Easy Light Beam Audio Transmitter: Only 3 Parts. Plus Bonus Preamp Schematics.

This light beam transmitter will use any head phone audio signal to modulate a beam of light. It is a modification of the Eveready 5109LS flashlight. This is a good flashlight for under $5 (battery included). After the modification it will still work as a flashlight and will have a significantly longer run battery run time. You will also need an audio jack. I used an RCA / Phono jack but a miniature or sub miniature phone (head phone) jack will work also. Last od all you will need an audio transformer. I used a 600 ohm to 600 ohm isolation transformer which is typically used with telephone equipment. The DC resistance is about 50 ohms on the primary and secondary windings. This worked out well since one coil or side of the transformer is wired in series with the LED. The flashlight is a little less bright. This is why the batteries last longer.

Step 1: Cut and Solder

The first picture shows the unmodified LED and mirror assembly. Cut either trace as shown. Solder the wires to either the primary or secondary. Glue the coil to the assembly.

There are lots of audio transformers that will work. I would probably not use a coil with less than 8 ohms DC resistance for the headphone connection. The DC resistance on the flashlight side directly effects the brightness of the flashlight. I plan on testing some transformers with lower DC resistance in the future.

Step 2: The Audio Input Connector

There is not a lot of space inside the flashlight housing. I has to cut off part of the center terminal of the RCA jack so the battery could be removed. The ridges on the outside get in the way of the connector so I removed them with a flat drill bit (for wood working) then drilled the through hole.

There you go. Connect your radio or CD player to this "flashlight". You are ready to transmit audio over a light beam.

Step 3: Light Beam Receiver

There are several ways to make a light beam receiver. One of the easiest ways is to connect a solar cell in series with a capacitor and then connect it to a battery operated audio amplifier. A lens will help a lot with light listening at a distance. The bigger the lens the better. Also a larger capacitor might work better.

The same Radio Shack amplifier can be modified so you can connect a photo transistor to the input. (Also a lens will help greatly).

The instructable is titled:

Modify the RadioShack Amplifier to Power a Condenser Microphone Element.

https://www.instructables.com/id/Modify-the-Radio-Shack-Mini-Amplifier-to-Power-a-C

Step 4: Photo Transistor / Lens Assembly

This is very simple. The photo transistor is soldered to an RCA jack. The lens is held in place on the other end of the tube. Any type of tube should work fine. My first assembly was a cardboard tube. The only issue making sure the focal distance between the transistor and lens are correct. I used to have a drinking straw attached to the top a sight.

Step 5: More Power

Need more power? You can connect an audio preamp in between the photo transistor assembly and the amplifier. This one has an input for dynamic (non powered) and condenser (powered) microphone elements. Use the condenser input for the photo transistor.

Step 6: Yet Another Preamp: Stereo

For even more power, here is a stereo preamp that uses an MPSA18 transistor for each channel. Note that for this preamp and the other one the transistors are biased with only two resistors. This gives the circuit greater gain. The two resistors are selected to give a voltage measurement of half the supply voltage across the resistor connected to the collector. There are lots of resistor combinations that will work. This stereo circuit should have a current drain of about one milliamp per channel with the resistors used. There is only one 100 - 300 uf capacitor connected from 9 volts to ground. The switch connected from 9 volts to the 10K resistor is switched on when using a photo transistor, CDS photo cell, or condenser microphone element. For maximum gain the output from one channel can be connected to the input of the other channel. How much gain can you get out of this preamp? I modulated a laser pointer with audio (using an LM386 circuit) that was pointed at a big gray transformer on a telephone pole about 100 feet away. I used the photo transistor / lens assembly and amplified it through the two preamp channels before sending the signal to the Radio Shack audio amp. I was picking up music from the laser pointer dot. Typically light beam communications are done line of sight. Picking up the signal off of a reflection means the source and the receiver do not need to be able to see each other. I'm thinking of modulated LED spot lights shining on the sides of buildings or clouds with people picking up the signals miles away. Or maybe pirate "radio" stations doing double duty as LED front porch lights, car head lights, and office lights. I hope I get the corner office on the 20th floor. My station will be easier to find.

Step 7: More Things to Try

Try putting a lens in front of the LED to narrow the beam for longer distances.

Try making the entire flashlight circuit from scratch and use an infrared LED in a tube with a lens. I did somalar testing with a different transmit circuit and with the received circuit pre-amplified. I got voice transmissions over a distance of about about 1000 feet.

How about a completely different form of alternate communications: Earth "Radio" (intructable to follow some time in the future): It is step 4 on my "scratch pad"

https://www.instructables.com/id/Luxstars-Scratch-Pad/#step4

Also check out step 14: Light Wave Communications: Notes for Class Room Demonstrations.

These were actual high school science classroom hand outs from about 20 years ago.