This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit is based on a 741 that activates a buzzer with oscillator. R5 adjusts the sensitivity.
Similar to other circuits found in this section, this circuit is from a 1996 Electronics World;...
We found this circuit in the 1983 edition of the Electronics Experimenter handbook. The circuit uses...
We found this circuit in a 1987 Hands On Electronics. It was indicated at the time to power...