Laser Communicator

Transmitting an audio signal over laser!

The best part of twenty years ago I came across a fairly simple diy project to build a means of transmitting and receiving an audio signal over laser.

It uses just a few components - chiefly a mini audio transformer, laser pointer and batteries for the transmitter and a solar cell, or light dependent resistor (LDR) and battery, for the receiver. Plus some cables with crocodile clips, an audio source of course and something to plug the receiver into. That's pretty much it.

When I first tried this, I failed to get it work.  Not only did I not hear any sound from the receiver (using  an LDR and battery connected to a crystal earpiece), I managed to blow the laser pointer - twice!  Obviously, the sound source was supplying too much extra voltage to the laser. On my third attempt, I carefully set the source volume at a low level, hoping the laser would be OK, while providing an audible volume.  The laser was undamaged, but still I heard nothing. At this point I gave up.

Fast forward  15+ years and I came across some revised instructions which advised adding a bi-colour LED across the transformer on the output side to protect cheap laser pointers, such as I had been using,  against excess voltage. 

Also, by this time it had dawned on me, having played a little with crystal radio sets in the meantime, that most likely I had been using a duff earpiece, a not uncommon occurrence with modern ones unfortunately.

So, newly emboldened, I got a bi-colour LED. For the receiver, I decided to use a solar cell from an old outdoor solar light and connected that to a crystal earpiece that I knew to be working.  I wired up the transmitter, noted that the LED flashed when the laser pointer was turned on and off and also when the source volume was turned up, presumably thereby protecting the laser. With crystal earpiece in an ear and phone connected to the transmitter, I hit the play button on the app on the phone, switched on the laser and pointed it at the solar cell... and heard music!

At first, I connected one stereo channel of the audio source to one leg of the secondary side of the transformer and the other channel to the other leg. It worked on some recordings, but not others! It slowly dawned on me why that was. This set up would output the difference between left and right channels. If there are no sounds/instruments/voices in common then all should be OK. On some stereo input with a common sound, say one instrument, then the sound of that instrument would be lost. Worse though was a mono recording with the same left and right channel. All sound would be common and all would be lost. What is needed is the sum of the left and right channels.

I then decided to make a compact package for demonstration purposes, as shown below. A few notes on construction:

• an LT700 mini audio transformer was used

• terminal blocks were used to connect components

• leads with crocodile clips were used to connect the laser pointer to the device

• at the transmitter, the crocodile clips were attached to bent paper clips as shown, so they wouldn't rotate. Ideally they would be wired direct to the terminal blocks, but I would have had to cut them to do that and I couldn't spare the leads.

• a stereo socket was used to connect the sound source, with ground attached to one leg of the transformer and left and right stereo channels to the other leg for a combined mono signal

• stereo socket and terminal blocks were fixed to a battery holder with adhesive pads.

For the receiver, I soldered a stereo socket to the two wires of the solar cell: socket ground to one wire and one socket channel to the other wire. This would be sufficient, as the intention was to connect the socket, via a stereo cable, to a mono amp with stereo input and a single speaker. (For a stereo amplifier, to hear the - mono - sound from both speakers, both channels will need to be wired together at the socket, as they were for the socket connected to the audio source).

As to what to plug the receiver in to: a cheap all-in-one external amp & speaker for a phone or MP3 player might seem like a good candidate, but will almost certainly be too quiet, the input signal from the solar cell and laser being much lower than that from a phone. This was indeed the case with such a device, based on an LM4871 IC, that I had to hand, the output being almost inaudible. And while a decent stereo system may be able to handle a variety of sources, it's probably unwise to use one here, in case it gets damaged! Luckily, I also had to hand a Haynes 'Build Your Own' mini amplifier and speaker, a configurable breadboard-based kit using an LM386 a mono amp IC. First, I built an amp for a lower signal-level 'standard' input. While appreciably louder than the other amp, output was still on the quiet side. However, one of the options available with this kit is to increase the standard 20 times gain 10-fold by adding a capacitor between pins 1 and 8 of the IC, thus making it suitable for low-level signal sources, such as a microphone, or in this case, the solar cell and laser.  I followed the instructions, but the amp was now very 'noisy', so I reverted to the lower gain version and made do with that. I revisited the problem later on after giving a demonstration of the device and thinking that I really should be able to fix the volume. A bit of digging around on the web indicated that I needed to connect the input to pin 3 of the LM386 and not to pin 2, as per the Haynes amp's manual. (See e.g. this  page).  Also, I found that an additional (drop-down?) 10k resistor placed in parallel with the output from the volume pot and ground, helped greatly with reducing noise. (See e.g. the illustration for Step 3: A Simple Circuit in the previous link: Audio Signal at left in my case  is the output from the volume pot). Sound quality and volume were now surprisingly good!

A few observations: 

• The laser produces a small dot, much much smaller than the solar cell. You need to experiment to find the best place on the solar cell to point the laser. In my case,  I have to point it at one of the visible sets of lines and hold it steady.

• To avoid distortion, I have to set the source volume at no more than 50%, somewhat below the level needed to cause the LED to flash. 

• The solar cell is best placed where there is little ambient light - in a bright location, there will be a noticeable hiss (bright daylight) or even a buzz (artificial light) due to the effect of background light on the solar cell.

So that's it. An impressive little project, almost guaranteed to generate a 'wow' when demonstrated!