Shop OBEX P1 Docs P2 Docs Learn Events
PIR misbehaviour? — Parallax Forums

PIR misbehaviour?

andresdrkandresdrk Posts: 3
edited 2010-02-03 03:07 in Accessories
Hi,

I'm currently developing a system using the PIR sensor 555 28027.

Lately I've found a strange behaviour of my sensor which I cannot explain and I would be very grateful if you could give me your opinion about it.

The PIR has been first tested in a very simple way: Connected to a 3.6V power supply, grounded and its signal output seen on an oscilloscope. Being this devices the only things present in a 2m x 3.5m x 4m room, windowless, without presence of heaters, A/C or any possible IR obvious source I could imagine. After the 10-60 sec warming up time of the PIR in a absolutely absence of people in the testing room, my output signal remains low as expected...but after a while, still in absolute absence of people or in my opinion a any IR source, my oscilloscope shows from 1 to 6 pulses as if some movement was detected...and then, randomly it will do it again. This fact ruins the measure because I'm not able to tell if really some movement was detected or if the signal was triggered by this misbehaviour.

I'll really appreciate your opinion.

Thanks.

Comments

  • SRLMSRLM Posts: 5,045
    edited 2010-01-14 21:32
    I'd test and compare with another PIR, and see if there is the same problem (and if it is synchronized).

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Powered by enthusiasm
  • FranklinFranklin Posts: 4,747
    edited 2010-01-14 21:41
    Two things, check for people from another dimension walking through the room or provide a 100K resistor to ground or power whichever is the off state.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • andresdrkandresdrk Posts: 3
    edited 2010-01-15 00:22
    @SRLM

    Same test was held today with 2 different PIR: same result and false triggers are not sync. These False triggers produce, when the jumper is set on H, a High level from 2,5s to 4s what becomes not negligible compared with the output produced by the movement of a hand as it was an "impulsive input" (time of the output 3-4 seconds on high level).

    @Franklin

    Measures with a spectometer were held to analyze the EM spectrum behaviour with no relevant detections, so I guess people from another dimension are not my problem here. (!) Anyway I'd like to ask you why do you think providing the 100k resistor could solve the problem? What do you think the problem is about, that can be solved adding this resistor? Tomorrow I'll try it and I'll tell you if it worked, but I'd really appreciate if you could provide me the idea behind the solution.

    Once again, Thanks for your comments, opinion and support.

    Andresdrk
  • FranklinFranklin Posts: 4,747
    edited 2010-01-15 04:27
    The resistor is called a pullup (pulldown) resistor and it keeps the pin from "floating" when it is an input. Check wiki or Google for pullup resistor for a better explaination but it is a common problem when there are false triggers.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • andresdrkandresdrk Posts: 3
    edited 2010-01-19 09:54
    100k resistor was added as pulldown. Unfortunately False Triggering continues. Thank you Franklin for providing the idea and the information; the problem is still present so further ideas are more than welcome.
  • william chanwilliam chan Posts: 1,326
    edited 2010-02-03 03:07
    I had false triggering with about 50% of the sensors when running at 3.3v using a switched regulator.
    But absolutely zero problems when running at 5v.

    I thought I was the only one having false triggering difficulties, until now.
    Currently to work around the problem, I test each sensor for a few days at 3.3v before using it.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.fd.com.my
    www.mercedes.com.my
Sign In or Register to comment.