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 and also by adjusting R3. This pot can be 47k. The speaker is either 4 or 8- ohm. Any phototransistor can be used in place of the original.
