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PIR Sensor trigger — Parallax Forums

PIR Sensor trigger

timadriatimadria Posts: 3
edited 2012-08-17 06:30 in Accessories
Hi,

I'm testing the PIR sensor (Rev A) in my project.
The output is triggering pulses constantly, eaven if there is no one in the room. I was wondering what's wrong? I tested both settings: L and H, no differance.

Kind regards

Comments

  • TymkrsTymkrs Posts: 539
    edited 2012-08-14 11:29
    What's your code and hook up look like?
  • timadriatimadria Posts: 3
    edited 2012-08-15 23:29
    My code is not imortand. I'm measuring the signal on my scope first.

    The hookup is like the datasheet tells me: + to +5V, - to GND and the puls is my output.

    Is it possible the PIR is to sensitive? I've bought 4 sensors and they all give these pulses when there is no one in the room.

    Tim
  • SRLMSRLM Posts: 5,045
    edited 2012-08-15 23:39
    Is there any current draw on the output pin (ie, are you driving anything)?

    Did you let it settle down? IIRC, it makes a map of it's view as soon as you turn it on, and then does comparisons against that to determine movement.

    It picks up movement of things with a different heat than the background. Do you have any fans running, or other (non-human but warm/cold) things moving?

    Also, is the power supply clean (no ripple)?
  • kwinnkwinn Posts: 8,697
    edited 2012-08-15 23:50
    SRLM wrote: »
    Is there any current draw on the output pin (ie, are you driving anything)?

    Did you let it settle down? IIRC, it makes a map of it's view as soon as you turn it on, and then does comparisons against that to determine movement.

    It picks up movement of things with a different heat than the background. Do you have any fans running, or other (non-human but warm/cold) things moving?

    Also, is the power supply clean (no ripple)?

    All good questions. I work with PIR sensors for lighting and security systems and have found that they can be constantly triggered by a source of air that is warmer or cooler than the ambient temperature. This includes being too close to an air vent/diffuser, having a laptop charging station nearby, or having a portable heater cycle on and off. Try putting a cardboard box over the sensor and see if it settles down. If it does not, the problem is most likely electrical in nature.
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2012-08-16 08:36
    If the false triggering is constant rather than periodic and random you might want to add a weak (say about 20K) pulldown between the output and ground. The output of the sensor is really intended for the typical impedance of a microcontroller input, not a scope. Also place a good size filtering cap across the power supply rails of the PIR. Even a desktop power supply can have ripples in it.

    But as others have said, many things can cause false triggers. It would be good if you defined "constantly." Is the false triggers once every 10 seconds, or 10 times a second?

    -- Gordon
  • timadriatimadria Posts: 3
    edited 2012-08-17 06:30
    Hey,

    The problem is solved after hacking the PCB.
    Regarding the datasheet of the BISS0001 chip (this is used on the PIR detecor) I changed the amplification of the input signal.
    When you look at the internal block Diagram and the schematic the OP1 functions as an intagrator for the input signal.
    Block.png

    Schematic.png

    By changing the R7 (1M on the Parallax PIR) to a smaller resistor the amplification reduces what makes the PIR less sensitive.
    For My application I changed this to 500k by adding an other resitor of 1M parallel to R7.

    Hope this helps everyone with the same problem.

    Tim
    Block.png
    664 x 390 - 30K
    Schematic.png
    660 x 370 - 18K
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