out of the way. You can also see how we used a ballpoint pen case to house the microphone capsule. 16 Next, drill the hole for the sensitivity pot and its locating pin, the photodetector hole, the power switch and the LED. Poke the LED through the 5mm hole in the case and use a dab of superglue to hold it in place. Secure the power switch with its washer and nut. A large pair of pliers helps here, but can be fiddly for younger hands. Use 10mm M3 machine screws, M3 flat and shakeproof washers and nuts to secure the speaker. Connect the battery, LED (with series resistor) and speaker to the receiver board, as shown in Fig.11. Testing First, check your wiring and ensure the black battery lead goes into the GND terminal of the power socket on both boards. Turn the transmitter power on, and you should immediately see the laser light up. Measure the voltage at TP1 by setting your DMM into voltage measurement mode, connecting the red probe to TP1 and the black probe to GND. You should get a reading between 0.8V and 1.2V. If the laser is not lit or the voltage on its cathode is out of the specified range, check that the laser has been connected the right way around. Put a meter across the laser diode on the mA range and measure the current. You should get a reading between 14mA and 22mA. Also you should check the voltages across diodes D1-D3. There should be about 0.6V across each. If this is not the case, check that they are the right way around. Then make sure that the 10μF bypass capacitor is the right way around. To verify that the current source is operating, check that the voltage on the base of Q5 (its middle pin) is about 0.6V relative to GND (its emitter) and that the voltage on the base of Q4 (middle pin) is about 1.2V relative to GND. If these are not OK, verify that you have fitted the right transistors and that they are in the right way around. The base-emitter voltages for transistors Q2 and Q3 should be about 0.6V. With the flat side towards you, the base is the middle pin and the emitter is on the right. If they are wrong, check that the transistor types are correct and that they are the right way around. The voltages Practical Electronics | February | 2025 Laser Communicator across the 1kW resistors should be close to 0.8-1V, with the one connected to Q2 being slightly lower than the other. Receiver testing Before switching it on, check your wiring and make sure that the battery is connected the right way around. Switch it on and measure the voltage between pins 4 (lower left) and 6 (one above lower right) of the LM386 IC; the reading should be very close to the battery voltage. If it is lower, check that the LM386 IC is the right way around and check your wiring and the switch. Next, measure the voltage on pin 5 of the LM386 relative to the GND terminal of CON4. This should be around half the battery voltage. If not, check that the electrolytic capacitors in the upper-right corner of the board are the right way around. My LED bench lamp causes substantial buzz when it is close to the phototransistor, and even LED room lights cause buzz at maximum gain. Such buzz indicates that the circuit is working. Try this with a mains-­ powered LED light in your house or lab. If that doesn’t work, check that the photodiode is the right way around. If the above works, move on to the setup stage. Otherwise, as a final test, monitor the voltage on the middle pin of the potentiometer with a voltmeter and turn the sensitivity pot up and down from minimum to maximum. In that case, you should see the DC voltage vary, especially if the phototransistor is illuminated. With the speaker connected, you could inject an audio signal of about 10-100mV at 1kHz (AC-coupled!) into the middle pin of the potentiometer with the volume turned right up. You should hear a loud (possibly distorted) tone from the speaker. Setup To set the system up, switch both the transmitter and receiver on, with them Figs.12 & 13: the receiver lid drilling (top diagram) is the most complicated of the project, with one large cut-out for the speaker that we made with a hole saw, plus six smaller holes to drill. Shown in the bottom section of the diagram are the locations of two holes that a cable tie passes through to hold the 9V battery in place. Practical Electronics | February | 2025 17 Constructional Project Parts List – Laser Communicator (Receiver) 1 single- or double-sided PCB coded 16102242, 80 × 37.5mm 1 UB2 Jiffy box, 197 × 112mm 1 100mm loudspeaker driver [eg, Farnell 2787070] 1 solder tag mini toggle switch (S2) 1 9V battery 1 9V battery clip with flying leads 2 2-way mini terminal blocks (CON4, CON5) 1 10kW 16mm single-gang logarithmic taper potentiometer (VR1) Semiconductors 1 LM386N 1W audio amp IC, DIP-8 (IC1) [Farnell 3116863] 1 BP2334 or TEPT5600 phototransistor (Q6) [Farnell 1458698] 1 red 5mm LED (LED1) Capacitors 2 220μF 16V radial electrolytic 1 10μF 16V radial electrolytic 2 100nF 63V MKT 1 47nF 63V MKT Resistors (all 1/4W 1%) 1 4.7kW 1 1kW 1 330W 1 10W Hardware 4 M3 × 10mm panhead machine screws 4 M3 flat washers 4 M3 star washers (toothed type) 4 M3 hex nuts 4 6mm-tall rubber feet 1 150mm cable tie 2 200mm lengths of light-duty hookup wire (red & black) 1 100mm length of 3mm diameter heatshrink tubing separated by at least a few metres. Align the laser onto the receiver. We always use this with a piece of paper with a target stuck over the hole for the phototransistor. That makes it so much easier to get a decent link and stops the laser from saturating the phototransistor. To align it, get the laser in the general vicinity of the receiver target, then adjust the screw jack so the laser dot is at the right height. Do this without holding the top of the transmitter, as that will mess up your aim when you let go of the box. Once the elevation of the aim is correct, gently change the laser’s azimuth by nudging the screw jack left or right. Again, don’t try to turn the transmitter by holding the Jiffy box, as everything will move when you let go. Just nudge it. If the sensitivity is high enough, you should hear the receiver go quiet once the aim is good. Adjust the sensitivity from minimum up until you get a clear(ish) link. With the gain right up, you will likely get feedback. Once you get feedback, you can back off the sensitivity on the receiver until you have a clear link. To aid you in this task, it’s a good idea to put something like a radio or smartphone playing music next to the microphone so you have a consistent sound to aim for. If the above are all good and you still can’t get sound from the receiver, switch the transmitter on and point the laser at a wall. Tap the front of the microphone repeatedly with your finger and watch the intensity of the laser spot. It should show brief and slight changes in intensity with each tap. If the variation is not apparent, check that the microphone is wired correctly. If you have an oscilloscope, check the voltage from the electret microphone at the MIC input on the PCB and the base of Q3. The signal should be easily visible on the 100mV/div range. Look for a similar signal on the cathode of the laser; it should be much the same signal as you saw on PE the input. Left: this photo shows how the battery, PCB, switch and speaker are mounted in the Receiver. The PCB is held to the rear of the lid by the potentiometer nut. An individual shot of the PCB is also shown. 18 Practical Electronics | February | 2025