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.
Suggested by Raytheon in 1967 documentation this circuit uses a lightbulb and LDR optical coupler of the...
Found in American documentation from the 90's, this circuit consists of a solid state relay with...
Indicated to be used at the sensor input of a PC, this circuit was found in an English...