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TIDA-00489: PIR sensor detecting pulse rate of LED ambient light

Genius 4205 points

Replies: 5

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Part Number: TIDA-00489

Hey guys,

we have some issues with the TIDA 00489 PIR sensor design. We noticed, that during tests the sensor was heavily influenced by a dimmed LED ambient light. We are counting the movements in a room for 5 mins. If the LED light is on we counted approx 30000 movements which fits to the 100Hz frequency of the LED lamp.

Do you have any suggestion how to proceed?

I would prefer a hardware solution like a low pass filter, where would you position that one and do you think a RC-filter is enough and how we should dimension it? (50 Hz bulb frequency in europe and 100Hz up to 50kHz for LEDs)

kind regards

Slev1n

  • Slev1n,

    Flashes of white light are known to be a cause of false triggers in PIR sensors.  In this case the sensor itself senses these flashes as a change in background temperature, basically blinding the sensor.  Since the LED is being flashed by the dimmer to produce the dimming effect, I suspect this may be the cause of what you're seeing.  If so, the proper place to filter would be before the sensor using a white light filter in front of the Fresnel lens of the PIR.  We tested in labs where fluorescent bulbs were used and did not notice any sensitivity to ambient light.

    A few questions:

    1) Have any of the filter values been changed in the amplifier stages of your design compared to the TIDA-00489 design?  Fresnel lens installed?

    2) What happens when the dimming frequency is increased?

    3) What does the test setup around the Sensor look like?  Could this possibly be a ground loop, or otherwise be picking up noise from the mains in the test setup?

    4) Is this repeatable on multiple sensor boards?

    Regards,

    David

  • In reply to David Stout:

    David Stout
    1) Have any of the filter values been changed in the amplifier stages of your design compared to the TIDA-00489 design?  Fresnel lens installed?

    No, we used exactly the hardware design given by TIDA-00489 design. The Fresnel lens is installed.

    David Stout
    2) What happens when the dimming frequency is increased?

    We cannot change the frequency with the current light source, but I will try to find a lightsource with higher modulation frequency.

    David Stout
    3) What does the test setup around the Sensor look like?  Could this possibly be a ground loop, or otherwise be picking up noise from the mains in the test setup?

    You can barely call it a test setup, a co worker had some sensors at home in his living room. There was a wight light ambient LED lamp approximately in 2m distance, the error occurs if the light is directly entering the lens or indirectly via reflections.

    David Stout
    4) Is this repeatable on multiple sensor boards?

    According to my co worker it is repeatable (light on or off) and all sensor boards he had were prone to this "error".

    A white light filter would be a good idea, maybe there are PIR fresnel lenses available having such filter already included. Actually, a lens with a material letting pass only IR light at 5-15µm would be the best, not sure if there are cheap available plastics having such properties. Do you guys know a good alternative?

  • In reply to Slev1n:

    Slev1n,

    Sorry for the delayed reply.  I have asked some colleagues about cheap available plastics as well as other options available to perform the optical filtering.  I am waiting to hear back from them.  However, depending on the amount of energy in the band of interest being produced by the LED, you could still have false triggers with the optical filtering included.

    Another option may be to use an ambient light sensor to sense rapid changes in the ambient light level and use that information to ignore pulses from the sensor which occur because of those rapid changes in ambient light.  Downside to this approach of course is increased power and cost.

    One recommendation would be to characterize how far away the sensor needs to be from the dimmable LED (or the appropriate angle) so as to avoid the false triggering.

    Regards,

    David

  • In reply to David Stout:

    Hey David,

    thank you for the response. Another disadvantage of measuring a high frequency signal is that every time a HWI is triggered a short current peak occurs drawing up to 2.4mA. But we will test how far away from the LED lamp we still have this behavior.

    But still, if you know about a replaceable lens having even better properties I would like to know.

    kind regards

    Slev1n

  • In reply to Slev1n:

    Slev1n,

    We have not been able to locate a replaceable lens which would improve the performance when operating near the dimmable LED lamp.  At this time the recommended solution would be to characterize and document the behavior or add the ambient light sensor to aid in filtering out sensor pulses due to rapid changes in ambient light instead of actual motion.

    Regards,

    David

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