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 other high stability thermocouple amplifier uses an 80's operating. The circuit can be used...
The mains cables work as antennas. We can take advantage of them with this circuit that consists of a...
This circuit was found in an American magazine in 1976. It doubles the frequency of TTL signals in the...