Need help with simple Photocell project.

Discussion in 'The Projects Forum' started by EvilRSA, Oct 2, 2009.

  1. EvilRSA

    Thread Starter New Member

    Oct 2, 2009
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    Hello everyone, I have a project that I keep getting stuck with on. Here's what I'm trying to do...

    I need something to act as a trigger when this project is exposed to average room lighting.

    The project is a 12vdc system that once triggered needs to power a car alarm siren. My first idea was to use a relay, looping the power out also back into the relay so that once it was tripped it would hold itself closed, and I still think this would work fine, but I'm hitting a road block on how to make the trigger.

    I'm pretty much a noob when dealing with photocells, but I don't think they can stand up to 12vdc.

    Any ideas are very much welcomed as I'm in the weeds at this point.
    I can pretty much buy what I need as long as its not too pricey.

    My project is going to be in a box, and when it's opened and light hits it, it needs to let power flow to my siren which is 12 vdc @ 1.25A

    Thanks!
     
    Last edited: Oct 2, 2009
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Use a cadimum-sulfide photo-cell from +12 to gate of N ch. MOSFET, a 1M pot gate to gnd., source to ground, sounder from +V to drain. P cell should be hi Z, several megs in dark.
     
  3. EvilRSA

    Thread Starter New Member

    Oct 2, 2009
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    So, just to check, this circuit will stay on once triggered till the power source is depleted, even if the light source is removed?

    If so, how's my shopping list shaping up?

    6V Heavy-Duty Zinc-Chloride Lantern Battery x2
    http://www.radioshack.com/product/index.jsp?productId=2103180

    CdS Photoresistors
    http://www.radioshack.com/product/index.jsp?productId=2062590

    MOSFET IRF510 Transistor
    http://www.radioshack.com/product/index.jsp?productId=2062618

    1M-Ohm Linear Taper Potentiometer
    http://www.radioshack.com/product/index.jsp?productId=2062297
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Sorry, missed the latching part. Think I would go for an SCR, but will need new triggering-tomorrow.
     
  5. EvilRSA

    Thread Starter New Member

    Oct 2, 2009
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    Okay, I look forward to seeing what you come up with on the rectifier.

    Just wondering though....it wouldn't be all that elegant but couldn't I also take a spst 12vdc 30a automotive relay (which I have right next to me here on the bench), take the +12 and MOSFET's Drain to the relay's coil, and the +12 to the open side of the relay, and the output would split to the relay's coil again to latch it open permanently and also supply the sounder?
     
  6. EvilRSA

    Thread Starter New Member

    Oct 2, 2009
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    Oh, I should also state, that this is meant to be destroyed, so elegance isn't really all that important.

    Me and my buddies prank each other pretty good, and this whole project is going to be placed in a small box, and be completely filled with crystal clear two part resin, so after my buddy opens the box the siren will go off and he'll have no way of stopping it until the batteries are exhausted or he breaks into this thing enough to break the circuit. ;)
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    396
    If you already have DPDT relay, try this:
    After testing , replace R2 with fixed resistor. After closing lid, snip the shunt to activate.
     
  8. EvilRSA

    Thread Starter New Member

    Oct 2, 2009
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    0
    Thank you very much, I'm not sure what I did to kill off my first MOSFET, because it was working for a few minutes, then stopped. I played around with it for a while trying to see what had happened, but after switching it out for a new one, its working like a charm again.

    Fixed resistance turned out to be 16K.
     
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