Update to Sound Transmission Using Light
The transmission distance can be increased greatly by replacing the transmitter's red LED with a cheap red laser diode purchased from a “$2 Dollar Shop”.
laser sound transmitter circuit diagram

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.

Do not apply any more than 5 volts to the laser diode. I find that 4.5 volts from 3 AA cells works well. At that voltage the laser draws about 30 mA allowing a cheap general purpose NPN bipolar transistor (such as the BC548) to work well in modulating the current through the laser.

The laser casing needs to be removed. I cut carefully along the metal casing using a fine hacksaw and then prise it off the laser barrel with long-nosed pliers.
laser sound transmitter

The photo alongside shows the laser barrel inserted in 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.

laser barrel mounted on angled plastic

This photo shows another way to mount the laser barrel - again using an off cut of plastic drainpipe.

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.

laser sound receiver mounted on wooden block

laser sound receiver inside a plastic box

The final photos show two identical receivers. One is mounted on a block of wood and the other is mounted inside a black plastic box.

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. Then the oscilloscope can display the different digital patterns when each of the remote control keys are pressed.

Please feel free to email me if you have any questions or improvements to suggest in this resource. Cheers, Denis.