This circuit was found in a Popular Electronics of the 1990s. The sensor is a rotating mechanical system with a propeller-driven magnet to count pulses with a reedswitch.
This circuit was found in an old Linear Technology documentation. The working principle can be...
Found in a foreign publication, this sensitive circuit triggers a relay. The voltage depends on...
This stereo meter-indicator was found in the August 1971 issue of Radio Electronics magazine. The...