We found this circuit in the September 1971 issue of Radio Electronics magazine. The sensor is an LDR and R1 adjusts the sensitivity. A relay with the appropriate voltage can be used instead of a buzzer.
Obtained from an English publication, this circuit actually needs a current limiting resistor,...
This circuit has its gain determined by the control DC voltage. The circuit is from a Radio...
This oscillator is a 1950s application of one of the first types of commercial transistors, the...