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How Does an Alarm Motion Sensor Work?



Many of us are familiar with motion sensors, as they have become a common part of most home security systems. These sensors are much like the motion detectors that open the doors to the grocery store or cause the sinks and toilets in the airport to activate without touch. We see motion-sensing devices everywhere. But how do they work?

There are two basic types of motion detecting systems. There are those with active sensors and those with passive sensors. Active sensors send out ultrasonic sound waves and passive sensors detect infrared energy.

Active Motion Sensors

Active motion sensors (also called radar-based motion detectors) send out ultrasonic sound waves - the same waves used in pregnancy ultrasounds - and these waves reflect back to the sensor. This pattern continues until it’s disrupted by an interference of some sort (e.g. a person walking past). This triggers the sensor to send a signal, which can open an automatic door, open a garage door or sound an alarm.

Passive Motion Sensors

There are two popular types of passive motion sensors. These are mostly found in residential homes.

Passive infrared motion detectors (PIR) detect emitted infrared energy – given off by humans and animals in the form of heat. When there is a sudden increase in infrared energy, an alarm is sounded. Small fluctuations in infrared energy aren’t detected by the sensor – which is fortunate because the alarm would otherwise go off as the sun rises and sets, changing the household temperature slightly.

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Photo sensor motion detection generally employs laser beams (or another source of focused light) and light sensors. The system sends out a beam of light. When an entity crosses the beam of light, the sensor registers this change in light level. The sensor in turn signals the control box, which can be programmed to emit a bell noise or sound an alarm.

Any combination of these three motion sensors may be found in a building’s security system, and they all play a vital role in keeping your home or business safe.

If you’d like to learn more about the motion sensing options offered by Crime Prevention, don’t hesitate to schedule a no-obligation appointment with one of our consultants today.


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