PIR sensor presence instead of motion

Thread Starter

TheMasterXXXXX

Joined Aug 23, 2022
29
Is it possible to cause PIR sensors (of this sort) to detect presence instead of motion? I would like for the sensor to detect the continuous presence of a body, instead of only triggering when the body moves. I thought occluding one of the internal detectors might work, but so far I have not been successful doing so.

If this is too problematic, are there better alternative sensors for this task? I intend to have several of these installed in the same room, so sensors that might interfere with each other (such as the ultrasonic proximity sensors) are not an option.

Any help is appreciated
 

Ian0

Joined Aug 7, 2020
6,312
The key to understanding how they work is the lens. The lens turns the field of view into zones of high sensitivity and areas of low sensitivity. When a warm object moves across the field of view, it pass through zones of high and low sensitivity and produces as AC signal as it moves.
With no lens, the output is a DC level determined by the average temperature across the field of view.
So, with a warm object within the field of view the output would be a higher DC voltage than if the warm object were absent.
But, and this is the crucial bit, in order to know if there is a warm object present, you would need to know what the output would be if that object were absent.
 

Thread Starter

TheMasterXXXXX

Joined Aug 23, 2022
29
The key to understanding how they work is the lens. The lens turns the field of view into zones of high sensitivity and areas of low sensitivity. When a warm object moves across the field of view, it pass through zones of high and low sensitivity and produces as AC signal as it moves.
With no lens, the output is a DC level determined by the average temperature across the field of view.
So, with a warm object within the field of view the output would be a higher DC voltage than if the warm object were absent.
But, and this is the crucial bit, in order to know if there is a warm object present, you would need to know what the output would be if that object were absent.
Not entirely sure how what you are describing aligns with the datasheet I linked to. The device I mentioned seems to be a mosfet that conducts when its two IR sensors detect a different signal (during motion). Perhaps what you are describing is a thermopile?
 

Ian0

Joined Aug 7, 2020
6,312
Not entirely sure how what you are describing aligns with the datasheet I linked to. The device I mentioned seems to be a mosfet that conducts when its two IR sensors detect a different signal (during motion). Perhaps what you are describing is a thermopile?
I'm describing a pyroelectric sensor. They do generally have two devices, one with an opaque filter for compensation; but the way it detects movement is exactly as I have described.
 

ericgibbs

Joined Jan 29, 2010
16,392
Hi,
It's a pity you didn't explain that in your earlier posts.;)

One method I have used is a pair of PIR sensors mounted in a stereoscopic manner, about 1mtr apart, both sensors need to be activated to enable an output.
A person standing in front of a picture, moving slightly, would trigger both the sensors.

E
 

Ian0

Joined Aug 7, 2020
6,312
Y
I don't need anything that sophisticated. I just want to detect when someone is standing in front of a picture frame so I can turn music on lol.
You might just be able to do that with a PIR sensor.
The background level will vary as the temperature of the room varies, so won't change very quickly.
A person in front of the picture will cause the level to change over a matter of a second or so then reduce when the person moves away, so comparing the output level with an average taken over a minute or so might just work.
You'd need to try it, but I'd say it was worth a try.
 

DickCappels

Joined Aug 21, 2008
9,319
Pyroelectric image sensors sense static images by placing an optical chopper wheel between the sensor and the lens. The chopper wheel is at one temperature and when a hole through the chopper wheel allows the detector to be exposed to the scene the scene can be sensed, even if nothing but the wheel moved.

This is very similar to a field mill used to measure static voltage charges.

A chopper wheel can enable the presence of stationary objects by a pyroelectric detector, though the signal may require processing beyond mere signal detection.
 

Thread Starter

TheMasterXXXXX

Joined Aug 23, 2022
29
Hi,
It's a pity you didn't explain that in your earlier posts.;)

One method I have used is a pair of PIR sensors mounted in a stereoscopic manner, about 1mtr apart, both sensors need to be activated to enable an output.
A person standing in front of a picture, moving slightly, would trigger both the sensors.

E
That doesn't work, because when you stand still they stop triggering. Only when you move rather overtly do they trigger.
 

Thread Starter

TheMasterXXXXX

Joined Aug 23, 2022
29
Y

You might just be able to do that with a PIR sensor.
The background level will vary as the temperature of the room varies, so won't change very quickly.
A person in front of the picture will cause the level to change over a matter of a second or so then reduce when the person moves away, so comparing the output level with an average taken over a minute or so might just work.
You'd need to try it, but I'd say it was worth a try.
That may work with a single channel PIR sensor, but the dual channel ones (such as the one I linked to in my post) only sense movement. It doesn't appear to be so easy to find one that works like that.
 

Ian0

Joined Aug 7, 2020
6,312
That may work with a single channel PIR sensor, but the dual channel ones (such as the one I linked to in my post) only sense movement. It doesn't appear to be so easy to find one that works like that.
No they don't. The output is proportional to IR levels. I explained in post #2 HOW they sense movement.
 

crutschow

Joined Mar 14, 2008
30,802
Put a delay on the signal so that the music stays on for some fixed time after it detects motion.

A sensor to detect a fixed presence is likely more complicated than you want since, to do that, you likely would need to recognize a shape.
 

ericgibbs

Joined Jan 29, 2010
16,392
Put a delay on the signal so that the music stays on for some fixed time after it detects motion.
hi MT,
The PIR module I often use, inexpensive, has Sensitivity and On Time Period trim pots.
Works from a 5V through 20V supply, the High output can directly drive a general type transistor/relay circuit.

E
Added basic amplifier/relay circuit, for this type of PIR module.
EG 1417.png
 

Attachments

Last edited:

LesJones

Joined Jan 8, 2017
3,813
I think using a sensor to detect a reflected modulated IR beam would be a better solution now that you have described what you are trying to achieve. (Depending on the layout it may be even possible just to use a beam break detector.)

Les.
 
Top