A Reed contact switch is a normally open contact which closes in the presence of a magnetic field. Each module contains a 10 Kohm pull-up resistor.
Specifications:

1. voltage: 5 VDC
2. connection: 3 pins, +(middle pin), ground and DO (data out)
3. dimensions: 25 x 15 mm
4. weight: 2 g

vma308_a4v01.pdf

ce_rohs_open.pdf