A Guide To Alarm Sensors

If you’re looking for an alarm system but you’re confused by all the various different options available to you, then you’re in the right place!

In this article, I’m going to explain about the five main types of sensor used in an alarm system, so hopefully by the time I’ve finished, you’ll have a greater understanding of what each one is, what it does,  where it’s good and where it’s not.

PIR Motion sensor

Probably the most popular sensor used in alarm systems these days is the passive Infrared motion sensor, or PIR for short. It works by sensing the movement of heat across its field of vision. Obviously we as han beings are warm blooded and therefore produce body heat and the PIR senses the movement of that heat across its field of view.

it’s good for covering  a room, especially a room which might have more than one window or door leading into it, because if you needed to cover each individual opening door or window, then things will get quite expensive, when one sensor would cover the whole room.

Where it’s not quite so great is where you might have draughts, which move pockets of warm air around unnaturally, places like garages, for instance, or warehouses.

Door & Window Contact Sensor

The next sensor to consider is the door sensor or door contact. These are  a magnetic sensor where you have the main sensor on the door and a magnet on the frame (or the other way around it doesn’t matter too much which). As the door opens, the magnet moves away from the sensor and it allows a switch within the contact to go open and as the door shuts, the magnetic field is restored and it physically pulls by magnetism, a switch closed within the electronics. In that way, it senses whether the door or window is open or closed. It’s a very simple concept, it’s been used many times in many alarm systems throughout the world.

Where it’s not so great is, as I said in the previous item, if you’ve got more than one door or window into an area, and you need to cover them all, then having a door sensor on each of those can get quite expensive.

Dual Technology Motion sensor

The next one to talk about is the dual technology sensor. I mentioned previously that a PIR isn’t quite so good in draughty areas like warehouses and garages, where you can get movements of pockets of heat, and that can confuse a standard PIR.

The solution to that originally was to use a microwave sensor, which sends out microwaves to sense movement. The problem with those is that they can snse beyond walls so you would sense the movement of something outside the area that you’re trying to cover.

To get the best of both worlds, they combine both PIR and microwave into a dual technology sensor. It’s a bit more expensive so you wouldn’t want to use it where a PIR would do, but as I say in garages, warehouses and larger spaces which have draughts in them, they are a great option.

Vibration sensor

The next sensor to talk about is the shock sensor, also known as the vibration sensor. In conservatories in homes, you wouldn’t want to use a PIR because again you’ve got a fairly wide space which is  susceptible to a quick build-up of heat, where the sun comes in through all the glass and heats things up. That can occasionally cause problems with a PIR. Again you’ve  probably got quite a number of different doors and windows that will be quite expensive to protect using a contact sensor on each one. Instead we can use a vibration sensor which will sense the vibrations of someone trying to break in. You would probably need a couple of shock sensors to cover an average size conservatory.

Glass-Break sensor

The last one I’m going to talk about today is the glass break sensor. This is an acoustic device thait listens and tries to recognise the sound of breaking glass, so in a conservatory again or in a shop where you’ve got large glass windows, if someone was to try and smash their way in through the windows, the sensor would recognise the sound of breaking glass and would trigger the alarm.

We once had a problem with one of those where a fire extinguisher bracket fell off the wall and the glass brake sensor thought that that was breaking glass, when actually it was the sound of metal hitting the floor. Not the most reliable but would do a job in certain circumstances.

There you have it, a brief whistlestop tour of the main types of sensors that we use in burglar alarm systems.