The frequency of the sound produced in the speaker depends on the light that falls on the phototransistor. This circuit is from a 2000 Poptronics, but it can be easily assembled today. The basic frequency of the circuit is given by C1 which can be changed over a wide range of values. The speaker is either 4 or 8-ohm. Any phot-transistor can be used in place of the original.
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