PIR motion sensor far too insensitive - why?

Discussion in 'The Projects Forum' started by ronwer224, Apr 21, 2016.

  1. ronwer224

    Thread Starter New Member

    Apr 8, 2016
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    1
    I need to make a PIR motion sensor for a museum project. The plan is that the PIR activates sounds when visitors enters certain displays/exhibits.

    I choose for the following solution, since it is a project that has been made with simple components, tested and proven functionally.

    PIR motion sensor.jpg

    To my great frustration it did not work as expected!

    The circuit is way too insensitive. Only moving my soldering iron closely to the PIR activated the circuit. My hand didn't do anything.

    Therefore I started to change things.

    I tried 2 different PIR sensors, one from eBay, the other one salvaged from a professional alarm system. Same result.

    R1 was changed into 1K, resulting in a slight increase of sensitivity.

    I changed R3 into respectively 2M and then 1M, but the circuit was unstable and prone to oscillations. And when stable it was still way too insensitive.
    I tried to change C2 and C3 to 100nF in order to try to get rif of the oscillations, but without success.

    I did not use the LM324 as in the original circuit, but choose the LM358 because that makes it easier to design the PCB. I assume these to be similar enough that this cannot be the problem. I tried several other opamps, all with the same or worse result.

    I can see on the scope that moving my hand in front of the PIR gives a pulse on pin 7 of U1b. The soldering iron gives a huge pulse.

    Now a few questions.

    What is better: decreasing R1 as opposed to increasing R2?
    Was it OK to replace the LM324?
    Can the problem be the window comparator?
    What else can I have done wrong???

    I hope somebody can help me!

    Ron Werner
    Norway

    PIR motion sensor pic.jpg
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If you are not using a focusing lens (fresnel or otherwise) in front of the sensor, you will see decreased sensitivity as a result of a lack of directivity.
     
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  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi
    As trace has pointed out you must use a Fresnel lens in front of the PIR.
    This pdf covers your circuit, note the lens feature. Page #5

    E
     
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  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
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    That should have made a big change to the output obtainable from U1a. How/where were you checking 'sensitivity'?
    Why do you have that 150u cap on the outputs of U1c/d? Try removing it.
     
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  5. ronwer224

    Thread Starter New Member

    Apr 8, 2016
    6
    1
    Thanks for these quick replies!

    I ordered lenses, they are on their way!

    But I don't need wide angle detection! I will build in the PIR sensors in a piece of tube. The whole sensor will be hung op at a height of about 3 meters, exactly above the entrance of the display/exhibit. Visitors need to actually walk under the sensor to activate the sound. The entrances are 1-2 meters wide. Outside that area I don't want to PIR to be activated.

    The 150uF cap was already reduced to 10uF and has the function to increase the time the "1" signal is on the output. I only need a short pulse, probably the pulse is already long enough without any capacitor at all!?

    But still, even without a lense, the PIR should easily react to my waving hand at a distance of 1 meter...or am I wrong!?

    Now with the extra amplification the PIR reacts at my hand only when I move it directly in front of the sensor, maybe max. 10 cm not more.
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi 224,
    The amplifiers are AC coupled which means that the detected body heating must be fluctuating, ie: a light dark light action across the sensor.
    Just moving your hand will produce a very weak slow change.

    To reduce the zone angle of operation use masking across the lens.

    Look here
    http://www.glolab.com/focusdevices/focus.html
    E
     
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  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    1,393
    You assembly looks good, but I find that the most often reason my projects don't work right off the bat is a wiring error. The use of LM358s instead of an LM324 is inconsequential.
     
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  8. ronwer224

    Thread Starter New Member

    Apr 8, 2016
    6
    1
    @tracecom...

    Accept my humble thanks! You inspired me to take a second second-look at the PCB, and yes, there it was! I had connected the 22uF cap to the "2" of U1a instead of "1". It's just a tiny little track that went wrong, but I suppose this must make a huge difference.

    (I don't use a PCB editor program, just CorelDraw)

    I will make a new PCB this evening, go back to the original component values and will post the result tomorrow!

    Everybody thanks for your assistance!

    Ron Werner
    Norway
     
  9. ronwer224

    Thread Starter New Member

    Apr 8, 2016
    6
    1
    Now the detector works excellent. Without a lense my moving body is detected at 1 meter at least.
    That means I can make the necessary units for this years exposition about log cabins.
    Thanks for your help. This forum certainly is the best I have seen so far!

    Regards,

    Ron
     
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