Seeed Grove - Physical Sensor Modules - Seeed Studio 101020038, The Grove Magnetic Switch from Seeed Studio contains a dry reed switch that can be activated by a permanent magnet or electromagnet.
The normally open switch is connected to a digital line and has a pull-down resistor giving a logic 0 signal.
When a magnet approaches, the contacts close and the output goes to a logic 1.
Seeed 101020038 Grove - Magnetic Switch SPST Encapsulated Reed Type