We found this circuit in the 1964 RCA transistors manual. The sensor may be a common LDR and the transistor BC558. The relay has an operating voltage according to the power supply.
This circuit was found in Electronics Now. With it the dialing signals of a telephone are...
This traditional configuration of a white noise generator that uses the junction of a transistor with a...
This circuit generates an accurate 1 Hz signal controlled by a crystal. The circuit is from 1995...