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 oscillator was found in a Raytheon documentation from the 60's. The phone must be high...
This circuit uses the flip-flops from the previous circuit to implement a 3-digit binary counter. The...
This circuit produces 1 kHz square signals using the CMOS IC 4070. The frequency is given by the...