A burglar alarm is a system that detects the presence of an intruder and gives an alarm. If the primary aim is to deter intruders, a bell, horn, siren, or light is used to give the alarm; if the primary aim is to apprehend intruders, a signal is transmitted by wire to a nearby police station or security office.
An electrical alarm system has circuits with pushbutton switches whose contacts are closed by the closing of doors and windows. When a door or window is opened, the switch contacts open and activate the alarm.
An infrared alarm system uses an infrared beam projected onto a
photoelectric cell either directly or by means of mirrors. If an
intruder interrupts the invisible infrared beam, the break is detected,
triggering the alarm. A wide area can be covered when the beam is
deflected by mirrors.
An electronic alarm system has two balanced electronic oscillators connected to an antenna mounted in the area to be protected. The presence of an intruder near the antenna causes the oscillators to become unbalanced because of his body capacitance; the unbalance is detected, triggering the alarm.
An ultrasonic alarm system constantly transmits and receives an ultrasonic wave pattern; any movement of an intruder causes the pattern of the received signal to differ from that of the transmitted signal; this difference is detected and used to trigger the alarm.