This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used over short distances with a lens system. The circuit is from a 1967 Japanese publication.
This circuit detects signals in a wide range of frequencies, indicating their strength in an...
This traditional two-tone alarm configuration with the 4011 integrated circuit was found in a 1998...
This circuit was found in the June 1980 issue of Radio Electronics magazine. The circuit uses an...