It is a 13mm single polar single throw clear glass magnetic reed switch . It's a passive electrical switching component that regulates the flow of electricity in a circuit. It consists of a glass envelope, containing two ferrous elastic reeds and an insert gas or a vacuum. Both reeds are normally apart but once the magnetic field approaches the sensor, they will come together, completing the circuit. Electromagnetic relays, weighing devices, level meters, and other devices can be used to detect if a washing machine, refrigerator, or photocopier door is open or closed. It works with Arduino, Raspberry pi, PIC, and other microcontrollers. It can generate only digital output not analog output.   

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