How to Transmit Audio With a Laser Pen

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Last Updated: September 24, 2022

Using a simple laser pen pointer, a few misc parts, and about 15 minutes, you can make a simple laser communicator that converts a sound source into light that travels across a room, and back into sound with very little quality loss.

1

Remove batteries from the laser pointer.

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You'll need easy access to the area where you normally attach the battery.

2

Connect a clip lead to the battery spring.

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Usually there is a small spring to which you can attach the clip lead. The other end of the battery usually connects to the case of the laser. Since there are many different styles of laser pointer, you may have to experiment with clip lead placement to get the laser to work with the new external battery pack. You may also have to hold down the laser's push button switch by wrapping a rubber band or some wire around it.
- Test the connection before you attach the transformer, to make sure the laser works with the new battery pack. If it doesn't light, try reversing the battery. Battery reversal will not harm the laser.

3

Connect the transformer.

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The 1,000 ohm side of the transformer has three wires coming from it. We only use the outside two wires. The inside wire is called a center tap and we do not use it in this circuit. Test the laser by attaching the battery. The laser should operate normally at this point.

4

Connect the earphone jack to the transformer.

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See the schematic of the transmitter.

5

Connect the receiver.

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You simply connect the solar cell to the microphone jack and plug it into the amplifier or stereo phono input. It does not matter which way the wires are connected to the solar cell. See the schematic of the receiver.

6

Plug the earphone jack of the laser into the radio.

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Then, connect the earphone jack of the laser into the earphone socket of the radio.

7

Connect the solar cell to the amplifier.

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Then, turn it down slightly until the hiss isn't noticeable. The volume control should be fairly high, corresponding to an ear splitting level if it was playing music.

8

Aim the laser across the room so it hits the solar cell.

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This indicates that everything is working fine at this point.]

9

Turn on the radio and adjust the volume.

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Slowly adjust the volume until you hear the radio station voices or music coming from the amplifier across the room. The radio should be just audible if the earphone jack is pulled out, not loud. If you can't hear the sound from the amplifier across the room, make sure the laser is shining on the solar cell, then try increasing the volume of the amplifier before you increase the volume of the radio.
- At this point you should be hearing the radio station coming in loud and clear in the amplifier across the room. Put your hand in front of the laser beam to break the connection, and notice that the music stops.

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Question

Do I have to have the transformer?

Community Answer

Yes, for impedance dropping to change from 1000 ohms to 8 ohms. A mismatch will cause no audio.

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Why is an amplifier required? Can I directly connect it to a speaker?

mars

Community Answer

A speaker driver on its own will likely not have the power necessary to operate from the solar cell. The voltage-modulated output from the solar cell will also not be significant without an amplifier. If the speaker has volume control, it contains amplifier circuitry and may work to an extent.

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Make sure your laser pointer does a dot instead of a shape, like an arrow.

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Warnings

WARNING: Any laser pointer may damage your eyes if you look directly into the beam.

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Laser safety glasses that are appropriate for the kind of laser you'll be using are recommended.

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Class III lasers are extremely dangerous to the eyes and Class IV Lasers can set things on fire in addition to causing severe eye and tissue damage. This would include any visible laser that puts out more than 5mW (that's 0.005 watts) Common laser pointers do not fall in to these categories, nor would this circuit likely be able to power such high powered lasers. Regardless, treat all lasers with respect, like you would a firearm, and keep them pointed in a safe direction at all times, away from eyes, reflective surfaces, and flammable materials.

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Infrared lasers can be especially dangerous, since their output is nearly invisible, but are still capable of inflicting serious eye damage. Do not look directly in to any laser, even if the output appears to be very weak. Infrared lasers are also inappropriate for this experiment since they do not qualify as "cheap" and would be nearly impossible to point correctly.

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Things You'll Need

For the Transmitter:
- A cheap laser pen pointer
- A battery holder that holds the same voltage as the laser needs. (Often 3 cells)
- A transistor radio or sound source
- An earphone jack that fits your sound source
- A transformer of the type known as an audio output transformer. It consists of an 8 ohm coil and a 1000 ohm coil. (Radio Shack 273-1380)
- Some clip leads (wires with alligator clips on the ends) to put it all together. At least one of the leads should be the type with a long, slender point (to connect to the inside of the laser)
For the Receiver:
- A small solar cell (Radio Shack 276-124) or photo resistor + battery (as shown below)
- A mono microphone jack that will fit the phono input of your stereo