magnet
2 pages across tutorials, modules, pinouts and guides.

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Hall Effect Sensor
An omnipolar Hall switch on a TinkerBlock board: bring either pole of a magnet near and SIGNAL goes LOW and a red light comes on. No contact to wear out, it works through plastic, and it looks at the field every 22 ms, so it is made for doors, lids and slow wheels.

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Reed Switch
A glass reed switch on a TinkerBlock board: two blades that a magnet pulls shut, from either pole, with no power needed to sense. It sits between VCC and SIGNAL with a 10 kΩ pull-down, so a magnet reads HIGH, and a red LED lights while it is closed. It draws nothing while open, and VCC must be your board's logic voltage.