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 was found in a 2005 Linear Technology application manual. The circuit monitors the...
This circuit is from a 1960 book by Hugo Gernsback. The transistors are germanium from the time....
This circuit, with printed circuit board, is from an article in Radio Electronics magazine from...