Hi Folks, I've been doing some experiments with HC-SR501 PIR sensors to detect the door opening inside a pickup truck canopy.
They work very well, detecting the door opening 100% in hot and cold weather. What I have found is they are not picking up the body heat from the person opening the door. Standing to the side and using a broom handle to open the door gives 100% response. Can anyone enlighten me to what the sensors are detecting ?

Cheers
Malcolm Moorhouse
Parys, South Africa

 

They actually have an IR camera . They trigger when enough pixels change in a short time.

Bob

 

I think "camera" might be an exaggeration. The common type PIR has two sensing elements and the differential between the two is the trigger. The lens is the critical part, it focuses the field of coverage in such a way that if a warm body (anything sufficiently different from the ambient moves across the field it will first encounter one, then the other sensor element. That change is what is detected.

It just has to be something hot, so if you are moving a cover and there is enough heat in general outside the cover, it will "light up" not element, then the other and trigger the sensor.

I am sure there are a lot of explanations online with diagrams. You might also try application notes for sensors which might have a theory of operation included.

 

Hi Malcolm,
Read this info link.
Radiant body IR
E
https://www.theengineeringprojects.com/2019/01/introduction-to-hc-sr501.html

 

Thanks for the response, I'm going to do more reading on the pyroelectric sensors themselves.
Everything I've read revolves around body heat which isn't the case in my application.
Even translating "body heat" to "IR source" doesn't seem to help.
I've done lots of tests and cannot seem to get a false trigger which is impressive considering their price.
In 30deg C heat they pickup the canopy door opening, even when the canopy is cooled in a sudden rain storm there is no false alarm. They detect the door opening in winter when vehicle has stood outside overnight and the temperature has dropped to 2deg C .
Let me dig a little deeper into the actual sensors and see what I find.

Cheers
malcolm

 

hi Malcolm,
A bit more info on the sensor element.

E

 

hi Malcolm,
A bit more info on the sensor element.

E

Thanks Eric,

Even your attached tech note talks about the sensor window being "tuned" to pick up radiation from a human body to reject reject environmental temperature changes. I've assumed opening the canopy door causes a swirl of air with a different temperature to the canopy interior which triggers the sensor. Yet flooding the canopy top with cool water in summer to simulate a downpour doesn't seem to cause a similar effect and give a false trigger. This is what puzzled me, the sensors seem to respond to IR radiation outside of the human 35deg c range. With these sensors being so cheap compared an alarm system PIR, I'm wondering if their specs to reject environmental IR emissions are so low that it works in my favour.

I make car alarms using radar sensors to detect an intrusion. One user asked what can be done to detect an intrusion into a pickup canopy. The radar sensor is no use with most canopies being fiberglass. The HC-SR501's were suggested, I said they won't pick up a moving door and have been pleasantly proved wrong.

There's a couple of sites I'm reading up about the PIR sensors and will post some feed back a little later.

Cheers
malcolm

 

If your movements is slowly enough then you can avoid the sensor to detect your body, I tested that before and I really cheated the sensor...

 

If your movements is slowly enough then you can avoid the sensor to detect your body, I tested that before and I really cheated the sensor...

I've heard of guys leopard crawling down a driveway and sneaking past out door PIR's.
With application of the, slow is not going to work, you've got to be in like Flynn, grab and RUN.

 

hi Malcolm,
On very wet days, my PIR system does not detect the Postman, who is usually soaking wet.

Also, the colder the weather the more sensitive the PIR alarms

E

 

I've heard of guys leopard crawling down a driveway and sneaking past out door PIR's.
With application of the, slow is not going to work, you've got to be in like Flynn, grab and RUN.

I have used the PIR Sensor with LEDs to detect the door, when I walk in at night then it will light up the LEDs automatically, but when the daytime that the sunlight is too bright, so the PIR Sensor will lose its function.