How do hidden defences detect burglars?

Medieval castles had all sorts of traps pitfalls to keep out intruders. Homes of today can have an equal number of defences, without having to resort boiling pitch. Modern defence devices include floodlights or alarms that are triggered when an intruder upsets a circuit monitored by hidden magnets or micro. chips, or by invisible beams.

The outer defences

The modern burglar might first have to face  a strategically placed invisible infrared detector that is affected by temperature changes caused by body heat. When anyone approaches the house, a sensor in When the detector switches on floodlights. If the caller is legitimate, the lights show the Way but anyone planning burglary will feel very exposed and less likely to continue.

The sensor is pyroelectric — that is, made from a ceramic material such as tourmaline, which, when heated, generates a voltage across it. The system is designed so that the sensor will respond to a temperature change caused by human body heat, but is less likely to be set off by changes in the weather.

 A burglar who dodges a floodlight – barrier may then face a door connected to a noisy alarm. A magnetic switch IL inserted between the door and its frame. When the door is shut, two contacts keep switch circuit closed. This switch is monitored electronically by an alarm circuit. If the door is opened arid switch circuit is broken, the alarm circuit triggers the alarm.

But a resolute burglar, out of sight of passers-by, might attack the door with a chisel or drill. This type of attack can be foiled by a vibration detector fitted to the door. This is a device in which a ball is disturbed by vibrations. The ball rests on sharp metal points wired to a microchip that is programmed to accept certain vibrations — such as those caused by wind or passing traffic — as normal. If the ball bouncing on the points sets up vibrations not in the program, it sets off the alarm.

The inner defences

If the burglar succeeds in getting through a door or window, he may face a battery of inner defences. These include pressure Dads concealed under the carpet and inked to an alarm circuit. They have two metal plates or foil sheets separated by a layer of spongy plastic. The two plates are pressed together if anyone treads on them, and this sets off the alarm.

Anyone who prowls around inside the house may be caught by a ‘magic eye’. This is a photoelectric cell with an invisible infrared beam shining onto it. If the beam is interrupted, the photocell triggers an alarm.

Other types of indoor detector use either ultrasonic waves (too high pitched for humans to hear) or microwaves (high-frequency radio waves) transmitted by

devices called transducers. They transmit the waves at a certain frequency (a given number per second), and the waves are reflected back to the unit from objects in the room. If anyone moves through the room, the reflected waves get bunched up or pulled apart, so their frequency is altered. The sensor detects the frequency change and feeds signals to a microchip which assesses the speed and bulk of the intruder. Anything assessed as typically man-sized makes it set off the alarm.

A commoner type of indoor detector uses an infrared system similar to the outdoor floodlight type. in the detector. a many-faced mirror or special lens creates a number of sensitive zones. If anything moving in and out of these zones is at a different temperature to the room surroundings, it generates a voltage. The detector electronically monitors the voltage, and is designed to set off the alarm if the temperature increase is likely to be caused by human body heat.

 

Picture Credit : Google