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.
The circuit presented is based on a mechanical tuning fork. The circuit is suggested by National...
This source, from a 1982 documentation, uses a triac to control the power and voltage applied to...
This circuit was found in a 1990 Electronics World magazine. The circuit can be mounted today by...