Author Topic: LED PIR security lights  (Read 775 times)


Re: LED PIR security lights
« Reply #1 on: January 13, 2019, 09:09:48 pm »
20w LED is about 150w incandescent.
We are making a New World (Paul Nash, 1918)

Kim

  • 2nd in the world
Re: LED PIR security lights
« Reply #2 on: January 13, 2019, 09:11:13 pm »
And if Big Clive's teardowns are anything to go by, they all lie about the wattage.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: LED PIR security lights
« Reply #3 on: January 13, 2019, 09:58:50 pm »
Timely this as I was contemplating researching a new exterior light.

I'd had a couple of 500W halogen lamps (which more than illuminated the exterior of a typical semi and driveway) with PIR switches, but decided that they were a failure as they were set off by bushes moving when it's windy - e.g. rose bushes, so they were regularly flickering on and off, and I eventually took them down.

What I needed was something triggered by a heat source - so anyone walking past would set them off.

If these LED lights have the same type of trigger, then IME they are useless where the area covered by the light has a lot of shrubs.

Someone tell me I'm wrong?

Re: LED PIR security lights
« Reply #4 on: January 13, 2019, 10:11:31 pm »
A PIR sensor *is* a heat source detector. They can only be triggered by movement of hot things. Either your sensors were incredibly sensitive, your shrubs were putting out an extraordinary amount of heat, or there was something else hot in the field of view.

(technically speaking a PIR sensor is essentially a thermal imaging camera with only two pixels. The lens is designed so that any infrared source moving causes a variation in the amount infrared hitting each pixel, which the electronics detect)

Kim

  • 2nd in the world
Re: LED PIR security lights
« Reply #5 on: January 13, 2019, 10:13:17 pm »
Possibly something warm behind the shrubs?  Or spiders in the detector is another popular failure mode...
To ride the Windcheetah, first, you must embrace the cantilever...

Re: LED PIR security lights
« Reply #6 on: January 13, 2019, 10:37:24 pm »
A PIR sensor *is* a heat source detector. They can only be triggered by movement of hot things. Either your sensors were incredibly sensitive, your shrubs were putting out an extraordinary amount of heat, or there was something else hot in the field of view.

(technically speaking a PIR sensor is essentially a thermal imaging camera with only two pixels. The lens is designed so that any infrared source moving causes a variation in the amount infrared hitting each pixel, which the electronics detect)
Point of order. Many PIRs have just one pixel. A variation of heat from what it was half a second ago is taken as movement.
Quote from: Kim
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Re: LED PIR security lights
« Reply #7 on: January 13, 2019, 11:17:52 pm »
Point of order. Many PIRs have just one pixel. A variation of heat from what it was half a second ago is taken as movement.
Fair point. I was wondering while I was writing that whether a single pixel one would work. Wonkipedia suggests two pixels ones have fewer false triggers.

Re: LED PIR security lights
« Reply #8 on: January 14, 2019, 07:26:50 am »
I recently replaced a couple of 500w halogen lamps with 35w LED PIR triggered ones.
They are *almost* as bright as the halogen ones were but are, in any case, more than adequate.
One of them is located a metre or so away from the central heating intake / exhaust.
Whenever the heating fires up the hot exhaust gases moving across the PIR can be enough to trigger the lamp.
The second lamp (on a different wall of the house) is unaffected.

Re: LED PIR security lights
« Reply #9 on: January 14, 2019, 08:10:20 am »
A PIR sensor *is* a heat source detector. They can only be triggered by movement of hot things. Either your sensors were incredibly sensitive, your shrubs were putting out an extraordinary amount of heat, or there was something else hot in the field of view.
That indeed was my understanding : Passive Infra Red, so they should detect heat.

I tried for ages to work out what was triggering them and had to conclude it was the movement of bushes.  For the life of me I was not able to identify any heat sources.  I was regularly up and down ladders tweaking the sensitivity.


In response to the OP, I have now researched this unwanted triggering and there is a lot on the net about this, many many people report that bushes, trees etc seem to trigger the lamp when you don't want it triggered.  Some 'experts' recommended cutting down trees and bushes in the range of the PIR.  Which suggest to me that the technology does not work in this situation.  It may be fine in some sterile industrial yard, but not in a domestic setting.

I have a number of bushes and trees around my house, and I can't see a solution, so I won't be trying a PIR triggered light again. Not unless someone can convince me they can be set up not to go off repeatedly on windy nights, which we get a lot of on the side of the Dee valley.

Re: LED PIR security lights
« Reply #10 on: January 14, 2019, 10:05:36 am »
Everything radiates IR, being hot compared to absolute zero.

Since the sensors are detecting changes in temperature, they will react to a bush that isn't at the same temperature as what's behind when the wind moves it, the same as they react to a person (or a cat) who isn't at the same temperature as what's behind.

All you can do is turn down the sensitivity, but then you risk not reacting to the clothing of a burglar who's wearing gloves and a hoody, with no actual warm skin in view

Re: LED PIR security lights
« Reply #11 on: January 15, 2019, 12:06:09 am »
Point of order. Many PIRs have just one pixel. A variation of heat from what it was half a second ago is taken as movement.
Fair point. I was wondering while I was writing that whether a single pixel one would work. Wonkipedia suggests two pixels ones have fewer false triggers.
The single pixel ones can desperately sensitive to any sort of noise. In a typical sensor the lens would typically give 5 bands of sensitive and 5 of insensitive in an arc of 120°. To pick up a person moving, the sensor has to detect stuff that changes in the sort of timescales of a person moving around in a room, so taking a second or two to move in or out of a sensitive area.

That leaves the electronics horribly sensitive to supply voltage variation, or flashing lights etc, and remembering the value from a second ago needs a big, and therefore leaky, capacitor. Doing it digitally is probably too expensive for the dynamic range needed.

I could see that having two pixels would reduce that tendency lots, so when the cooker turns on, the supply voltage dips, and both pixels show a dip, the electronics ignores it.

I think there would still have to be some memory of recent levels of IR, or a hot pipe in the view of the sensor would set it off.
Quote from: Kim
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Re: LED PIR security lights
« Reply #12 on: January 15, 2019, 12:11:33 am »
A PIR sensor *is* a heat source detector. They can only be triggered by movement of hot things. Either your sensors were incredibly sensitive, your shrubs were putting out an extraordinary amount of heat, or there was something else hot in the field of view.
That indeed was my understanding : Passive Infra Red, so they should detect heat.

I tried for ages to work out what was triggering them and had to conclude it was the movement of bushes.  For the life of me I was not able to identify any heat sources.  I was regularly up and down ladders tweaking the sensitivity.


In response to the OP, I have now researched this unwanted triggering and there is a lot on the net about this, many many people report that bushes, trees etc seem to trigger the lamp when you don't want it triggered.  Some 'experts' recommended cutting down trees and bushes in the range of the PIR.  Which suggest to me that the technology does not work in this situation.  It may be fine in some sterile industrial yard, but not in a domestic setting.

I have a number of bushes and trees around my house, and I can't see a solution, so I won't be trying a PIR triggered light again. Not unless someone can convince me they can be set up not to go off repeatedly on windy nights, which we get a lot of on the side of the Dee valley.
It is easy to see where a leafy bush or tree would have leaves very close to air temperature, but the ground has much more thermal mass than a leaf, so it takes much longer to respond to temperature changes, so there could easily be a difference there.

If the leaves are in sunlight, they could well be very much hotter than the ground, but for a PIR light, that's not likely to be a problem as they probably won't be set to come on if there is enough ambient light to make the PIR one redundant.
Quote from: Kim
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Kim

  • 2nd in the world
Re: LED PIR security lights
« Reply #13 on: January 15, 2019, 12:14:26 am »
PIR detectors are basically witchcraft.  I know the theory, but there's no reasonable way something like that should work as well as it does.  (See also:  Photocopiers.  Bicycles.  Anything with a radio in it.)
To ride the Windcheetah, first, you must embrace the cantilever...

Re: LED PIR security lights
« Reply #14 on: January 15, 2019, 09:41:12 am »
Be aware that some cheaper generic models have been guilty of misquoting their rating and include drivers half the stated output as well as lacking correct earthing. Video is dated 2014. I have a 50w flood that is very bright indeed replacing a 500 watt halogen and which is remote controllable.
https://www.youtube.com/watch?v=vbt2ojkXPuo
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CAMRAMan

  • Formerly A Warwickshire Lad
Re: LED PIR security lights
« Reply #15 on: January 15, 2019, 11:26:47 am »
I have a 20W PIR LED floodlight and it does a reasonable job of lighting my back garden.
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tonycollinet

  • No Longer a western province of Númenor
Re: LED PIR security lights
« Reply #16 on: January 27, 2019, 12:54:08 pm »
I got the 40W version of this - more than bright enough. Could have managed with the 23W one I'm sure, but the brightness is nice (illuminates car parking space so excellent for loading/unloading).

Having said that - I only paid £35 inc. shipping at the time. I think it was on a 1/2 price offer.