Found in a Popular Electronics from the 90's, this circuit detects the approach of objects by the reflection of the radiation emitted by the LED. The relay is a 12 V sensitive type.
This heartbeat sensor circuit was found in the July 1992 issue of Radio Electronics magazine. The...
This amplifier circuit with a low power MOSFET was found in Popular Electronics from the 90s. The...
This circuit, which can be built with several CMOS gates, was found in the March 1996 issue of...