This circuit was found in the Electronics Experimenter Handbook from 1991. The sensor is a diode and the circuit has a relay according to the voltage used in the supply.
This ultra-low distortion driver was found in 80's Analog Devices documentation. The circuit has a...
This second-order filter was found in a documentation from the 90's. It can be implemented with...
This circuit goes into operation producing a sound on a small speaker when the supply voltage...