Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.
The purpose of this 1965 Radio Electronics circuit is to observe cycloid waveforms on a standard...
This 555 timed touch switch was found in a 90s Hands-On Electronics. It triggers a buzzer, but a...
Found in a 1969 documentation, this circuit consists of a multi-oscillator with neon lamps....