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This is fun to me because I have rigged up simple circuits using photoresistors and dual 555 timer chips to make sounds. Mine was more like a simple optical theremin. One photoresistor controlled the audio frequency tone of a fast square wave. The other controlled a slower pulse rate at which the audio tone turned on and off.
So you could use one hand to change the pitch of the notes, and the other hand to change the rate at which notes were played. Or you could stand near it with a flashlight and control it that way.
But this gadget takes the whole idea so much further!
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