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Transmitting Soundvia Laser Light

Sound Transmission Using Laser Light

This webpage is an update to my "Sound via Light" webpage. The light transmission distance can be greatly increased by replacing the transmitter's red LED with a cheap red laser diode purchased from a “$2 Dollar Shop”.
The circuit diagram shows the modified transmitter circuit.

The circuit design makes a compromise between laser brightness and sound quality and loudness. This is because the laser needs to be bright enough so that it is easy to locate the beam when positioning the receiver and for students to see that the laser beam is really there and ‘carrying’ the sound.
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Do not apply any more than 5 volts to the laser diode. I find that 4.5 volts from 3 AA cells works fine. At that voltage the laser draws about 30 mA allowing a cheap general purpose NPN bipolar transistor (such as the BC548) to modulate the current through the laser. This modulates the laser light output brighter and dimmer in rhythm with the audio signal.

The laser casing needs to be removed. I use a fine hacksaw to cut carefully along the metal casing and then prise it off the inner laser barrel with long-nosed pliers.
The photo alongside shows the laser barrel inserted into a black plastic pen casing which is itself mounted in a small wooden block glued to the larger block.

In the foreground a battery casing contains the three AA cells and has an integral off/on switch.

The transmitter circuit, built on Veroboard, is in the background along with the input socket which is mounted on an off cut of white plastic drainpipe.
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This photo shows another way to mount the laser barrel. Again, I have used an off cut of plastic drainpipe to do this.

The photo also shows where to solder the positive and negative leads to the laser. This bypasses the laser's off/on switch shown on the left.
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The final photos show two identical receivers. One is mounted on a block of wood and the other shown below is mounted inside a black plastic box.
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The Banana sockets and switch on the side of the plastic box allow the receiver output to be connected to an oscilloscope.

This is particularly useful when a remote control is ‘shone’ at the receiver. The oscilloscope displays different digital patterns when each of the remote control keys are pressed.

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