Which of the following is not a main type of sensor used in alarm systems?

Pressure switches are typically used in specific applications, such as in industrial contexts for monitoring pressure levels or in some fire detection systems, but they do not fall under the main categories of sensors commonly associated with standard alarm systems like residential or commercial security systems.

In contrast, motion detectors, glass break sensors, and heat detectors are integral components of most alarm systems. Motion detectors identify movement within a designated area, which helps in detecting unauthorized access. Glass break sensors are designed to detect the sound or shock of breaking glass, alerting to potential intrusions through windows. Heat detectors sense changes in temperature, which can indicate fire hazards. Together, these sensors form the primary line of defense in typical alarm system setups.