The frequency of the square signal produced by this circuit depends on the light falling onto the sensor. Frequency range also depends on C1 and C2. Values from 10 nF to 470 nF can be used for these capacitors. The circuit operates with voltage sources from 3 to 12 V and current drain is very low. A piezoelectric transducer can be wired to the output to convert the signals to sound.
Photo Astable Multivibrator
