Magnetic Reed Switch Door Sensor

The reed switch contains a pair (or more) of magnetizable, flexible, metal reeds whose end portions are separated by a small gap when the switch is open. The reeds are hermetically sealed in opposite ends of a tubular glass envelope. The basic glass reed switches are relatively fragile that’s why we have sealed them in a rigid plastic housing for easy usability. When the device is exposed to a magnetic field, the two ferrous materials inside the switch pull together and the switch closes. When the magnetic field is removed, the reeds separate and the switch opens.  This makes for a great non-contact switch.

One thing all of these switches have in common is the two-terminal interface. But whether those terminals are normally open or closed is another question. The reed switch we’ll be using in this example is normally-open. That means “normally”, when the switch is unaffected by a magnetic field, the switch is open and does not conduct electricity. When a magnet comes close enough for the switch to activate, the contacts close and current can flow.

Just as your magnet may have two poles, the reed switch’s pair of ferrous contacts are also polarized. The position, distance, and orientation of your magnet all play a role in determining how the switch activates. Both parts of switch should be connected as following ;