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A solenoid is a device which converts an electrical current into mechanical motion.
The picture shows a solenoid. There are other styles, but they all work the same basic way.

Wire Coil

Moving armature

Connecting wire

This diagram shows the motion of the solenoid when an electrical current passes through the coil.

Q: How does the electrical current produce movement?

A: When a current is passed through a wire, a magnetic field is produced. If the wire is wound into a coil, the field is increased sufficiently to pull an iron plunger.

This diagram shows a suggested link up between a solenoid and the pallet of an organ. The pallet blocks the airway from the windchest to the organ pipe until pushed down.

Another method, is to install the solenoid inside the windchest, so that it acts as a combined solenoid/pallet.

An interesting fact about the electro-magnetic process, is that it is reversible. If a magnet is moved near a wire, a small current/voltage is produced. If the magnetic field is moved near a coil of wire, a larger voltage is generated. The voltage produced is directly proportional to the rate of magnetic field change. When the solenoid is driven with a current, during playing, a magnetic field is created, but when the current is removed, the magnetic field doesn’t simply ’evaporate’, it collapses at a very fast rate, inducing a large voltage at the terminals of the coil. This can sometimes be in the order of a few hundred or even thousands of volts, which can lead to severe damage to any electronic driver circuits. This is referred to as the ‘Back EMF’.

The standard technique to prevent this is to connect a diode across the terminals of the solenoid, polarized the opposite way to the applied voltage. Fortunately, devices like the ULN200x have diodes fitted internally (although some would argue that the diode should be connected at the coil end of the circuit to improve the solenoids performance!).

If an organ is being converted to operate electronically, it is usually desirable to leave the original keyframe mechanism in place, so that the organ may be played in the ‘traditional’ way, using a book/roll of music. In this case, an arrangement needs to be made to control the air feed to the puffs on the windchest. This is usually achieved with an external valve box operated by solenoids.