The most basic sensors of any alarm system is a door and window Surface Mount Magnetic Contact Switch. This type of contact switch is for normally closed circuits; this means that the when the magnets are lined up, the switch is closed. This type of setup is ideal for monitoring doors, windows, and sliding doors. Measuring 2.48" long and half an inch high, they attach with wood or sheet metal screws (not included) to the frame of the door or window and the case or jam. This Normally Closed Surface Mount Magnetic Contact Switch is also available in a 10-Pack. A Normally Open Surface Mount Magnetic Contact Switch is also available individually and in a 10-Pack.The switch is waterproof and mounted in a high-impact plastic case. The rhodium plated contacts handles up to 100mA at 100 volts DC and are rated for 50 million openings and closings. It can reliably operate in temperatures between -15F and 160F. The sensor's maximum operational gap is ". Mount the magnet on the door or window so that the two are in contact with each other when the door or window is closed. Slotted mounting holes make it easy to adjust the distance between the magnet and the sensor.