Using an analog microamperimeter this circuit has a phototransistor adapted to the wheel as a sensor. The circuit is from American documentation from the 1980s, but the components are common today.
This Popular Electronics circuit can be found in various versions on this site and in this...
The purpose of the diode in this oscillator circuit is to limit the amplitude of the output...
This circuit can be used as a shield based on 555. The calculation of the timing network is done...