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PIR sensor protective lense — Parallax Forums

PIR sensor protective lense

BrianseBrianse Posts: 3
edited 2014-09-20 02:49 in Accessories
I'm working with a PIR sensor (555-28027) to sense sweet corn ears on a conveyor. It's inside a PVC pipe to limit what it sees but I need something to close the end of the pipe that the sensor can "see" through. The lense needs to be washdown proof (able to be sprayed with high pressure water),

I glued a piece of plexiglass on the pipe and it protected the sensor fine, but it couldn't "see" though it. I also tried Lexan (polycarbonate), and anything else I could find lying around the shop. The only thing I've found that the PIR can see through is a 4 mil sheet of polyethylene. I don't think the poyethylene is sturdy enough to take a direct spray at washdown and I haven't found a glue that would stick to it either.

Does anybody know what frequency IR the sensor uses? I don't see it on the datasheet.

Any suggestions of another material to try??

Does anybody know what the white plastic shield on the sensor itself is made of??.

Except for this problem, the sensor is perfect for what I need it to do.

Thanks. Brianse


  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-01-20 07:53

    Welcome to the forums.

    I'm intrigued that you would use a PIR to detect ears of corn. Usually, PIRs are used to detect the motion of warm bodies, such as people or animals, in a case in which the warm bodies are detected moving across a background that is not warm. The PIRs use the movement of the difference in body thermal radiation to determine motion. So if the ears of corn are not warm, or if the ears of corn are warm and the conveyor is also warm, your system might not "see" anything at all. Have you tested your system with actual corn under the conditions at which it will be operating? If the corn is warm, will the conveyor belt start to warm up over time?

    I think most PIRs operate at a wavelength around 5 to 14 microns.

    Are you sure you don't want to use some regular IR beam-breaking type of counter?

    What are your washdown conditions? Hot water and bleach? I'm not sure polyethylene (PE) is rated for bleach. I know polypropylene is good at higher temps and I think it can handle bleach, and I'm not sure the PIRs can see through it. And you will probably need to find a mechanical way to affix polypropylene to the PVC pipe since I'm not sure I would trust most adhesives. Probably anything you use will have to be FDA approved so you don't splash some toxic adhesive onto the corn.

    I'm fairly sure that near IR, as used with IR LEDs like those used in beam-breaking applications, can shine through polypropylene.
  • Duane DegnDuane Degn Posts: 10,450
    edited 2012-01-20 08:14

    Have you tried a thicker sheet of polyethylene? Maybe from an old milk jug?

    If the sensor can see through a sheet thick enough to protect it, you could use Gorilla Tape to hold it in place. You might need to replace the tape once in a while, but that stuff is surprisingly strong.

    I don't know to which wavelenths the PIR is sensitive, but I found this on Wikipedia:
    This filtering window may be used to limit the wavelengths to 8-14 micrometres which is closest to the infrared radiation emitted by humans (9.4 micrometres being the strongest).

    Edit: I hadn't seen ElectricAye's post.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-01-20 08:32
    There's a nifty chemical compatibility app here:

    Based on that, I'm having second thoughts about polypropylene being compatible with chlorine.
  • Duane DegnDuane Degn Posts: 10,450
    edited 2012-01-20 08:39
    There's a nifty chemical compatibility app here:

    Based on that, I'm having second thoughts about polypropylene being compatible with chlorine.

    Why's that? Bleach isn't chlorine. It's Sodium Hypochlorite. PP is rated excellent at < 20% concentrations and good at 100%.

    Edit: I think PE would also be fine for modest concentrations of bleach.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-01-20 08:56
    Duane Degn wrote: »
    Why's that? Bleach isn't chlorine. It's Sodium Hypochlorite. PP is rated excellent at < 20% concentrations and good at 100%.

    Edit: I think PE would also be fine for modest concentrations of bleach.

    I'm glad you saw that. I had thought PP was great with bleach, but what confused me was the compatibility of PP with "chlorine water" when I looked it up. So I guess Chlorox bleach would work fine with PP.

    I also see on Mcmaster that there are PE sheets that are rated for washdown applications, so maybe PE is the way to go after all. Now if we could only solve the adhesive problem....
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2012-01-20 09:17
    Getting back to the PIR sensor, I don't think it's the right one for the job. PIRs detect difference in infrared across two adjacent cells, which can come from any kind of motion, including the conveyor. I would suspect the sensor isn't "seeing" the conveyor for some reason (cold, not reflective, etc.), but that may not always be the case.

    I'd try a simpler light sensitive sensor that can tell the difference between light reflecting off the conveyor and the corn. You can then fine-tune how much of a change between the conveyor "background" and the corn "foreground" you want simply by adjusting the value of a couple of resistors, and an analog threshold. Obviously this works best if the conveyor is dark, because the corn will be relatively light in color.

    These kind of light sensors typically work in the near IR region of 800 to 1000 nm, and most any protective covering will work, even glass (only a specialized glass will work with the longer IR that a PIR is sensitive to). You want a pretty strong IR light source, so look for an "ultra bright" IR LED. You can combine LEDs to make it even brigher. Use the camera on your cell phone to visualize the relative brightness and coverage of the LED(s).

    You wouldn't even need a protective covering over these. Just pot around the emitter and detector (silicone caulk maybe). That'll make it waterproof. Then put a baffle around this gloop to reduce the field of vision.

    Anyway I think it's worth a try.

    -- Gordon
  • kwinnkwinn Posts: 8,687
    edited 2012-01-20 15:58
    You can use a PVC fitting with a threaded end and an end cap that threads onto it. Make the lens the same diameter as the threaded fitting, drill a hole in the center of the end cap that is the same or close to the inside diameter of the threaded fitting and use it to hold the lens in place. Also makes replacing the lens much easier.
  • BrianseBrianse Posts: 3
    edited 2012-01-21 08:55

    Thanks for all the comments.

    Concerning the use of IR verses color or beam breaking:

    I'm working with corn that's been cooled, so there is a temperature differential again a "warm" background.

    It's in a husker, so it's a "dirty" enviroment and the lense will get covered with green husks/ yellow silks. The colors I'm trying to sense. The PIR is nice because it's always compensating for changing background, so as the lense gets dirty it still works (up until it gets really thick). I've already tried it with the 4 mil polyethylene.

    The PIR is also nice because I can look down from the top with only one sensor (as opposed to needing two sensors across from each other).

    The PIR also can drive a long wire, though a couple of disconnects, so I can get the rest of the electronics away from washdown.

    Concerning chemistry, it's an organic system so bleach is taboo (at least at concentrations that are effective). I'm using peracetic acid.

    A milk jug isn't something I've tryed yet, thanks Duane.

    That chemical compatability app is something I didnt' know about, thanks ElectricAye. Though peracetic acid isn't on it, PE does seem compatible with hydrogen peroxide and acetic acid.

  • Tracy AllenTracy Allen Posts: 6,561
    edited 2012-01-21 10:20
    Take a look at the plastics from Fresnel Technologies. Also download their Fresnel lens guide.

    Consider using a basic PIR element behind a fresnel lens acting as a cover, perhaps with the smooth side out toward the work area. Eclectic E6000 adhesive (Goop) sticks to it passingly well, especially if the bond surface is roughed up a little and well cleaned. Or the lens could be clamped with a gasket.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-01-21 10:59
    Brianse wrote: »
    ......The colors I'm trying to sense.....

    I'm a little confused about what you mean by colors here. But, heck, if the system works even as temperatures fluctuate, then go for it.

    Brianse wrote: »

    Concerning chemistry... I'm using peracetic acid.....

    it appears that peracetic acid is often shipped in PE containers, so I'm guessing it's compatible:
  • Mark_TMark_T Posts: 1,981
    edited 2012-02-04 18:02
    Most organic compounds all have strong IR absorptions (in fact IR spectroscopy is very useful in identifying chemical structure because of this). The most transparent compounds are those with no functional groups and as non-polar as possible. This leaves simple non-aromatic hydrocarbons as about the only good choice for IR windows, polythene is normally used. Thinking about it the grade of polythene may matter - probably has to be free of plasticisers.

    Common salt is also a possible choice if there's no moisture at all! IR spectrometers often have polished NaCl cell windows because it doesn't absorb the organic solvents used in analysis.
  • GiuliaGiulia Posts: 1
    edited 2014-09-18 06:45
    Hi, i just came across this blog as I am working with cameratraps that are triggered by a PIR sensor, but since I use these cameratraps in emerged marine caves and in proximity of seawaves and seaspray I need to construct a protective casing that will protect my cameratraps in case wave action during heavy winter storms temporarily hits the cameratraps.
    the question is: what type of material do i need to build my protective case in so that my PIR sensor will continue to trigger with the same sensitivity?

    I found Brianse's post interesting but was wondering if anyone else or if Brianse had succesfully arrived at the correct solution. Unfortunately the companies which build the cameratraps are not interested in solving the problem so I need to find a fix-it solution on my own.
    any suggestions?
  • Tracy AllenTracy Allen Posts: 6,561
    edited 2014-09-18 09:37
    Hi Giulia
    Welcome to the forums. I'm curious what it is that you will detect in the sea caves? "Release the Kraken!"

    The marine environment is hostile to electronics. (You know that already!). The protective case has to be good, but do you think the window or lens will be the biggest issue?
  • Steph LindsaySteph Lindsay Posts: 766
    edited 2014-09-18 10:48
    Interesting applications and discussions here! I just wanted to point out the recently-released PIR Mini, which I don't think was mentioned yet on this thread. It might be easier to use than our regular PIR sensor in some applications where the electronics need to be sealed away, since the PCB is thinner than, and perpendicular to, the lens. It is designed to drop in a drilled hole in an outer case, and I imagine a sealant could be used between the case and the outer lip of the lens. I'm not sure it will do everything the OP needs though.

    195 x 300 - 45K
  • NWCCTVNWCCTV Posts: 3,629
    edited 2014-09-18 17:24
    Just curious. What would applying thin coats of lacquer over a PIR sensor do to it's ability to sense?
  • Steph LindsaySteph Lindsay Posts: 766
    edited 2014-09-19 10:10
    NWCCTV wrote: »
    Just curious. What would applying thin coats of lacquer over a PIR sensor do to it's ability to sense?

    I do not know! Ever since we spend an evening running around a Beverly's Craft Store with two SumoBot robots testing the IR absorption/albedo of different black fabrics, papers, and so forth, and sniffing for IR interference from the store's light fixtures, while the clerks watched us with deep consternation, I no longer make guesses about what physics is going to do with light and materials interactions! I'd be inclined to simply experiment with the lacquer you have in mind and test it, since I imagine that differences in brands and number and layer thickness might all make a difference.
  • BrianseBrianse Posts: 3
    edited 2014-09-20 02:49

    The milk jug actually ended up working the best. I remember glue was a problem holding it on though. I think a hot glue gun might have been what worked best, but it's been awhile since I did it.

    The polyethylene film worked fine too, if that's durable enough for your application you could just put it inside a bag made out of it.

    That new sensor I saw on the thread looks like it has some advantages, for me at least. Thanks parallax.

    Good luck with your application Giulia.
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