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PirSensor

Discussion in 'Sensors and Actuators' started by Zarain, Apr 6, 2018.

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  1. Zarain

    Zarain

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    Nov 10, 2017
    Is there any way to decrease the time delay of my pir?. datasheet says it has a minimum delay of 0.3s but once my pir senses my hand (for example) it becomes high for about 5s. I just want it to become low as soon as i remove my hand out of its sight.Is it possible?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I assume it's a PIR module.

    Figure out what chip is used, look up the datasheet, figure out the circuit, and maybe you'll be able to see what to change to alert the delay.
     
    hevans1944 likes this.
  3. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Two things. First, the turn-off delay might be programmed into the controlling chip and not adjustable by changing an external resistor or capacitor. Second, the delay might not be a controlled parameter at all.

    The sensor warms up (accepts radiation) based on the field strength of that radiation, but it cools off (sheds thermal energy) based on its construction and the temperature/movement of the ambient air around it. So the 5 s might just be how long it takes to cool off enough to go back to the "nothing detected" state.

    To me, based on other thermal work in various MIL and industrial systems, 5 s feels like its natural response time to removing the heat source. That it is over 10 time the turn-on response time is entirely possible.

    ak
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    PIR (Passive Infra Red) sensors are actually two pyroelectric IR sensors mounted side-by-side with two slightly different fields of view provided (usually) by Fresnel lens arrays. The sensors are connected so their signals subtract. That means any change in sensor signal caused by heating from the image field does not produce a significant differential output signal, nor does any change in ambient temperature. When a object warmer (or colder) than the background radiation enters the field of view (FOV) of both sensors, it will enter (or leave) the FOV of one sensor before the other because the Fresnel lenses present slightly different points-of-view (POV) to each sensor. This POV difference causes a momentary difference in the differential output of the two sensors, which is further processed electronically to "trigger" the PIR sensor head "ON". This action occurs very rapidly because the thermal response of the pyroelectric sensors is very fast.

    What happens when the object being sensed is no longer in the FOV of both sensors? The differential output returns to zero. It is up to the PIR sensor designer to decide how long to wait, after the differential signal decreases below a threshold level, to declare that an object is no longer detected. A long delay helps to prevent false negatives, that is, declaring no object is detected when an object is actually in the FOV.

    If you take a PIR sensor apart, you can gain direct access to the pyroelectric sensor elements and do your own signal conditioning. However, considerable experience is required for success because pyroelectric sensors are extremely high impedance current-source devices, much like electret microphones which operate on similar principles. Easier to let the Asian manufacturers do all the heavy lifting. Cheaper, too.
     
  5. Bluejets

    Bluejets

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    Oct 5, 2014
    Might be more suitable to use an LDR or an ultrasonic sensor.
     
  6. Zarain

    Zarain

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    Nov 10, 2017
    Thankyou btw i'm using pir sensor hc-sr501.
     
  7. Zarain

    Zarain

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    Nov 10, 2017
    Thank you very much sir, what component/s do i need to change if i am using hc-sr501?
     
  8. Zarain

    Zarain

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    Nov 10, 2017
    I am also considering this two sensors but I think Pir sensor is the best option to my project bc it detects only person/s (bc of their heat) or sometimes animals.
     
  9. Bluejets

    Bluejets

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    Oct 5, 2014
    Any heat changes really.
    Depends on sensitivity as to how the relay reacts.
    For example they can be upset by trees blowing in the wind.

    Way back when, there was the story of the spider that somehow deducted that certain movement would trigger the relay and as a result, turn on the light which then brought moths to his dinner table.
     
    hevans1944 likes this.
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Check out this site which explains the use of this module.

    Most importantly, this site identifies the IC used in this module. It is the BISS0001.

    Googling "BISS0001 datasheet" will take you to a number of sources for the datasheet.

    It looks to me that changing the minimum resistance connected to pin 6, or reducing the associated capacitor would allow you to reduce the minimum activation time. There is a similar control for the lockout after triggering, so you may want to change that too. You may also want to play with retriggerable vs non-retriggerable mode.
     
    davenn and hevans1944 like this.
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