Electronic Parts

Discrete Components

Sensing and Automatically Operated Switches

A switch does not have to be operated directly by a person's hand. A machine can press it, a magnet can operate it, or a change in position, pressure, liquid level, or temperature can cause contacts to change state.

Typical sensing-switch symbols

Limit switches

A limit switch detects that a moving part has reached a particular position. The actuator may be a plunger, lever, roller lever, whisker, or other mechanical form. Limit switches are common in machinery, doors, gates, lifts, printers, and other equipment where the position of a moving part must be known or limited.

Microswitches

"Microswitch" is commonly used for a small snap-action switch. The internal spring mechanism changes the contacts rapidly after the actuator reaches a particular point, rather than allowing the contacts to creep slowly together. A typical unit has COM, NO, and NC terminals and can therefore be used as an SPDT changeover switch. The point at which the contacts snap over and the point at which they reset are usually slightly different. That difference helps prevent chatter when the actuator is near the changeover point.

Reed switches

A reed switch contains thin ferromagnetic contact reeds sealed inside a small glass capsule. A nearby magnetic field causes the reeds to move together or apart. Since there is no external mechanical actuator entering the sealed capsule, reed switches can be useful for door and window sensors, position sensing, counters, and other low-power applications.

A magnet does not have to touch the reed switch. The distance and orientation of the magnet matter, and the contact rating of a small reed switch must still be respected.

Reed switch or Hall sensor?

A reed switch is a true mechanical contact operated by magnetism and can often be tested like an ordinary switch. A Hall-effect sensor is an electronic device that detects magnetic field and produces an electrical output. Both may be used for similar position-sensing jobs, but the Hall sensor needs power and should not automatically be treated as a pair of dry switch contacts.

Float switches

A float switch responds to liquid level. Some operate a mechanical switch through a lever; others place a magnet in the float and a reed switch in a sealed stem. They are used for tank level indication, pumps, alarms, and overflow protection.

Temperature and pressure switches

Thermostats and pressure switches convert a physical change into contact motion. They are switches rather than continuously variable sensors when their output is simply one contact state or another. Many include hysteresis so that the turn-on and turn-off points are intentionally different, preventing rapid chatter near the threshold.

Choosing the normal state

In sensing and safety circuits, the choice between normally-open and normally-closed operation can be important. A normally-closed loop can make a broken wire appear as an open circuit, which can be useful when the circuit should fail in a detectable or safe direction. The right choice depends on the complete system design.

Sensing versus safety-rated switching

A switch that can sense a position is not automatically suitable as a safety interlock. Safety applications may require specific contact construction, forced-opening action, redundant circuits, environmental ratings, and system-level fault monitoring. The component must be selected for the actual safety function, not only because its electrical contacts appear to work.

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