This circuit comes from a Gernsback Publications of the 90s. The circuit uses a general-purpose transistor as a sensor. The power supply must be symmetrical.
The circuit shown is from Motorola's Linear Interface ICs from 1992. The circuit uses a...
This nice function generator circuit with the XR2206 was found in the May 1977 Radio Electronics...
This voltage inverter allows you to raise the voltage of a battery to values as high as 140 V...