How are eddy currents used in switching devices?

Proximity switches often involve eddy currents. These switches have the advantage that they have no (internal) moving parts or electrical contacts. This is an advantage because springs and bearings can wear out and because electrical contacts can become corroded.

Proximity switches are activated by the proximity of a conductor, such as a human finger. You will have noticed them in the control panels of lifts, for example. If the proximity of your finger throws the switch but the proximity of a non-conductor does not, the switch may work on eddy currents (although it could also work by capacitance).

Proximity switches of one type work like this: The switch contains an oscillator using a resonant circuit operating at radio frequencies. The coil sets up a high frequency magnetic field. When a conductor enters this field, eddy currents flow in the conductor.

This can have two effects. First, it changes the impedance of the coil, and so changes the frequency* of the resonant circuit. Also, ... more.

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