Found in old documentation, this circuit uses a double gate MOSFET. The operating frequencies depend on the coil, and the indicator is a microammeter.
The circuit is from the 1994 National Semiconductor Linear Applications Handbook. The component used is...
This circuit, which is present in other points of this website, was found in a 90s Hands On...
This simple circuit produces a tone when triggered by any vibration picked up by the sensor. The...