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.
The difference between this circuit and the previous one is that it uses PNP transistors. The...
Found in Radio Electronics from the 80's, this circuit can also be controlled by a microcontroller acting...
This circuit was found in a 1976 documentation. We can assemble it with equivalent transistors, for...