Solenoids

A solenoid is like a motor, in that both are “actuators”, i.e., a device
that makes something else move. The difference is:

This packet shows how to control either one of two solenoids, both of
which are integrated into water valves:

Both of these devices are extremely power hungry. The Arduino cannot
possibly provide the current to make them work. However, as in the
case of high-powered speakers, the Arduino can
be used to control when/how the current from another device to the
solenoid is turned on or off.

Because of the high power requirement, the solenoids in this example
are controlled by a MOSFET rather than a transistor, because the MOSFET
has extremely low resistance and therefore won’t get so hot when
controlling a high current flow.