Reed Relay / Magnetic Sensor (Reed Switch)
A simple switch based on two ferromagnetic reeds inside a glass tube that touch each other, making contact, under the field of an approaching magnet. It needs no supply voltage, works with two wires and behaves like a dry contact. It is the cheapest contactless solution for end-position detection.
The appeal of a reed switch is its simplicity: a small magnet is fixed to the moving part and the switch to the body; when the magnet comes close, the contact closes. Since there is no active electronics you do not need to run a supply cable.
- Current limit: The contacts are thin; they are suitable for signal currents on the order of a hundred milliamps. They are not used to switch the motor directly, only to trigger the driver's digital input.
- Contact bounce: Because of the mechanical reed structure, closing produces multiple pulses on a millisecond scale; a software delay on the driver side or a capacitor on the input suppresses that bounce.
- Magnetic environment: The field of a nearby motor or contactor can cause false triggering; verify the mounting distance by trial.
In a system using current-sensing termination a reed switch is often unnecessary; but in delicate mechanisms that cannot withstand the stall force it is useful as a second limit that cuts the motion before the current builds up.
Where this term is used: DC Motor Driver Selection Guide