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 metal detector was found in a 2001 Poptronics, but it can still be easily assembled, as the...
We found this circuit in a 1980 documentation. The flasher uses 3 transistors in the configuration of an...
This current meter with many scales was found in a 80s American documentation. The operational amplifier...