This physical line servo control uses components that are not currently common. The circuit is from an 80's documentation and is powered by a voltage according to the servo.
The recommended distance between the emitter and the phototransistor is 10 mm for this...
This circuit was found in a 1994 documentation. Speed measurement pulses are obtained by a reed...
Here is a high-quality intercom for demanding readers. The circuit is from a 1982 documentation. Of course,...