The purpose of a float switch is
to open or close a circuit as the level of a liquid rises or falls.
Most float switches are “normally closed,” meaning the two wires
coming from the top of the switch complete a circuit when the float
is at its low point, resting on its bottom clip (for example, when a
tank is dry).
To complete a circuit, float
switches utilize a magnetic reed switch, which consists of two
contacts sealed in a glass tube. When a magnet comes close to the
two contacts, they become attracted to each other and touch,
allowing current to pass through. When the magnet moves away, the
contacts demagnetize and separate (breaking the circuit), as shown
In a float switch, the magnetic reed switch is hermetically
sealed in a stem, most often made from plastic or stainless steel.
The float encases a sealed magnet, which moves up and down the
length of the stem as a fluid level rises and falls. As the magnet
passes by the contacts in the encased reed switch, they touch and
complete a circuit between the two lead wires, as shown in the
Properly used, float switches can
deliver millions of on/off cycles, for years of dependable
operation. Failures are normally due to overloading, frequently
caused by spiking voltage. Before purchasing a float switch,
read our page on
Curing Spiking Voltage with a Relay.