B
breezer
Pir stands for Passive Infra Red. (Please note the word Passive)
It uses a pyroelectric sensor to detect infra red heat. (aka sensor)
The sensor is looking for infra red heat given off by pets people, etc, you and I see light, a pir can only see heat
The heat it does see is really small, so it has some signal amplification to improve the signal produced by the sensor.
When you amplify anything you also amplify the bits you don’t want, a good pir will filter all this out, a not so good pir will try but is not as good.
The sensor is fixed and can not move, but to allow it to see a wide area there are two options used in the design of a pir
1) The sensor is mounted backwards looking at a lot of small mirrors (like if you were sitting in a car and you can not move at all, so to assist you I come along and put say 30 rear view mirrors on the windscreen, (I put them in 3 rows of 10*) you can now see in all directions behind you)
2) The sensor is pointing forwards and you have a Fresnel lens (the flat funny piece of plastic you find in the back of some large vehicles, to enable the driver to see at “odd angles” again I come along and stick a few of these up so you can see in various directions) again in 3 rows of 10*
A pir will only activate when it sees the infra red heat move from one “area” to another.
Right now you know how it sees and what it sees. I will point out some common mistakes / misconceptions
Being as it sees heat, almost anything solid will stop the pir from seeing the heat source, I say almost anything because they can see through polythene, (what did you think the lens / cover is made of?)
You and I see reflected light, light travels through glass, heat doesn’t (Well not enough for a pir to see you)
How come people say you break a pir beam and it then goes off?
Remember I said (Please note the word Passive) well here is why.
The word passive means to do nothing (close enough for this description) so a PIR emits nothing! I is however receiving infra red heat.
The breaking a beam theory comes from way back when they were first marketed. In order for the person installing it, the makers thought it a good idea to show where the pir can see (remember the lens / mirrors) so they drew a plan view with 10* lines to show where it can see in relation to the lens / mirrors.
They also showed a side view with the same thing but only 3* lines
So as most people (I opened THAT window and asked) never RTFM they only look at the pictures, so they assumed (wrongly) they that were breaking a beam.
There is such a thing as an AIR (yes its Active Infra Red) it has 2 boxes one side of a wall the other on the opposite side of the wall, but they must be in line of sight as one sends a beam of infra red light to the other, interrupting or breaking this beam will cause an activation, and since these were one of the first detection sensors people thought this must be how a pir works, it isn’t
· I chose 30 it can be more but most often is less
Pirs do not like draughts, radiated heat, extreme cold, water, as these can cause “false alarms”
If you stand still in front of a pir it can not see you as you are not moving (remember I did say it can only see when the heat moves.)
The viewing lenses /mirrors are a arranged a bit like a pack of 20 cigarettes
7-6-7 (assuming it has 20)
Each row is not aligned with the one below, and each row is at a slightly lower angle than the one above
It uses a pyroelectric sensor to detect infra red heat. (aka sensor)
The sensor is looking for infra red heat given off by pets people, etc, you and I see light, a pir can only see heat
The heat it does see is really small, so it has some signal amplification to improve the signal produced by the sensor.
When you amplify anything you also amplify the bits you don’t want, a good pir will filter all this out, a not so good pir will try but is not as good.
The sensor is fixed and can not move, but to allow it to see a wide area there are two options used in the design of a pir
1) The sensor is mounted backwards looking at a lot of small mirrors (like if you were sitting in a car and you can not move at all, so to assist you I come along and put say 30 rear view mirrors on the windscreen, (I put them in 3 rows of 10*) you can now see in all directions behind you)
2) The sensor is pointing forwards and you have a Fresnel lens (the flat funny piece of plastic you find in the back of some large vehicles, to enable the driver to see at “odd angles” again I come along and stick a few of these up so you can see in various directions) again in 3 rows of 10*
A pir will only activate when it sees the infra red heat move from one “area” to another.
Right now you know how it sees and what it sees. I will point out some common mistakes / misconceptions
Being as it sees heat, almost anything solid will stop the pir from seeing the heat source, I say almost anything because they can see through polythene, (what did you think the lens / cover is made of?)
You and I see reflected light, light travels through glass, heat doesn’t (Well not enough for a pir to see you)
How come people say you break a pir beam and it then goes off?
Remember I said (Please note the word Passive) well here is why.
The word passive means to do nothing (close enough for this description) so a PIR emits nothing! I is however receiving infra red heat.
The breaking a beam theory comes from way back when they were first marketed. In order for the person installing it, the makers thought it a good idea to show where the pir can see (remember the lens / mirrors) so they drew a plan view with 10* lines to show where it can see in relation to the lens / mirrors.
They also showed a side view with the same thing but only 3* lines
So as most people (I opened THAT window and asked) never RTFM they only look at the pictures, so they assumed (wrongly) they that were breaking a beam.
There is such a thing as an AIR (yes its Active Infra Red) it has 2 boxes one side of a wall the other on the opposite side of the wall, but they must be in line of sight as one sends a beam of infra red light to the other, interrupting or breaking this beam will cause an activation, and since these were one of the first detection sensors people thought this must be how a pir works, it isn’t
· I chose 30 it can be more but most often is less
Pirs do not like draughts, radiated heat, extreme cold, water, as these can cause “false alarms”
If you stand still in front of a pir it can not see you as you are not moving (remember I did say it can only see when the heat moves.)
The viewing lenses /mirrors are a arranged a bit like a pack of 20 cigarettes
7-6-7 (assuming it has 20)
Each row is not aligned with the one below, and each row is at a slightly lower angle than the one above
but a pir for an alarm really can not see through glass.