This General Electric circuit was found in a 1964 documentation. The circuit triggers a relay when the sensor is activated. Trigger adjustment is done in a trimpot.
This circuit for audio applications was found in an English magazine from the 80's. The circuit...
This circuit uses a JFET at the input to increase the impedance. The circuit has a gain voltage...
This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even...