sound via light

Posted on Feb 7, 2014

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. People transmit information all the time when they use remote controls for TVs, DVDs, stereo systems, etc. , but they usually do not

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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. 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. 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. the oscilloscope trace that monitors the phototransistor ! (This forms a feedback loop and shows the principle of the "light pen" used on old...

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