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Following on from my post outlining the modifications I wanted to make to my alarm system, it seems that one of the PIR sensors has started to play up. I tested the 9V battery which was flat and reading 6V or so.

After replacing the battery, it still seemed to have some problems – when it detected movement, it would keep thinking it detected movement and switching the LED on and off every 4 seconds for about a minute or so (or maybe even longer).

Above is the picture of the PCB for reference. I started measuring voltages and compared it to another PIR sensor but there wasn’t a difference in voltage that stood out.

However what I did notice is that on the point shown above, when the sensor was activated, it would jump down from 3.3V to 1.5V and then jump around 1.5V to 1.6V then rise back up to 3.3V soon afterwards. I found that on faulty sensor it would turn back on when it was starting to rise to 3.3V – so it never got to go rise back fully to 3.3V.

I checked out the BISS0001 datasheet to take a look at what pins I should be looking more thoroughly at. The IN- and IN+ looked like they might be the inputs from the PIR but compared to another PIR it seemed to be normal.

Next I tested the 2IN- and 2OUT pins in which I found that when 2IN- rose up to 1.7V it would trigger the sensor, the strange thing was that when the multimeter was connected it would sometimes stop the fault from happening.

Knowing that the multimeter has some input resistance but I didn’t know how much, I decided to connect a 1 Megaohm resistor from 2IN- to ground. It did in fact stop the fault from happening but then it only activated when you were a few centimeters away from it. Next I tried 2 Megaohm but the fault came back – so half way in between should be the sweet spot.

I added a 390K resistor to the 1 Megaohm resistor and now it seems to work well. I don’t quite know what made this PIR sensor start playing up – will it happen to the other PIRs too? Who knows.

2 Responses to “Fixing an issue with a PIR sensor”

  1. Martin says:

    I think you will end up building your own alarm system from scratch, including the PIRs – they are available as break-outs 🙂

    • Alex says:

      Hehe, probably! I did order one from Ebay and it works quite well :).

      I also ordered an ultrasonic detector which I may think about using outdoors which could perhaps trigger a more quieter alarm – something that would give them feedback that an alarm system is active inside the home.

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