This circuit is from the 1990 Motorola Data Databook. The circuit can be implemented with equivalent transistors. The power supply must be symmetrical.
This circuit was found in a Linear Technology manual from the 1990s. The component is the LTC1453...
This circuit emits a sound when a magnetic field, of a magnet, for example, approaches the sensor....
This interesting sine wave oscillator circuit for the audio track uses an incandescent light bulb...