This lockable two-position servo control system was found in documentation from the 1990s. The error amplifier can be an operational amplifier and the sensors are phototransistors.
This circuit is from 1977 documentation, but can be assembled with BC548 for Q1, Q2 and Q4, BC558...
Found in American documentation from the 90s, this circuit uses components that are still common...
This high impedance input amplifier is suitable for the use with piezoelectric sources such as...