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Author Topic: PIR sensor operation questions  (Read 4038 times)

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Offline celbanTopic starter

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PIR sensor operation questions
« on: May 25, 2010, 01:19:26 PM »
Hi all,

I've been reading plenty of threads in these forums regarding PIR sensors, specifically, their application to flame detection. I'm working on a candle flame detection robot and I just received my Parallax PIR sensor. I'm testing out the PIR sensor using http://www.ladyada.net/learn/sensors/pir.html as a guide.

My problem is that the PIR seems to be too sensitive! I want to mount the PIR on a servo that scans a particular area but even the slightest change in position sets off the sensor, regardless of any heat source in its field of vision. I'm wondering if this is the proper operation of the PIR and what to do if it isn't.

Thanks!

Offline Razor Concepts

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Re: PIR sensor operation questions
« Reply #1 on: May 25, 2010, 03:28:15 PM »
PIR sensors only sense change in heat. They have two pyroelectric sensors sideby side and hte circuitry only triggers when it senses difference. It would be difficult to do flame sensing if the pir is moving since it always will sense change despite no flame.

Offline waltr

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Re: PIR sensor operation questions
« Reply #2 on: May 25, 2010, 03:30:56 PM »
That sensor has a wide angle Fresnel lens. Maybe reducing the angle it 'sees' would help. Block off most of the lens with aluminum foil.

It is possible to reduce the sensor's sensitivity by putting something over the sensor to attenuate the source.

Keep experimenting.

paulstreats

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Re: PIR sensor operation questions
« Reply #3 on: May 25, 2010, 04:27:33 PM »
Quote
I want to mount the PIR on a servo that scans a particular area but even the slightest change in position sets off the sensor

Of course it does. Its designed to. When the servo moves,the sensor is responding a change in surroundings.

Basically what you have to do is> move the servo -> ignore the readings until the sensor settles down(this should be in the datasheet) -> once the sensor is settled, take a reading or wait to see if the sensor is triggered -> move the servo again.

 So you have to stop the servo, reset the sensor - THEN take the reading. then move the servo to its next position.

Offline Soeren

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Re: PIR sensor operation questions
« Reply #4 on: May 25, 2010, 05:43:35 PM »
Hi,

Pyroelectric differential sensors, as used in PIR detectors, can be used by covering one of the detectors (half the window) with a metal foil and then use the remaining as a magnitude sensor, but it takes a different circuit after the sensor and will need a bit of experimenting to get it right with whatever sensor you have - you may even have to adjust it for ambient temperatures right before using it.

Alternatively, get the right kind of sensor - some pyroelectric sensors are made for the exact purpose and they're quite efficient.
When I was around 6 years old, my father took me on a visit to a friends job. This guy was the Major on a large fire station (or what it's called) and demonstrated all of their new "toys" for my father and me, among them a pyroelectric based pocket size glow detector (for finding wood glowing inside without visible signs, used when the flames were gone and everything has to be positively quenched). This "toy" detected my fathers cigarette (lit, but not smoked, i.e. a minimal glow) 4 to 5 meter away -
that was around 1964, so it should not be hard detecting a much hotter candle flame today, with 46 years of improvements to electronics.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline celbanTopic starter

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Re: PIR sensor operation questions
« Reply #5 on: May 25, 2010, 10:46:00 PM »
Thanks for the advice, I'll post my results here.

 


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