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 uses two Raytheon RC4558 operational amplifiers that are similar to the 741. The...
This small stereo amplifier for headphones was found in a documentation on integrated circuits from...
This is a MOSFET version of a grid dip meter for frequencies between 1.8 MHz and 150 MHz,...