This project was found in a 1981 documentation. It consists of an interesting audio oscillator that comes into operation when a beam of light hits the LDR. It is an astable multivibrator with two NPN transistors for general use whose frequency is basically determined by R1-C1 and R2-C2. The LDR is connected to the collector of one of the transistors acting as a load for it. In the dark, with the high resistance that the LDR has, there is no collector polarization for the transistor and the oscillator cannot work. With the lighting of the LDR, its resistance drops to the point of allowing the polarization of the transistor and thereby producing oscillations. The transducer used is a crest phone! or a high impedance magnetic phone (2000 ohms). A crystal capsule can also be used as a transducer. The circuit is supplied with a voltage of 3 V, that is, two small batteries connected in series.
