Description:
The magnetic contact switch (3562) is nice and very small and is designed for surface mounted installations.

he switch handles gaps of up to 12mm (1/2 inch) between the magnet and the switch. The switch will turn the digital input "on" when the magnet is within 12mm of the switch, and will turn "off" when the contact is broken by moving the magnet more than 13mm away.

Specifications:
Sensor Type: Proximity (Magnetic)
Detecting Distance Max: 12 mm
Detection Hysteresis Max: 8 %
Sensor Output Switch Type: Normally Open
Cable Properties: 450 mm and 22 AWG
Switching Current Max: 500 mA
Switching Voltage Max (AC): 100 V AC
Switching Voltage Max (DC): 100 V DC

Resources:
More information to be found in the Data Sheet