The purpose of an alarm system is to let the user and the appropriate authorities know whenever there is an emergency. This includes an intrusion, fire, environmental problem or medical crisis. The system identifies these emergencies using various sensors. Some popular sensors for security systems include door and window contacts, motion detecting sensors, glass break sensors, smoke and heat detectors, flood sensors, temperature sensors and security key fobs. Sensors can be hardwired into a security system, or they can communicate wirelessly through the use of a wireless receiver.