This circuit, from an old publication, can employ silicon transistors like the BC548 and the transformer is obtained from an unused transistor radio. The phototransistor is of any type and the supply can be made with voltages from 6 to 9 V. The frequency of the emitted sound changes according to the light incident on the sensor. The speaker can be 4 or 8 ohms and the capacitor can have its value changed.
