Sound Transmission Using Light
These relatively simple circuits can be used to transmit information across a small distance. The "information" I normally use is music from a radio, iPod or CD player - you could also try a microphone and amplifier.
transmitter circuit diagram for sound via LED light

People transmit information all the time when they use remote controls for TVs, DVDs, stereo systems, etc., but they usually do not think much about the transmission as a real event.

Here, the medium of transmission used is a fluctuating beam of red light which can be seen and interrupted. The fact that music or speech is being transmitted and received adds to the impact on students.

This system gives surprisingly good clarity of sound. If you wish to try distances greater than 50 cm between transmitter and receiver then you will need to focus the light from the transmitter's red LED.
receiver circuit diagram for sound via light

Using a small lens from a disposable camera works well. I mount it on a small wire frame. The frame can be bent to position the lens a focal length away from the transmitter LED. It would also help to put another
lens at the receiver end. You could also experiment with fibre optic cable (the plastic demonstration type - 1 mm in diameter) placed between transmitter and receiver. Another possibility to investigate is how well the unit works if the transmitted red light is reflected off mist or smoke or dust in the air.

Investigating light sources:
The receiver can also be used on its own as a detector for investigating other sources of light and infrared. You could show visual waveforms of the light being investigated by connecting oscilloscope inputs across the phototransistor or across the output of the LM386 integrated circuit with the speaker disconnected.

Interesting sources to investigate are:
  • infrared remote controls (How is each key on the remote encoded?)
  • incandescent bulbs, fluorescent tubes, new electronic fluorescent lamps
  • TV and computer monitor screens
  • candle flames
  • displays such as DVDs and bedside clock radios
  • Infrared from auto focus cameras (How accurate is the focusing optics?)
  • the oscilloscope trace that monitors the phototransistor !!! (This forms a feedback loop and shows the principle of the "light pen" used on old computer screens.)

Component list:
transmitter veroboard layout for sound via LED light

receiver veroboard layout for sound via light

Transmitter:
  • 1 x 100R
  • 1 x 220R 1/4 watt resistors
  • 2 x 1K0
  • 1 x 10K skeleton potentiometer
  • 1 x 1µ0 capacitor (electrolytic or mylar)
  • 1 x BC548 transistor (or equivalent)
  • 1 x 5mm diameter high brightness clear red LED (Dick Smith Z-4021)
  • 1 x single pole, single throw switch (SPST)
  • 1 x 9 volt battery and battery clip
  • 1 x 3.5 mm jack socket
  • 1 x connecting lead with 3.5 mm male jack plugs at either end
  • vero board, connecting wire, case, etc.
Receiver:
  • 1 x 330R
  • 1 x 1K0 1/4 watt resistors
  • 1 x 4K7 skeleton potentiometer
  • 1 x 10K potentiometer
  • 1 x 1µ0 capacitor (electrolytic or mylar)
  • 1 x 10µ capacitor (electrolytic)
  • 2 x 100µ capacitors (electrolytic)
  • 1 x LM386 audio amplifier IC
  • 1 x phototransistor
  • 1 x 5mm diameter red LED
  • 1 x single pole, single throw switch (SPST)
  • 1 x 9 volt battery and battery clip
  • 1 x 8 ohm loudspeaker
  • 1 x 8 pin IC socket (for LM386)
  • 1 x knob (for 10K potentiometer)
  • vero board, connecting wire, case, etc.
Note: All components can be purchased from Dick Smith Electronics (New Zealand) with the exception of the phototransistor which is sold by Jaycar (catalogue number ZD1950).

I'm happy to correspond with anyone over modifications, improvements or further uses of these circuits.