1. Home
  2. Electronics Repair
  3. How motion sensors work

How motion sensors work

I'd like to get responses, ideally, from folks who actually know how these things work, and not the usual Usenet speculation.

I install a lot of motion-detector lights for clients. A lot of these installations are problematic. I just adjusted one for the third or fourth time because the light was staying on all the time, even though the sensor unit (I replaced the entire unit with a Heath-Zenity sensor recently) was working properly.

It might help for me to know how, exactly, these sensors work. By "sensors" I mean the actual sensor, as well asw the entire unit with the support electronics.

I know *generally* how these work: so far as I know, the sensor detects heat, so I'm guessing it's an infrared receptor of some sort.

What would help are knowing such things as what the actual "angle of view" of these sensors are (I know this is usually given on the box, but I wonder how accurate those figures are and how much is hype when they say the sensor has a 270° range), what their effective distance range is, what kind of response curve (distance/temperature of radiating body/response) they have, and what kinds of effects due to reflection one needs to be aware of.

Of course, it's possible there's already a document out there somewhere in Internet-land that explains all this; if there is, and if you know about it, I'd appreciate a link.

It might even be fun to get one of the "raw" sensors and play around with it on a test bench, maybe try to characterize it myself. Remember, my goal here is to be able to install these things (motion-activated lights) so that they actually work correctly. As things stand now, it's often a crap shoot. And yes, there are often defective components. This last installation was typical, where the sensor unit had simply failed and needed to be replaced. (And if anyone knows which units are better than which others, that too would be valuable information.)

Thanks in advance.

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl
Loading thread data ...

If you really want a detailed explanation, you can find several online

- there are different detection technologies and there are dfferent types of lenses, which affect the field of view.

My experience is that aim is the key to reliable operation, regardless of the motion sensor brand. Unless you are very fimiliar with a specific unit under different conditions, it's unlikely that you will get the aim correct the first (second/third) time.

Example: There is a Zenih motion sensor light on the corner of the house. It faces a side street which is about 5 feet lower than the ground at the side of the house. If the motion sensor is aimed for people detection at 10-15 feet, a vehicle turning into the side street can trip it. If the motion sensor is aimed down enough to avoid vehicle trips, it doesn't see people until they are within about 5 feet of the sensor, which is about 10 feet off the ground.

I've used a similar motion sensor light on level or up-sloping ground and had much more reliable operation (longer range sensing, fewer false detections).

John

Reply to
news

On 2/21/2010 5:12 PM snipped-for-privacy@jecarter.us spake thus:

Well, sure, aim seems to be about 85% of the trick here. And certainly having a sensor looking out on level or upsloping ground is ideal. The problem with my last installation was that the light with attached sensor was under the eave of the house looking straight down on a driveway below. Apparently the main problem was that the heat from the lights was keeping the sensor on; turning the lights away from the sensor seems to have fixed it. (Makes a good case for using CFLs, except that they're bad on two counts: take too long to come up to full brightness, and aren't supposed to work correctly with most sensors, being a non-resistive load.)

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl

On 2/21/2010 4:12 PM David Nebenzahl spake thus:

So I found this circuit out there in the wild:

formatting link

It uses something called a PIR. Does that stand for piezoelectric infrared? I've seen other references to piezo sensors: how do those work? Do they both send and receive a signal? (The device shown here is a 3-terminal thingie.)

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl

On 2/21/2010 5:41 PM David Nebenzahl spake thus:

My guess was wrong; it's "pyroelectric", not piezo.

Hey, whaddya know; actually found useful stuff Googling, like this page that has a pretty good explanation plus a schematic:

formatting link 

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl

"David Nebenzahl"

See:

formatting link

.... Phil

Reply to
Phil Allison

Motion sensors are weird. The sensor itself is a piece of piezoelectric plastic that has been used to make tweeters (most notably by Pioneer).

Oddly, these sensors generate a voltage when exposed to infrared. The fluctuations in this voltage (as a warm body moves in front of the sensor) can be capacitor-coupled to a FET for amplification.

Reply to
William Sommerwerck

On 2/21/2010 5:47 PM Phil Allison spake thus:

Sorry, I don't rely on Wikipedia, "the 'encyclopedia' that any pimple-faced 7th grader can edit", for any credible information.

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl

Passive Infra Red, meaning that it doesn't have its own IR source. It works from a pair of IR sensors, and looks for a difference in the IR level on the pair. That is why it takes a few seconds to a minute for them to settle when first powered up. Some early PIR detectors came with several sets of lenses to adjust the beam width & height. You looked at the manual and picked the one you needed for the desired operation.

--
Greed is the root of all eBay.
Reply to
Michael A. Terrell

Good for you. The first PIR for the consumer market was a motion sensor for burglar alarms. It ran on a lithium battery, and was sold as a "Passive Infrared Detector". I tested one from the first production run, decades ago.

--
Greed is the root of all eBay.
Reply to
Michael A. Terrell

"David Nebenzahl"

** No need to *rely* on wiki - but it is usually a good starting point in a quest for the basic facts on a great many topics.

Other web sites are JUST as likely or MORE likely to contain serious errors or give misleading information.

In any case, I never post a link unless I have read it through and reckon the info is good.

"the 'encyclopedia' that any pimple-faced 7th grader can edit", for any credible information.

** You are in NO position to decide what is credible and what is not.

Cos you are a trolling, brainless cretin.

..... Phil

Reply to
Phil Allison

And /your/ qualifications are...?

Reply to
William Sommerwerck

FWIW, the detector's field/pattern can be changed with opaque material-black tape placed on the lens (sides and/or top/bottom).

For a narrow field, applying pieces of vertically-oriented tape to the sides of the lens area reduces the field to a more narrow pattern.

Putting the tape (or opaque paint) on the inside would look more professional, IMO, but the tape or other material needs to be secure so it doesn't peel away and block the sensor. An opaque mask could probably be fabricated and glued securely to the inside of the lens holder portion of the enclosure.

Those are some informative links you posted.

-- Cheers, WB .............

Reply to
Wild_Bill

On 2/21/2010 6:17 PM William Sommerwerck spake thus:

Heh; I see we both got our tickets for the Phil Allison ride here. Great fun for free; just sign up at RoomForInsults.com and experience the adrenaline rush as the invectives start flying!

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl

PyroElectric or Passive InfraRed sensor.

Typical sensor:

Tutorial:

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

He is a mentally ill Aussie who won't stay on his prescribed medication.

--
Greed is the root of all eBay.
Reply to
Michael A. Terrell

Close--pyroelectric, and all pyroelectrics are also piezoelectric, afaik. Porch light sensors use a pyroelectric split cell, with the two halves wired in series, and a MOSFET connected to the point in the middle. The MOSFET usually has a 10M gate leak resistor.

The Fresnel lens in the front is made of high density polyethylene (HDPE), which has about 40% transmittance in the 8-14 um band. It is actually arranged as segments of several lenses (about a dozen or so), so it casts a dozen images of everything in the field. Warm things that move horizontally cause warm images to move across the split detector, making an AC waveform. The FET's output is AC-coupled into another amplifier that drives the triac.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Reply to
Phil Hobbs

Piezo? Pyro? I don't think so.

PIR is Passive InfraRed. Ie, it detects infrared (heat) without actively sending out any infrared illumination.

Reply to
greenpjs

Phil Hobbs wrote in news: snipped-for-privacy@electrooptical.net:

NOPE. PIR=Passive Infra-Red.

--
Jim Yanik
jyanik
at
localnet
dot com


> Porch light sensors use a pyroelectric split cell, with the two 
> halves wired in series, and a MOSFET connected to the point in the 
> middle.  The MOSFET usually has a 10M gate leak resistor.
> 
> The Fresnel lens in the front is made of high density polyethylene 
> (HDPE), which has about 40% transmittance in the 8-14 um band.  It is 
> actually arranged as segments of several lenses (about a dozen or so), 
> so it casts a dozen images of everything in the field.  Warm things that 
> move horizontally cause warm images to move across the split detector, 
> making an AC waveform.  The FET's output is AC-coupled into another 
> amplifier that drives the triac.
> 
> Cheers
> 
> Phil Hobbs
>
Reply to
Jim Yanik

On 2/23/2010 5:56 AM Jim Yanik spake thus:

Nope.

They can be called that, but they are also called pyroelectric (look it up if you don't believe me). And no, I don't think they have anything to do with piezoelectricity; they are, after all, passive elements.

--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
Reply to
David Nebenzahl

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.