Found in documentation from the 1980s, this circuit can have several adaptations to use modern transistors and LEDs instead of the lamp. The power supply voltage can also be changed.
This circuit is from the 70's by A. Fanzeres. We kept the original text. It has already been said...
Found in 1965s General Electric documentation, this circuit is intended for the control of servos...
This circuit is from the Linear Circuits Application of Texas Instruments, using an operational amplifier...