This interesting sensor for RF signals was found in an article in the June 1989 Radio Electronics magazine. It is a field strength meter with an LED bargraph indicator.
This 4-function, high-pass, low-pass, band-pass and rejector filter was found in French...
Passing between a light source and the sensor will trigger on the relay by a time interval...
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...