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.
With this circuit it is possible to keep the current in an LED constant even if the supply voltage...
This circuit was suggested by Motorola in its manual of components for optoelectronics (1987)...
This sensitive circuit can be designed with FETs such as BF245 and MPF102. The sensor is a silicon...