PIR through frosted plastic?

Other Hardware Sensors
1

Hi there

Thanks in advance for any help you can give on this topic.

I’m looking to place a Passive InfraRed sensor inside a frosted plastic cube as part of a community art project for kids and have run into a problem- the PIR doesn’t pick up any reading through Polyvinyl Chloride Polymer, the material that was being used for molding the frosted cubes.

I am expecting that this is because the frosted plastic is also diffusing the IR light or blocking it entirely. If I trigger the PIR from within clear plastic containers or without any covering, it works and I get a reading.

The challenging factor is that the cube itself needs to be frosted in order to diffuse the light from 4 RGB SMD5050 LEDs. The final result should look like this: Pixel Mosaic

Ordinarily the lens would simply be placed on the outside but this is an artwork and it would be so much more impressive if it isn’t obvious how it actually works.

My question:
Does anyone know how I can get a PIR sensor to respond inside a 3mm thick frosted plastic cube?
Is there a type of frosted plastic that will let IR pass through?
Is there a change to the circuit that could be done (currently just directly going to an ATTiny85 and being amplified in software)?
I am considering an amplification circuit concept but I really don’t know if I’m heading down a futile path with the type of plastic I have. Is it even possible?

The cubes: Photo of plastic cubes

PIR sensors trialed:

  • D203S
  • D203B
  • D204S

PIR Lenses:

Thank you for your help.

Kind Regards
Michael.[/list]

2

Hi,

they don't even work through glass sometimes!
I had a similar project and in the end i used an X band sensor

its like a motion detector/ radar that even works through walls!

Cheers,

Oli

3

Thanks for your reply Oli. I hadn't seen an x-band motion detector before and searching on it brought up a similar problem someone else was trying to solve for their art project:

X-Band Motion Detector + Arduino

Unfortunately, as reading the above thread and a quick search suggests:

  • They are very expensive
  • They detect movement toward not across
  • They are bulky

I did find a cheaper item that appears to be the same thing for 1/6th the price however they mention it is very sensitive to damage:

Please use a low voltage soldering iron and connect it to a reliable grounding. This device is sensitive to ESD and it may be damaged during soldering. The IF output of the HB100 module is especially sensitive...

I'm concerned that when combining the cost, fragility, space constraints and sheer quantity I need, they may not a viable option. I'll try to source some cheaper units and do some tests.

With the x-band motion detector, do you know how quickly it responds? or more aptly, how many samples are needed to remove false positives? I'm looking for an indiscernible delay in the interactivity (300ms max).

Do you have any other suggestions?

Is IR really not going to be possible without putting holes in the cubes or putting them on the front face

Thanks

4

I'll add to that about x-band, whilst it may seem paranoia to suggest this, I don't think there has been any study as to the health implications of placing 375 x-band sensors in close proximity to each other and young kids. I would have to consult a radiologist prepared to confirm the safety of this.

At the frequencies of operation of these radars, the penetration of the energy into tissue would be very limited, perhaps ... no more than a centimeter for X-band.38-40 Thus, the greatest exposure that could occur is for an individual to place the aperture of the radar antenna very near the body. Such an exposure would result in localized deposition of some energy in tissues very near the surface of the body in the region intersecting the radar beam... Nevertheless, the determination that present-day exposures are low does not entirely eliminate the questions of historical exposure nor does it directly address the effects of long-term, low-level microwave exposure because so little research has been done on chronic low-level effects

excerpt from this article

5

Update: According to Daniel Gonzalez from ePastics, Plexiglas 2447 White allows IR light to pass through.

Unfortunately it's quite transparent. I'm ordering some samples to test with and getting some costs and whether it's still available for custom molds that would typically have Polyvinyl Chloride Polymer poured in.

Still keen to hear ideas from other people as to other ways this problem could be tackled. The key is 1 sensor per cube and 375 cubes in a 3m x 1.8m display.

Thanks!

6

According to Daniel Gonzalez from ePastics, Plexiglas 2447 White allows IR light to pass through.

Yes but what wavelength of IR?

The sort that a PIR sensor uses ( wavelength 8-14 micrometres ) and the sort an IR remote control uses (wavelength 0.94 micrometres ) are miles apart. So you need a bit more detail on that statement.

7

There is another problem: A PIR lens consists of a bunch of fresnel lenses. So sometimes your body heat is focused on the detector, sometimes it is not.

So as you move across the field, the heat it picks up goes in and out of view. This is how it detects a warm body.

So you cannot use a cover that is translucent to IR. It must be clear for IR light, although it can be opaque to visible light, as the PIR covers are.

More info:
Sparkfun How PIR Sensors Work

Have you considered capacitive sensing?

8

Thanks for your replies.

@Grumpy_Mike: Yes but what wavelength of IR?

I have contacted ePlastics for specs on that plastic. I found this article but there's a lot of general information in there so I was unable to work out what wavelength is specifically allowed through from reading that document. There's quite a bit of content I didn't fully understand (but would like to).

@polymorph: So you cannot use a cover that is translucent to IR

My understanding of the Plexiglas product is that is allows IR light to pass through undisturbed by the diffusion being applied to the visible light.

In some very basic early tests I was able to use etched frosted glass. Under a high power microscope I could see that the etching process had lots of tiny stippled dots where it was completely opaque and then completely transparent- this could mean that it was allowing the IR to pass through despite looking diffused. They may be doing that? I'll make sure I update this thread when I find out more.

@polymorph: So as you move across the field, the heat it picks up goes in and out of view

Would you see a problem in using a narrow fresnel lens with a hole in the front of the plastic that is smaller than the sensor?

I actually want a really narrow field of view- just straight ahead. This has been a challenge with PIR as most fresnel lenses are designed for wider viewing angles.


*the dotted line is where the sensor with the lens would be, behind the plastic. The small white circle in the top middle is a hole for it to see through.

So, does the sensor really need to be on the outside of the box if I only want what is directly in front of the sensor to be noticed?

@polymorph: Have you considered capacitive sensing?

Yes, I started with capacitive sensing. I need a range of 2-3m out in front so capacitive just isn't going to do that from my experience.

I was able to get about 30cm reliably with a variety of home made coils, sheets and surfaces and 10M of resistors but it had a very unstable baseline reading. Sometimes it was just climbing with no obvious stimulus and other times I would trigger it and it would take at least 20 seconds before it was back into normal ranges again. Furthermore capacitive isn't directional, well not that I know of.

9

You can probably use the sensor without the fresnel lens cover, with a narrow view like that. The hole takes the place of the multi-zone fresnel lens.

If the plastic is clear at IR, great guns.

Capacitive sensing can be made -somewhat- sensitive, but not like what you seem to be shooting for.

There is also ultrasonic. An ultrasonic rangefinder is fairly directional. But... it requires that it be outside the box or a hole through it. So I guess that is out.

10

I said:-
The sort that a PIR sensor uses ( wavelength 8-14 micrometres )
1 micro meter is 1000 nano meters. So we are looking at a wavelength of 8000 to 14000 nm for PIR radiation.

Figure 6 in that sheet you linked to showed zero transmission at those wavelengths although it doesn't go down as far as that you can see it is all zero by 2800 nm.

It says:-

Colorless Plexiglas sheet is entirely opaque to infrared wavelengths from 2800 nanometers up to 25,000 nanometers in thicknesses of 0.118" or greater.

My understanding of the Plexiglas product is that is allows IR light to pass through undisturbed by the diffusion being applied to the visible light.

For the IR that a PIR detector uses this is incorrect.
So if you are getting Acrylic sheet it will not do what you want.

11

Figure 6 in that sheet you linked to showed zero transmission at those wavelengths

Thanks @Grumpy_Mike for seeing that in the document and saving time on another fruitless testing and sampling process.

@polymorph: You can probably use the sensor without the fresnel lens cover

Oh, no, because the range is extremely short about 6" max without a lens on it. It's also quite erratic in the baseline reading.

I also just received this reply from a PhD candidate at the local university who I reached out to last week that may help anyone else looking at this thread:

Straight out PVC with absorb most IR light, so your best and cheapest option is polyethylene. Also, acrylic might work, or might not. It really depends, as acrylics describe a whole range of plastics, and often have various impurities such as chlorides etc. so stick with polyethylene.

Below is a graph of the transmittance of PE over the wavelengths your diode with read (5-14um). There are other materials that have much better transmission, but they are much more expensive and some are very toxic. There is high density PE and low density PE. Get low density PE because this will mean there is less electrons in the way per size of the material. This will help reduce the absorption of IR light in the material. Polycarbonate should do the job also, however, I think LD-PE is cheaper and is already white-ish, while polycarbonate is clear to start with. Any colour additives with change the IR absorption profile.

The next thing you need to worry about is the transmission. The transmission through the diode is 75%, I assume that this means that only 75% of the light that hits the surface of the diode is actually read. So your accumulative transmission is AxB, so at 4um, it is 0.8*0.75=0.6, so only 60% of the light will actually be read. However this sample transmission is of a PE film, so for the increase in thickness there will be an increase of absorption. The best I can tell from an almost unreadable article I found on it, is that transmission at 1.8um drops to about 70% at 1mm thickness, thought the sample was Ultra high density PE. Assuming a linear decrease over thickness (which is fairly safe), at 2mm the transmission would be around 50%, so your transmission would reduce to around 35%.

In summary, my recommendation is to use 1-2mm thick low density polyethylene.

I'm going to pursue this material and advise of the results although given the combination of the content on this thread, I'm not expecting a favourable outcome and may have to resort to external sensors or a hole as per my diagram. Not terrible but certainly not as aesthetically appealing.