PIR Sensors and plastic cases

[gregebert]

To my surprise, the PIR sensor that I had inside a clear-plastic case was not detecting any motion. After drilling a large hole for it, no more problems.

Anyone else see this before ?

[theold...@gmail.com]

Yes, I had the same issue with a case made from acrylic I cut with my laser cutter.

 

Bill v.

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[Adrian Godwin]

You can only cut the acrylic with a laser cutter because the material is somewhat opaque to the laser light. It absorbs it and is melted. If the material were as clear to IR as it is to visible light, it would be very hard to cut because the light would pass right through without giving up any energy to the plastic.

Yes, opacity to infra-red by materials such as acrylic, some of which are highly transparent to visible light is well known and very material dependent. Similar materials that are transparent do exist though - consider the thin material covering a PIR, or the often apparently black material covering the IR receiver of a remote control. Some properties are astonishing. For instance, the silvery metal Germanium which has the appearance of a mirror to visible light is used as a lens material for thermal cameras.

Infra-red covers a very wide band though, of a few decades, and materials have varied characteristics. In many cases it is possibe to find something with the properties you need. Refer to articles such as https://www.edmundoptics.com/knowledge-center/application-notes/optics/the-correct-material-for-infrared-applications/ for further details.

  

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[Adrian Godwin]

Here's a description more appropriate to plastics. It's interesting to see the transmission-by-wavelength graph for plexiglass there, which has some very fast transitions in it as well as very variable response to the wavelength of interest. It's not a simple low-pass or even bandpass effect (though most of the transparency is in the region of 700 to 2200nm..

https://physics.stackexchange.com/questions/407900/what-plastic-like-materials-allow-ir-to-pass-through-them

PIRs and thermal cameras operate at about 10um

Common desktop laser cutters operate at about 1000nm (1um)  (other wavelengths are used for metal cutting)

Remote controls operate at about 700-900nm, bordering on visible light

[newxito]

I had the same problem, I wanted to hide the PIR in the calculator behind a brown acrylic but that didn't work at all. I also tried red and orange without success.

[Terry S]

Yep had the same issue. Eventually mounted the PIR on the side of the case.

[Mac Doktor]

On Aug 27, 2024, at 2:37 AM, Adrian Godwin <artg...@gmail.com> wrote:

You can only cut the acrylic with a laser cutter because the material is somewhat opaque to the laser light. It absorbs it and is melted. If the material were as clear to IR as it is to visible light, it would be very hard to cut because the light would pass right through without giving up any energy to the plastic.

I have one quibble with your highly informative posts. Acrylic doesn't melt, it burns just like wood. There are other materials that you might call "plastic" that do melt and are tricky to impossible to cut using any wavelength. Many of them also give off toxic chemicals and can catch on fire. 

Here are some lists of what you can and cannot burn:

https://makermade.com/blogs/resource-articles/laser-cutting-engraving-materials-list

https://www.troteclaser.com/en-us/learn-support/faqs/unsuitable-materials-laser-processing

Avoid PVC like the plague. The chlorine with damage not only your lungs but also machine parts and the copper traces on the PC board.

Titanium Dioxide melts. It's extremely white but absorbs the light anyway—somehow. On ceramic bathroom tile it turns black and fuzes into the glaze permanently.

If you have a laser engraver and run the exhaust outside be sure to put the business end up high. You don't want the neighbors to call the FD every time you burn something. 

 

Terry Bowman, KA4HJH
"The Mac Doctor"

https://www.astarcloseup.com

Edward R. Murrow: “Who owns the patent on this vaccine?”
Dr. Jonas Salk: “Well, the people, I would say. There is no patent. Could you patent the sun?”—See It Now, 12 April 1955

[Nicholas Stock]

Microwave sensors work fine through acrylic....

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[Mac Doktor]

On Aug 27, 2024, at 12:13 PM, Nicholas Stock <nick...@gmail.com> wrote:

Microwave sensors work fine through acrylic....

Aluminized acrylic? I suppose you could fire a maser at it to find out.

Terry Bowman, KA4HJH
"The Mac Doctor"

https://www.astarcloseup.com

“Reality is that which, when you stop believing in it, doesn't go away.”–Philip K. Dick, "I Hope I Shall Arrive Soon"

[Adrian Godwin]

Yeah, melting is not accurate. No residue dribbles from the cut. However, I believe it actually vaporises. There is no charring on the cut.  Polycarbonate does appear to burn, it makes a horrible mess. Wood, as you'#d expect, certainly burns.  Totally agree about PVC. 

Maybe Titanium Dioxide is another wavelength thing ? It looks white in the visible spectrum but what does it look like in IR ?

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[Mac Doktor]

On Aug 27, 2024, at 2:49 PM, Adrian Godwin <artg...@gmail.com> wrote:

Yeah, melting is not accurate. No residue dribbles from the cut. However, I believe it actually vaporises.

Yes. Nothing left except gas and smoke. Some "problematic" materials can be cut if they're really thin. An eight of an inch is too much.

There is no charring on the cut. 

Other materials like wood and paper will char. This is why air assist is essential. It also keeps the soot away from the lens protector. Just a couple of years ago that was typically a modification you had to do yourself, usually reinventing the wheel in the process. Now every manufacturer has a big aquarium pump hooked to a nozzle on the business end of the laser module. Note that light charring can sometime be brushed off or removed with a "magic" eraser.

Maybe Titanium Dioxide is another wavelength thing ? It looks white in the visible spectrum but what does it look like in IR ?

It works perfectly with a 450nm diode laser at medium power settings. I have no clue as to how it's even possible.

Terry Bowman, KA4HJH
"The Mac Doctor"

https://www.astarcloseup.com

"If only you could see what I've seen with your eyes."—Roy Batty, Blade Runner