Home Alarm PIR signal question

Discussion in 'The Projects Forum' started by Zaplady, Feb 3, 2016.

  1. Zaplady

    Thread Starter New Member

    Oct 17, 2012
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    Hello
    I have six unused petite 101 PIRs left over from an old alarm. I want to use them to trigger cameras but am having problems reading the signal from the circuit. My understanding is that when the PIR is tripped it will open the circuit between N and C (the alarm contacts). It says it is a "Positive looped normally closed circuit".
    I have found a digital copy of the PIR leaflet at
    https://www.google.co.uk/url?sa=t&r...SZdkQwabZ9hK4f-6g&sig2=_k6iAgip2TC15xaueJimbg
    When the PIR is triggered the LED (on the PIR board) lights for a short time and goes off but there does not seem to be any change in the signal on the alarms contacts N/C.
    I have looked on the internet and a lot of people seems to use the LED circuit but I want to use the N/C contacts.
    I have tried measuring the resistance between N and C
    and the voltage difference but there does not seem to be any change.
    If I just connect a voltage (12V or 5V or 3.3V) to C and and an LED followed by a resistor on N and go to ground then the LED stays on. (either way round)
    I thought maybe it was because the signal was too short so I read N using an interrupt on an Arduino (with a pull down resistor) and there is no change.
    I have tried 5 of the 6 PIRs and get the same result so I don't think they are faulty.
    Am I missing something? What is this signal supposed to look like?
    Thanks
    Mrs Z
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Yes you need to use the NC( normally closed) terminals these will be a short circuit when the pir is powered up and not detecting anything,set the link to 1pulse, then wave your hand over the pir, the led lights up and the NC terminals will go open circuit.

    Note the NC contacts will also be open circuit with no voltage applied to the pir.
     
  3. hp1729

    Well-Known Member

    Nov 23, 2015
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  4. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
    2
    Thanks for this Dodgydave. I have just tested all six of the PIR and the N/C circuit is closed when nothing is attached to the PIR. Does this mean that all of my PIRs are malfunctioning or do they work in some other way?
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    It sounds like you know what you're doing so far. What was the pulse count set to? How long does it take to "warm up" before it is ready to operate? Good 12 V?
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Are
    Are you sure its the NC terminals and not the Tamper, as tamper only opens when the lid is removed.
     
  7. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
    2
    Hi hp1729. Thanks for your reply. I have tried 1,2 and 3 pulse counts. The results are the same. The LED lights to show the PIR is triggered but the N/C contacts remain closed. It takes up to a minute to warm up. The 12V is good. It seems so odd that they all 6 do detect change in the room but none of them open the circuit. They have never been used before.
     
  8. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
    2
    HI Dodgydave. Thanks for your reply. I have not mixed up the tamper circuit. The tamper circuit works fine on all of them. It is only a simple switch. Do they have a shelf life?
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The pirs use reed relays, its possible the reeds are stuck closed, if you measure the voltage across its coil which is approx 5v, the contacts should close and open when no voltage applied, you can hear the reeds clicking if you hold it to your ear.
     
  10. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
    2
    Thanks Dodgydave. I got out my magnifying glass and cannot find anything on either side of the circuit board that looks like a reed relay, let alone is large enough to make a noise I could hear. This is bizarre. Is there some other way they could work? I bought them about 20 years ago.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Take pictures of the pir component side and track side.
     
  12. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
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    [​IMG] [​IMG]
    Thanks for taking so much time to look at this Dodgydave.
     
  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    It appears to be an un occupied spot. I don't think the board has a relay installed.
     
  14. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    They're too dark, can you make them lighter...
     
  15. Zaplady

    Thread Starter New Member

    Oct 17, 2012
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  16. hp1729

    Well-Known Member

    Nov 23, 2015
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    Bad news. It sounds like you have considered all the possibilities and not found the problem. :)
    So we are down to the switch itself. How are you checking the switch contacts? I couldn't follow your earlier descriptions.
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Looks like there's no relay, the NC terminals are shorted out,it uses a transistor and 100 ohm resistor, you need to use one nc terminal and the positive supply to create a loop for the alarm panel.

    You could put a relay in if you wanted.
     
  18. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
    2
    Hi thanks for your reply. This is how I checked the switch contacts:
    1) I put my multimeter leads across the two terminals and measured the resistance and the voltage difference while the PIR was triggered and not triggered.
    2) I connected the C to a voltage source and the N to an LED in series with a resistor then connected to ground. The LED stayed on throughout even though the LED on the board lit several times as the PIR was triggered.
    3) I put 5V on C and connected N to Arduino pin 3 (with a pulldown resistor). I attached an interrupt that did not trigger. I tested the code by pulling the wire from N out to verify that it worked. I did this in case the signal was short and I was missing it with the LED.
    I am slowly coming to the conclusion that the PIRs are faulty.
     
  19. Zaplady

    Thread Starter New Member

    Oct 17, 2012
    11
    2
    Thanks for this, Dodgydave. I am not sure what you mean. Please could you explain further? My knowledge of electronics is limited.
     
  20. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The pir is designed for a specific panel that uses positive loop detection, the transistor pulls the nc terminal to ground via a 100 ohm resistor when no detection is present, then when it detects an heat source like a person, it removes the ground and goes open circuit.
     
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