We found this circuit in a 1980s General Electric documentation. It appears from other sources in this section. The circuit makes use of diacs and SCRs of the time.
This circuit was found in an old Linear Technology documentation. The working principle can be...
This circuit is intended for PWM speed control of a motor up to 500mA, using integrated circuit 555....
The operational amplifier for this circuit, found in a 1979 publication, may be the 741....