A reed switch is an electromagnetic switch used to control the flow of electricity in a circuit. They are made from two or more ferrous reeds encased within a small glass tube-like envelope, which become magnetized and move together or separate when a magnetic field is moved towards the switch. The switch effectively works like a gate, or a bridge, in an electric circuit so when the two reeds are in contact, electricity can flow around the circuit operating a device. Unlike mechanical switches they do not require something or someone to physically flick them on or off, they are controlled completely by invisible magnetic fields.
This video will show you how magnetic contact switch sensor works and simple project that can be made using it.
This is the hardware connection and the components needed.
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