PIR question

Discussion in 'General Electronics Chat' started by mbohuntr, Jan 2, 2011.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I finally got around to testing the camera trigger circuit using two 7555's. The circuit works great. After connecting the PIR trigger, the camera wakes up and triggers perfectly. I them checked the current draw and it was fluctuating continouously from 28mA to around 10mA. I then disconnected the trigger wire, and the 555 trigger and output voltages stabilized with the trigger pin held high, and output near zero volts. The circuit, including the PIR drawing 7mA at rest.
    Apparently the PIR trigger wire is causing instability on the trigger pin of the 555! Does the pull-up need to be smaller?, or perhaps a resistor between the PIR trigger and pin 2... (The timers will trigger if I touch pin 2 with a bare wire in my hand, so I am near the threshold...) The trigger wire is pulling current, while not dropping the trigger voltage enough to cause instability. I hope this makes sense... Thanks!!
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,761
    924
    Check the easy stuff first and verify WHERE the problem is.


    Check the cable itself first for shorts or open shields. Rule out the cable and any non soldered connections you have made(something might be loose)

    That way you can be sure to not waste your time trying to fix a good circuit.
     
  3. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    If I manually disconnect the trigger wire immediately after pin 2 is pulled low, the circuit will cycle normally. I first thought that the output was going low at the same time as pin 2, causing an illegal operation. I increased the pulse time to over 2 seconds by bumping up the R/C time. Then I tried to lower the pullup resistor from 10k to 4.7k, nope... I tried various caps in series with the trigger wire without success... I really hope I don't have to drop the pullup down to 1k, as that would bump up the current draw by 2mA... Any suggestions??

    The circuit is simply 2 7555's (monostable) cascaded and delayed by a R/C delay on pin 2 of the second chip. The outputs are ran into IR emitter/detector pairs to trigger the camera.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I know it is on other threads, but could you show the schematic you are using?

    The 555 monostable configuration without the input signal conditioner has some idiosyncrasies. When pin 2 is brought low and kept there the 555 continues timing, but if pin 2 is kept low the output can not go low, even if it has timed out. When the input goes high and the 555 has timed out the output will go low, and the 555 will be ready for another trigger pulse.

    The input conditioner in the traditional 555 monostable circuit prevents pin from going low and staying there. Eventually the input capacitor charges and pin 2 returns to a high state.

    [​IMG]
     
  5. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks for the reply Bill, I will redraw the circuit and post it tomorrow. Funny thing, I built the same circuit on a breadboard to better troubleshoot it, minus the inverter and second stage, and it seems to work fine???
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Did you make a PCB? Post that too. Many eyes do eliminate bugs make.
     
  7. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
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    Here is the schematic, I'm pretty sure I got everything copied correctly. Sorry about the appearance, Multisim isn't the best tool for this.

    The concept is pretty easy, The PIR sends the trigger low on the first stage, in turn triggering a 1 sec pulse to trigger the refresh to the camera. (U1A)

    The rising edge also reverses the state of the inverter that keeps Q1 saturated, and the second stage trigger high. R2 and C3 slowly decay and delay the triggering of the second stage pulse. This gives the camera a chance to wake up before the second pulse triggers the shutter. (U2A)

    The falling edge of the first stage then changes the inverter state and pulls the second stage trigger high and recharges the RC delay.

    By itself, the circuit works well and draws very little current (7mA) I tested it using LED's to visualize the sequence. When I connected the modified PIR http://www.sparkfun.com/products/8630, The circuit triggered fine, and after a 5 minute wait(to allow the camera to go into sleep mode), sensed my motion, refreshed the camera and took another pic. When I connected a meter in series to measure the total current draw, it was dancing between 12 and 28 mA. The output in the first stage was fluctuating around .8 -1.8 volts when it should have reset to 0v. This is too much for battery use. I was hoping for 10-20 days.

    I tried increasing the pulse time on the first stage, then tried a series cap and 10k pullup conditioning trigger similar to your last pic. The circuit didn't trigger, so I may have done something wrong there... The circuit will trigger and reset fine if you remove the trigger wire immediately after the trigger.... so I'm thinking it's in need of a better trigger filter circuit. Thanks for the help, I'm at my limit of knowledge here. No PCB, I haven't tried making one of those yet, just a messy and full protoboard.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Word is always the worst choice to use to display a schematic. I translated it for you...

    [​IMG]

    Several things jump out at me, lack of signal conditioner on U1, the lack of tying the unused gates down on the 7404 (the inputs should be grounded). Let me mull over it a bit and I'll get back with you.

    *****************

    Quick note, look at your pin numbers. They show a 556 (dual 555), but they don't actually match the outputs of a 556 chip.

    http://forum.allaboutcircuits.com/album.php?albumid=41&pictureid=308
     
    Last edited: Jan 5, 2011
  9. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks Bill, U1 and U2 are actually Radio Shack Emitter and Detector pairs, I used the opto symbol for clarity. I'm not sure that will make a difference or not.... I have not had any education in signal conditioning, this ought to be a good opportunity.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    OK, A1 and A2 then. The number are way off.
     
  11. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
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    I think I may have it..... I was wondering why I wasn't seeing a voltage drop at pin 2 using your series cap, I put the second 10k pullup resistor in parallel with the cap and triggered it. Pin 2 dropped to 1/2 VCC, with no instability. Ah-HAA!!! I dropped it to 2.2k, and pin 2 drops to around 1 volt. The cap now seems to function as a smoothing cap. More testing on my days off... Thanks for the help Bill and Kermit!!!
     
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