Reed Switch: Working Principle and Applications Explained

This article explains how a reed switch works, including its construction and circuit symbol.

What is a Reed Switch?

• A reed switch is a magnetically activated mechanical switch.
• It consists of two metallic contacts enclosed in a small enclosure, usually a glass capsule.
• The contacts are magnetic and move in response to a magnetic field. A permanent magnet is needed to activate the switch.
• There are two main types of reed switches: SPST (Single Pole Single Throw) and SPDT (Single Pole Double Throw). The most common type is SPST, which can be either normally open or normally closed. Bistable versions are also available.

Figure 1: Reed Switch Structures (Make type and Change Over type)

How Reed Switch Works

Figure 2: Reed Switch working

Let’s understand how a reed switch operates using an example: an alarm system triggered when an intruder enters a building (home/office).

• In this system, a magnet is attached to a door or window, and the reed switch is attached to the frame very close to the magnet. Both the magnet and reed switch are typically enclosed in sealed plastic enclosures.
• As long as the door/window is closed, the magnet activates the reed switch. This is known as the “normally closed” condition.
• When the door/window is opened by a stranger or intruder, the magnet moves away from the reed switch. As a result, the contacts relax and the circuit is broken, triggering the alarm.
• Reed switches are commonly supplied with axial leads, allowing them to be easily connected in series. If any of the switches are opened, the alarm will be triggered.