PIR Motion sensor delay question

by Mike724 on Mon Oct 15, 2012 9:23 pm

I'm looking at this product here:
http://www.adafruit.com/products/189

But I was wondering what the delay means exactly... The websites says the delay is "0.3-18 seconds" which I'm guessing .3 is the lowest and 18 is the highest and you turn the time "dial" that is on the board to choose.

So my question is, does it take 0.3 seconds for it to notice movement or 0.3 seconds for it to take an "image" to compare to check for differences (and then movement is detected instantaneously?)?

With what I'm doing it is crucial that the arduino knows about the movement almost instantaneously. Time is a big factor.

Thank you very much. :)
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Re: PIR Motion sensor delay question

by adafruit_support_bill on Tue Oct 16, 2012 5:10 am

It will trigger very quickly when it first detects movement. The delay is the time it takes to reset after triggering. It will not trigger again during the reset period.
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Re: PIR Motion sensor delay question

by ywQ on Tue Oct 16, 2012 3:15 pm

Hi,
The information is helpful. But would like to ask further questions re this PIR sensor.

Could you explain the 'sensitivity' of this sensor? How does it work? What is the difference between when it is set to Max. and when it is set to Min.

When turning the screw/dial to increase the sensitivity of the sensor, somehow it seems that it doesn't make it easier to trigger initially, but makes it so that once triggered, it never turns off.

Also, when you say the PIR sensor triggers very quickly, how much time is it?

Thank you!!
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Re: PIR Motion sensor delay question

by adafruit_support_bill on Tue Oct 16, 2012 4:12 pm

There are several variations on this sensor. It sounds like the 'sensitivity' pot on yours is really just delay in reverse. If you turn it to the "maximum sensitivity" it is really "minimum delay" so there is no delay before re-triggering.

The spec sheets do not have a timing spec for detection time. From my own observations, I would estimate < 100mSec.
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Re: PIR Motion sensor delay question

by ywQ on Tue Oct 16, 2012 7:47 pm

There are two dials on this PIR, as the picture on the Adafruit shows. One changes Time and other Sensitivity. So, how do these affect the 'behavior' of the sensor then?

Time is the delay time and Sensitivity is...
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Re: PIR Motion sensor delay question

by adafruit_support_bill on Wed Oct 17, 2012 4:49 am

On that one, the sensitivity controls the gain on the pulse detection. See the tutorial for an explanation of the optics involved: http://www.ladyada.net/learn/sensors/pir.html
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Re: PIR Motion sensor delay question

by ywQ on Wed Oct 17, 2012 9:18 am

Does it mean that if the sensitivity is at max, the PIR is in the 'non-trigerring' mode?

In which case, having maximum sensitivity makes the sensor to stay on (HIGH) while multiple movements are detected, isn't it essentially delaying the response time of the sensor?

I am not so sure how these two dials -- sensitivity and time, work together or affect each other.

So, if I want to make the sensor very sensitive with the shortest delay, I would naturally think that the sensitivity dial set to max. and time dial to minimum. But given the explanation, that causes more delay. Should the sensitivity dial set to minimum and time dial to minimum as well, then?

Sorry, if I sound so obtuse. I appreciate your kind assistance.
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Re: PIR Motion sensor delay question

by adafruit_support_bill on Wed Oct 17, 2012 10:47 am

Setting sensitivity to MAX is causing it to be triggered by background noise. For maximum 'usable' sensitivity, you need to adjust it to the highest setting that doesn't give you false triggers.

The delay is a completely separate control. It determines how long after it triggers before it can be triggered again.
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Re: PIR Motion sensor delay question

by ywQ on Thu Oct 18, 2012 1:31 am

Thank you very much! I will experiment further with these dials, based on your articulation.
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