Help with 556 triggered by PIR sensor

Discussion in 'The Projects Forum' started by czibesz, Jan 17, 2010.

  1. czibesz

    Thread Starter New Member

    Feb 2, 2009
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    This is my first post, and although I have found some threads related to my project, I am new enough to electronics to find myself stuck in the particulars of what I am trying to do.

    I am trying to design a timed circuit that uses a PIR motion sensor to trigger a moving object for about 1 minute. After it times out, it is at rest until motion is detected again. I replicated the circuit as posted on the parallax forum here (first attachment in the post):

    http://forums.parallax.com/forums/default.aspx?f=21&m=370512

    This part is just a matter of putting it on the breadboard, and it works fine. My problem is that the PIR needs to be on the moving object; since the PIR needs to adjust for a period of time (about a minute), I figure that I need a second timer that starts as soon as the first one times out. This timer can run for about a minute or so (the PIR adjustment period), and then when it times out, it allows the PIR--if it senses movement--to start the motion in the first timer again.

    My guess is to interrupt the signal from the PIR to the first timer input somehow? (It obviously does not work to interrupt the power to the PIR, which only defeats the purpose of the adjustment period.) Can anyone help with the best way to do this?
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    You may have a problem; one function of adjusting time is for PIR to establish a refference background- a little hard to do if PIR is moving.
     
  3. czibesz

    Thread Starter New Member

    Feb 2, 2009
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    Exactly my problem!

    I was thinking that after the first one times out, it could trigger the second timer--during which the PIR sensor is NOT moving, allowing the it time to adjust. When the second times out, it would re-allow the PIR to decide whether or not to trigger the first. Therefore, after movement, there would always be a 60 sec adjustment time for the PIR to do its job.

    I am trying to figure out how to isolate the signal of the PIR for the adjustment period. Right now, its signal passes through a SPDT relay NC contact. When timer two goes, its output goes to the relay coil and breaks the PIR signal, but not its power. This seems to work OK.

    The next problem is how to have the end of the first timer trigger the start of the second timer, but only once. I know 555s are good at this, but I can't seem to sort it out.

    Right now I am putting together a circuit diagram to post.
     
  4. czibesz

    Thread Starter New Member

    Feb 2, 2009
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    Here is what I have for a circuit so far, but I still need some help. I am sure that some of my circuit symbols look funny--that's because I hardly know what I am doing! (It has been helpful to read through Bill Marsden's work on 555s to get me this far. Thanks! Also, next time I will use PaintCAD!)

    In the circuit, the parts that lead "to motor circuit" and "to speaker circuit" work fine. I omitted them for simplicity.

    I basically guessed how to configure T3 and T4 (by duplicating the transistors laid out in the first part of the timer). I also guessed that by triggering the second timer from the DPDT relay, the second part of the timer would run first (that is, it would interrupt the PIR and the first timer would never receive a signal to start) but I guessed wrong! It seems that the first timer always runs first?! I am not sure I understand why...

    Anyway, it seems to be working, but not 100% of the time. Could it be the moving parts (and a moving breadboard) could be giving false triggers?

    Also, I am curious whether any of the circuit is redundant and/or not necessary? Much of this has been guess work.
     
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