Suggested by a 90s Popular Electronics this circuit is intended to receive volleys or whistles from thunderstorms in the wave range below 20 kHz. The coil consists of 300 turns of wire 26 in a ferrite rod.
This circuit is from American documentation from the 1980s. The transistors are general purpose...
This circuit was found in a Popular Electronics of the 1990s. The sensor is a rotating mechanical...
The purpose of this circuit is to sense the water level in the windshield wiper reservoir. The circuit...