Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.
This circuit changes the wave shape of a square signal according the values of C1 and R1. Other...
This circuit from a 1978 American publication is used on vintage vehicles that still have contact...
This circuit was obtained from an English magazine from 1977. The magazine no longer exists, but...