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hc-sr501 sensitivity

guy_c

Oct 31, 2023
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Hello everyone,
Ive noticed lastly that there are much too many excitations of the pirs on my west wall since i replaced them. What i found out is that with a lense of a north wall pir the sensitivity of the new west wall is much reduced and that an old north wall pir become very sensitive with a new lens. Both old and new lens being thouroghly cleaned. My conclusion is that electronically, both old & new sensor are alike but optically they differ just because of the lens. Question - how do i make the lens "less sensitive"?
 

Delta Prime

Jul 29, 2020
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Welcome to Maker Pro.
Here's a little snippet.
You can attenuate by using thin sheets of polyethylene. Polyethylene is the same material that the PIR lens is made of. It will pass, but attenuate the wavelengths you are interested in. Find something around .015" and begin stacking until you hit your desired range.
 

guy_c

Oct 31, 2023
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Thanks. From my experience, Increasing the sensitivity resistance (cw), increses sensitivity
 

guy_c

Oct 31, 2023
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Welcome to Maker Pro.
Here's a little snippet.
You can attenuate by using thin sheets of polyethylene. Polyethylene is the same material that the PIR lens is made of. It will pass, but attenuate the wavelengths you are interested in. Find something around .015" and begin stacking until you hit your desired range.
Thanks! I should try this

Can you tell where can i find the PE sheets you recommend? Do they sell on AliExpress/eBay?
 

Harald Kapp

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optically they differ just because of the lens.
The material of the "lens" is probably the same, but other parameters may differ, e.g. thickness. That is what the sensitivity adjustment potentiometers are for.

From my experience, Increasing the sensitivity resistance (cw), increses sensitivity
The directions for increasing or decreasing sensitivity are afaik not standardized. Have you tried ccw?
 

guy_c

Oct 31, 2023
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The material of the "lens" is probably the same, but other parameters may differ, e.g. thickness. That is what the sensitivity adjustment potentiometers are for.


The directions for increasing or decreasing sensitivity are afaik not standardized. Have you tried ccw?
@Bluejets - Sensitivity or detection distance varies as the sens. pot resistance.

In my case, with the new lens, zeroing the sensitivity pot still senses my body at 10m and cats at 3 meters
 

Harald Kapp

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zeroing the sensitivity pot still senses my body at 10m and cats at 3 meters
This doesn't sound like a problem but like an inherent feature of how a PIR sensor works. Unless you are cloaked in heavily insulating clothes, your body should emit more heat than a cat and should therefore be detected more easily aka at a geater distance than a cat. Completely normal behavior on the sensor's side.
 

Harald Kapp

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Ok but i want it to detect me at 3 meters but not cats
This is imho not an issue of sensitivity per se.

The only idea I have is to adjust the alignment of the sensor such that it detects objects only above 1 m height - assuming the cats are on the ground only.
Maybe add some ir-intransparent shielding below the sensor. to block the heat signature from cats.
 

Delta Prime

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Thanks! I should try this

Can you tell where can i find the PE sheets you recommend? Do they sell on AliExpress/eBay?
In the kitchen you'll find plastic wrap probably along with the aluminum foil
Common household plastic wrap is made of polyethylene. Plastic sandwich bags you know those ziplock things.... polyethylene. The big cooking bags for chicken and turkey... polyethylene. That's what I use and that's what I would recommend. wrap it around your lens, little Elmer's glue, and you're good to go... have fun now. But don't go wrapping your cat in it. That would be a "Cat"astrophe!
I have made common passive infrared sensors less sensitive by placing a tiny piece of plastic sandwich bag over the window works like a charm.
1698846124607.png
 
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guy_c

Oct 31, 2023
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@Delta Prime and how did you fix the piece of plastic to the window? There is space between the lense and the window so unless you glu the foil to the metal it might float. Now assume you do glu it, unless glued to the metallic perimeter it may be at random distance and have non deterministic non repeatable behaviour. I have 13 around the house and everything was (almost) just fine with older lenses: no cat detected and yes person detected. I say almost because i did have oscillaion especially at high humidity. I intended to resolve that with confirmal coating
 

Bluejets

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Ok but i want it to detect me at 3 meters but not cats
............so give the cats an ice bath......
Really, you are asking the impossible and it's typical of those who have little understanding of the processes.

Get a home camera system perhaps...........:)
 

Delta Prime

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There is space between the lense and the window so unless you glu the foil to the metal it might float
No no not foil!
In the kitchen you'll find plastic wrap probably along with the aluminum foil
Common household plastic wrap is made of polyethylene
I may have misspoke . I usually store my plastic wrap adjacent to (right beside)aluminum foil in the kitchen. :)
My technique was required when I was a child playing with electronics and I was lucky it worked. In my particular situation I I use both a cooking plastic bag that you place Chicken in, to cook in an oven and that thin plastic Saran rap.I cut small round pieces the same diameter of the metal sensor.
To to make the plastic stick to the sensor I got a toothpick and a glue stick that you find in a child's project box. I would place the tiniest amount of glue that would fit on the very tip of the toothpick and fix one layer plastic and readjust the potentiometer it was still too sensitive added additional layer the same way, still too sensitive,so a third, not sensitive enough so I took the third one off and put a thin piece of plastic Saran wrap and viola it worked for my situation. I took everything off and applied a little bit of super glue onto each layer at the edges (don't apply to much super glue or capillary action will occur) and it worked all year round in all weather conditions. But what I'm getting at is... it is a trial and error process do it in layers. It was only later that I found out polyethylene was used and molded into a fresnel lens. For inexpensive PIR sensors.
I have 13 around the house and everything was (almost) just fine with older lenses: no cat detected and yes person detected
Do you have the old lenses is there a way you can fix them to the new sensors? Just a thought.
I say almost because i did have oscillaion especially at high humidity. I intended to resolve that with confirmal coating
If you attend to resolve it with conformal coating then I would suggest more research is necessary.
 
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guy_c

Oct 31, 2023
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............so give the cats an ice bath......
Really, you are asking the impossible and it's typical of those who have little understanding of the processes.

Get a home camera system perhaps...........:)
Wondering how this contributes to the discussion. Personally, when i cannot help i simply dont. Fact is that i have a good understanding of this and other physical processes as well as of social manners
 

guy_c

Oct 31, 2023
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@Delta Prime Yes, this is what i did yesterday, replaced the 3 west walls sensor's lenses with old ones. There is 1 or 2 tenths of mm difference in the position of the 4 plasic pins so i cut 2 and once fixed with screws was nicely fitted. Result this norning from ino server:
night pirs =2, meaning 2 excitations
Max density =1 meaning never more than one excitations in my sliding time window. Threshold of alarm is 5.

I tried to follow your explanation on how you adjusted the sensitivity with layers of (potatoes , im vegan) cooking bags. I did not fully understand but this should hardly work in my application where the sensors are submitted to harsh suns uv during the summer.

As for oscillaions, im am not the only one to suffer from. Read they've been prouced even on the bench by a guy. Only correlation i found was humidity.... And of course, i always replace the pots with resistors; pw with a short, sensitivity with 470k. Leaving the pots=> oscillation guaranteed
 

guy_c

Oct 31, 2023
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@Delta Prime ... forgot - with new lenses i had
Night pirs in the order of 20
Max density often ≥ 5
Totally unusable!
 

Delta Prime

Jul 29, 2020
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If you're open to suggestions?
You are going to have to upgrade. That means money! But it will eliminate the frustration and give you the security you seek. It'll keep you and those precious little kitty cats safe.
For detecting only human presence you can look for D6T sensor (omron) which is used to detect only the presence of humans based on the heat emitted at a particular wavelength. It also eliminates false triggering and unwanted thermal emissions.
 

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Delta Prime

Jul 29, 2020
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Wondering how this contributes to the discussion. Personally, when i cannot help i simply dont
I can only imagine who you are talking about but it's a 50/50 shot win win either way cuz my access is limited. People with close minds should always keep their mouths closed as well. It's why the human gene pool needs a lifeguard.
 
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