This circuit was found in a 1967 General Electric manual. It is intended for the control of high voltage Nixie pipes used at the time. SCRs may be equivalent.
This 1969 Philips documentation circuit makes it possible to control an engine through the sound...
We found this circuit in a 90's Radio Electronics. The circuit can still be easily assembled as all...
This circuit was found in the July 1992 Radio Electronics magazine. Sensitivity adjustment is made...