PIR question

Discussion in 'General Electronics Chat' started by mbohuntr, Oct 28, 2010.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I am finishing the connections on my camera trigger project, and ran into this?? The directions said to use a pullup resistor on the alarm output of the PIR, so I connected it to pin 3 of the 555 along with a 1M ohm resistor and it stays low? (The 555 trigger pin gets pulled low) Is my resistor too large?? Sorry for the crude drawing, I'm too tired for multisim.

    http://www.sparkfun.com/commerce/product_info.php?products_id=8630
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Yeah 1 meg sounds too high, without knowing how much current the pir can sink (not in data sheet) it's just a crapshoot. I'd try something like 10kΩ
     
  3. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks, one of the message posters on that link said something similar. I'll keep you posted.
     
  4. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
    32
    No good, I think something else is going on.. This unit uses a 5v regulator. If I am supplying 5v from the benchtop, then would the regulator stop working? I went from a 1M pullup gradually down to 10K, but I don't dare go lower. It is still pulling the voltage down. If it's working correctly then it should swing from 5v-0v right?
     
  5. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    GOT IT!! The regulator doesn't work properly until over 7 volts or so. I guess I'll use a separate power supply for the PIR. Thanks for the help.
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I love it when a plan comes together...
     
  7. campeck

    Active Member

    Sep 5, 2009
    194
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    yeah, regulators have a dropout voltage you need to be aware of.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I may be nuts [confirmed] , but it looks like you have two outputs connected together-PIR to 555 pin 3. 10k would be about right.
     
  9. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    You are absolutely right! I should have wrote a 2 instead. The end of a long day. How did you guys calculate that value??
     
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