Misc. Circuits: Magnet Motion Detector using TL082 Op-Amp
Here’s a little circuit that illustrates using a biased coil as a sensitive detector of changing magnetic fields.
The high-input-impedance TL082 dual op-amp makes a great comparator, which can respond to tiny voltage changes induced in the coil by the motion of a nearby magnet.
This is an illustration of Faraday’s Law of Induction in action. The voltage in the coil depends on the time rate of change of the magnetic flux linking the coil. So the polarity of the induced voltage depends on which pole of the magnet is approaching or receding from the coil, and the amplitude of the voltage depends on the speed at which the magnetic field changes. The LED indicates these changes quite clearly.
Two variants of the circuit are described, with the last one having crazy sensitivity (but some tendency to self-oscillate).