Burglar Alarm

colinw

This might seem like a daft question but can a burglar alarm which has infrared sensors in the rooms be set off by ordinary lights?
We are off on holiday soon and want to put the alarm on. We also want to have a couple of lights connected to timer switches coming on when it gets dark.
Will the alarm go off or does it look for a particular type of heat?
Sorry if this is a daft question.

m00nie

they work off movement, the infared is the beams it sends out and if brokien the alarm goes off, so you will be ok to have the lights come on

colinw

Cheers for that

quoia

m00nie wrote:

they work off movement, the infared is the beams it sends out and if brokien the alarm goes off, so you will be ok to have the lights come on

Sorry but you are incorrect.

PIR detectors "sense" heat.
They shouldn't be pointed at windows since the sun shining on them can set them off.

Re having lights coming on and off in your absence, you should be ok providing the lamps in question are not in the PIR's field of view.

If the PIR is looking at a cold light bulb that suddendly switches on and gets very hot, YES it will set the alarm off.

SEE the relevant bits from:

http://home.howstuffworks.com/burglar-alarm2.htm

http://home.howstuffworks.com/question238.htm

YorkshireBoy

quoia wrote:

Sorry but you are incorrect.

PIR detectors "sense" heat.
They shouldn't be pointed at windows since the sun shining on them can set them off.

Re having lights coming on and off in your absence, you should be ok providing the lamps in question are not in the PIR's field of view.

If the PIR is looking at a cold light bulb that suddendly switches on and gets very hot, YES it will set the alarm off.

SEE the relevant bits from:

http://home.howstuffworks.com/burglar-alarm2.htm

http://home.howstuffworks.com/question238.htm

I'm sorry but the above is totally incorrect, poor advice, and should not be followed by the OP.

Passive Infra Red (PIR) detectors need heat AND motion (AND indeed, speed of motion) passing BETWEEN their invisible IR "beams" to activate the circuitry.

I was going to substantiate my statement above, but I found that I couldn't simplify it any more than one of your own referenced links does!

"...More advanced security systems include passive infrared (PIR) motion detectors. These sensors "see" the infrared energy emitted by an intruder's body heat. When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly...."

I hope this clarifies the situation for the OP and that he does indeed secure his property when he goes away on holiday.

YB
(Electrical Engineer)

quoia

YorkshireBoy wrote:

I'm sorry but the above is totally incorrect, poor advice, and should not be followed by the OP.

Passive Infra Red (PIR) detectors need heat AND motion (AND indeed, speed of motion) passing BETWEEN their invisible IR "beams" to activate the circuitry.

I was going to substantiate my statement above, but I found that I couldn't simplify it any more than one of your own referenced links does!

"...More advanced security systems include passive infrared (PIR) motion detectors. These sensors "see" the infrared energy emitted by an intruder's body heat. When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly...."

I hope this clarifies the situation for the OP and that he does indeed secure his property when he goes away on holiday.

YB
(Electrical Engineer)

BUT YOU'VE TOTALLY MISSED THE POINT.

Re the link data I listed and you've quoted!!!

When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly ...."

Motion is NOT required when something that is ALREADY IN THE PATH SUDDENLY CHANGES IN TEMPERATURE.

PIR's are NOT TRUE MOTION detectors. They are not "radar", ultrasonic or otherwise, do not use sound or the Doppler effect.
They are CHANGE IN TEMPERATURE detectors.

They work on the principle that if the area they are monitoring has a sudden rise in infrared energy, then 99.9% of the time, something "HOT" MUST HAVE MOVED into the area and hence there was "motion".

However there is the other 0.01% and a PIR looking at an electric light bulb that is powered off and hence COLD, will TRIGGER if it is turned ON and GETS HOT !

The P in PIR stands for PASSIVE.
They are NOT ACTIVE.

They GENERATE absolutely nothing.
They are ONLY RECEIVERS.
There are no "invisible beams" being broken by things moving about.
They are basically THERMOSTATS !

YorkshireBoy

quoia wrote:

BUT YOU'VE TOTALLY MISSED THE POINT.

Re the link data I listed and you've quoted!!!

When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly ...."

Motion is NOT required when something that is ALREADY IN THE PATH SUDDENLY CHANGES IN TEMPERATURE.

PIR's are NOT TRUE MOTION detectors. They are not "radar", ultrasonic or otherwise, do not use sound or the Doppler effect.
They are CHANGE IN TEMPERATURE detectors.

They work on the principle that if the area they are monitoring has a sudden rise in infrared energy, then 99.9% of the time, something "HOT" MUST HAVE MOVED into the area and hence there was "motion".

However there is the other 0.01% and a PIR looking at an electric light bulb that is powered off and hence COLD, will TRIGGER if it is turned ON and GETS HOT !

The P in PIR stands for PASSIVE.
They are NOT ACTIVE.

They GENERATE absolutely nothing.
They are ONLY RECEIVERS.
There are no "invisible beams" being broken by things moving about.
They are basically THERMOSTATS !

I concede I made a poor choice in terminology, when using the analogy of the "beams". What I was trying to illustrate was the detection of the heat source on the PIR's lens (all PIR instruction leaflets show an arc of coverage/"beam" format, eg 90 deg-180 deg, with some specialist units even having 270-360 deg detection, and this is what I was referring to).

I still maintain that to activate the PIR the "light bulb" (as you call it) would have to be physically moved from one place in the room to another. To illustrate my point, I've copied some text from another website. I believe the key sentence in here is...

"As a warm body or heat source moves from one facet to another,..."

The DPC referred to at the end of the text is the number of "knocks" (as in double-knock) that are required to activate the sensor. Generally, there are 3 settings (1,2, & 3). I have my PIR's set to 2 - this means that there must be detection on two different "lens facets" within x milli-seconds to activate the sensor.

Anyway, have a read of the text below and see if you agree.

=============================
PIR's work by detecting the movement of heat radiation within a protected area. The PIR has a lens, which directs and magnifies heat energy, on to a very sensitive pyro-electric element on the PCB (Printed Circuit Board), such as that emitted from a warm-blooded body. This lens has multiple lens facets, which focus different parts of the room space on to the Pyro Element. As a warm body or heat source moves from one facet to another, the electronic circuitry amplifies the very small changes in heat and therefore detects the movement of the intruder. An alarm output is only generated when specific parameters are met. The detected heat source must fit a profile, which includes, heat magnitude and motion speed, finally, a selectable number of detections must be detected in a set time before an alarm output will take place; the number of detections can be set using the Digital Pulse Count (DPC) jumper
=============================

Regarding your original post, obviously we are not talking about ceiling lights because the PIR will not "see" them. We must, therefore, be discussing table lamps on timers plugged into wall sockets.

Are you seriously suggesting (and indeed maintaining) that a standard tungsten filament "light bulb" (or lamp as we call them in the trade) will activate a passive INFRA-RED detector? Bearing in mind that each operates at a different wavelength in the spectrum.

YB

quoia

YorkshireBoy wrote:

Regarding your original post, obviously we are not talking about ceiling lights because the PIR will not "see" them. We must, therefore, be discussing table lamps on timers plugged into wall sockets.

The vertical detection area of many PIRs is within the realm of being able to detect a ceiling light that hangs from a flex that is 10 to 15 inches long. (perhaps not a flush fitting lamp unit similar to a fluorescent light)

YorkshireBoy wrote:

Are you seriously suggesting (and indeed maintaining) that a standard tungsten filament "light bulb" (or lamp as we call them in the trade) will activate a passive INFRA-RED detector? Bearing in mind that each operates at a different wavelength in the spectrum.

You'll be telling me next that an incandescent lamp doesnt emit infrared !!

I suggest you read the fitting instructions for the standard single detector PIR units that are fitted in most alarm (especially household) systems.

Should NOT be pointed at heat sources or windows or reflective surfaces.

I know of an office where they occasionally had the alarm go off in the middle of the night. The alarm system always identified a particular circuit triggered by a single PIR. They had Rentokill in to check there were no mice in the building. The alarm company did a full diagnostic, checked for insects and spiders in the PIR and ultimately replaced the PIR to eliminate. The problem persisted intermittantly. After further investigations (and warnings about false alarms from the police) the PIR was removed from the room and repositioned into a corridor. The problem went away. Almost 1 year later the problem returned but a different circuit in a different room was always now the cause. My friend who worked in these premises was telling me about this and I was intrigued by the mystery that was baffling the alarm "experts".
I looked at the room that previously had had the problem and the one suffering from it now, asked a few questions about what changes, if any, had been recently made. What became apparent was that there had been a recent office relocation by several members of staff and the functions of certain rooms had changed. Indeed the "new" room had recently taken over the function of the "old" room that previously had caused the false alarms.

A few more questions didn't add any further clues but it left me thinking how strange. A few days later they had another false alarm and my mate was telling me about another visit from the police in the early hours and them not being very happy about it. I asked my mate if they kept a log of all the dates and times of the false alarms. He said yes so I asked to look at it.
It took me a few hours to see a pattern but there was one. I had to discount a few of the entries (because they didn't fit a pattern) but what was left DID ! Indeed it was only when I saw an entry that had BST against the time that I saw it. Correcting all the times to GMT, where some had happened in the summer months and the time was BST, made it all too clear that time was a significant factor and then some dates formed a pattern.
I asked my friend what happened on these days that was different to most of the others. People working later than normal or anything special? Nothing that he could think of.

Whilst I couldn't be 100% certain, because some false alarms were outside the pattern, I predicted when the next one was likely to be.
Sure enough I got it right. I asked him to ask everyone what was different on that day to the previous 3 weeks. Again loads of blanks faces and no help.
I went round that lunchtime and, over a cup of coffee, "interogated" a secretary that was usually first in and last out of the "PIR" room in question. Whilst chatting there was a beeping noise and she excused herself for a few minutes. When she returned she explained that it was the fax that had ran out of paper. Then she went on complaining about the waste of fax roll that had happened due to an "advertising flyer" for some company based in the USA that had arrived overnight. Apparently this happened about every 5 or 6 weeks and was rather annoying since it was usually 20 or 30 pages long.

Suddenly the penny dropped. I asked to see the fax machine.
It was in the "false alarm" PIR room. A little over a year before the same machine had been in the other "problem" room. The fax was quite small and inconspicuous due to the fact that the paper was hidden inside and appeared out of a small slot at the back, close to the wall before dropping to the floor.
The unit itself was quite hot since it was a thermal paper type and had just been printing for a few minutes.

I told my mate I'd solved the problem but needed half an hour to show him.
I unplugged the fax and took it outside to cool down back to something like room temperature. 20 minutes did the trick and I brought it back in again. We put the alarm system into diagnostic/test mode and closed all the doors etc.
I phoned my office and got them to send a fax to my mate. The alarm system displayed a PIR activation on that circuit about a minute later. We went and looked and the fax had not even started to print. BUT it was going though its HEATING UP CYCLE in preparation.

My mate's boss immediately took us both down the pub and plied us with a large amount of alcohol.
He arranged for the fax machine & telephone line to be moved into a room not monitored by a PIR and they never had a false alarm again !

YorkshireBoy

Wow, thats a long story!

I'm trying to compare your fax machine heat source theory with situations I may have come across. The nearest comparison I have is a PIR mounted so it views one of our office corridors longtitudinally. At the end of the corridor is a coffee machine which has been programmed to switch on at 5.30am. To date we have not had any false alarms on the intruder system.

With regard to your incandescent lamp emitting infra-red light, I agree there may be a very small crossover on the spectrum (they lie side by side with IR wavelengths in the order of 0.01mm and visible light typically 400-700nm) but, as in my previous quotation, the "detected heat source must fit a profile, which includes, heat magnitude [SIZE OF BODY] and motion speed [THE TIME BETWEEN DETECTION ON THE INDIVIDUAL LENS FACETS".

Anyway, Its clear we're not going to reach a conclusion on the technical aspects of this thread so, with regard to the OP's fundamental question, and assuming that any table lamp in the typical domestic room WILL be in the PIR's "line of sight"...

...I STILL propose that the OP sets his alarm, AND uses his light timers to increase his properties security, when he goes away on holiday. What say you?

YB

colinw

I seem to have started something here mind. My plan is to use the lampson timer switches and have the alarm on. However I think I will do a test before I go away and set the bulbs to come on when the alarm is on. Actually the lampshouldnot be in view of any of the sensors. Thanks for opinions anyway and especially the long story!

YorkshireBoy

colinw wrote:

I seem to have started something here mind. My plan is to use the lampson timer switches and have the alarm on. However I think I will do a test before I go away and set the bulbs to come on when the alarm is on. Actually the lampshouldnot be in view of any of the sensors. Thanks for opinions anyway and especially the long story!

You're very welcome Colin.

By the way, we don't have any light timers ourselves. However, my parents, who live next door but one, have 3 or 4 of these timers running table lamps in various rooms.

I fitted their alarm system some 13 years ago (bungalow with a PIR in every room except the bathroom). For as long as I can remember, they've been going out 2 nights a week dancing and take 5 or 6 holidays a year, always using the timers and setting the alarm, and they've NEVER had a false alarm!

HTH
YB