When the magnet approaches, N flux inside the bifilar coil increases. An opposing flux is produced by the coil and an EMF is generated across L1 and L2. The EMF across L1 applies + voltage to B-E terminals of Q1 and turns it ON. Q1 saturates rapidly and a high current is withdrawn from B1. This current builds a strong magnetic field inside the coil. Since L1 and L2 share the same core, an opposing flux is produced by L1 and the magnet is pushed away.
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As the magnet goes away, the flux inside the coil decreases. An EMF is induced across L1 that applies - pulse at B-E terminals and Q1 is turned OFF.
Consequently, collector current is cut sharply. As the magnetic field inside L2 collapses, a high voltage spike is produced by L2 that charges B2.
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