|MadSci Network: Engineering|
Hi Felix: Most security systems have a portion of circuitry to handle sending an alarm when triggered. What triggers that circuitry is what you are interested in. From your questions, I gathered two particular types of trigger mechanisms (sensors as we call them). One is a activated by light, and the other, well, the other will take a little longer. The entry beam detector is made up of two components: a light source, and a detector. The light source (beam) is usually placed on one side of the door, and the detector is placed on the opposite side of the door. The light beam is focused with lenses and the beam is directed towards the detector. Most systems work in such a way that as long as the light beam is not broken, the system is not triggered (call it 'status OK'). When a person walks through the doorway, the light beam is momentarily blocked by the person's body. The alarm system is then triggered by this momentary disruption of light (call it 'alarm condition'). Now, on to the first question. The door switch proximity detector is a bit ambiguous. You can have a simple switch as the sensor for the alarm, as the door is opened, the alarm triggers. And/or you can have a proximity switch as the sensor. A proximity switch may take many forms. There are magnetic proximity sensors (metal detectors), light proximity sensors (wash-room automated faucets and flushers and the light beam detector above), capacitive proximity detectors (certain light switches, and your TV antenna), and then there are thermal sensors which have a branch of their own (most body heat detectors). The last one is most commonly known as motion detectors, sometimes referred to as PIR detectors (Passive Infra-Red). Most of these sensors are used with circuitry that has adjustments for 'normal conditions' or 'status OK'. Once this normal level condition is set, anything that causes the conditions to exceed the normal level will trigger the alarm. As an example, on a capacitive sensor that is attached to a door knob, the effect of your hand approaching the knob would be like grabbing the TV antenna, and is then detected as an alarm condition. That is, by the way, one of my favorite type of sensor as it is harder to defeat. Yes, false alarms are possible. The more common motion detector/proximity detector is activated by heat or infra-red emissions from a live body. These PIR motion detector systems may be defeated, by cooling the sensor, far below the trigger level. This can be accomplished by cranking the air-conditioning up really high, then walking very very slowly past the sensor. I don't know how much electronics experience you have, or if you wish to try a circuit or not. If you wish to get some good reading on the topic, try a book entitled 'The alarm, sensor, and security circuit cookbook', by Thomas Petruzzellis. It's one of the few books with sample circuits that actually work with out too much difficulty. Good luck, and let me know if you need more specific information. Abtin Spantman SPANTMAN@EXECPC.COM
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