Any Circuit Designers In Here?

Discussion in 'Shop Talk' started by Oxankle, Nov 16, 2006.

  1. Oxankle

    Oxankle Well-Known Member

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    Burglars in my barn burn my behind. That being the case I have installed an alarm triggered by a motion detector. A home-made device utilizing one of the common motion dector lights, a power supply and a siren.

    Here is my situation:
    The motion detector lights are rigged for a maximum of 300 watts. by wiring the power suppy into the light circuit I overload the motion detector and have now burned it up.

    Now I propose to add a relay. The relay operates on milliwatts, a tiny load that will not tax the motion sensor even with lights installed. On the output side of the relay I will install a receptacle for a `12 volt power supply, and the power supply will be permanently wired to the siren. A third grader handy with a screwdriver and wire stripper could do it.

    Question: This will be a lightweight setup, to reiterate the coil will draw only milliwatts. When the relay opens will the collapsing field be likely to tear up my motion sensor's circuitry? Do I need to stick a diode in there?
    Ox
     
  2. Bearfootfarm

    Bearfootfarm Hello, hello....is there anybody in there.....? Supporter

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    Im not much when it comes to electricity but wouldnt it be easier to just use smaller bulbs to keep it below the 300 watts? Or does the power supply take too much to do that?
     

  3. GREEN_ALIEN

    GREEN_ALIEN Sunny, Wet, Tornadoey SD!

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    It would be easier to answer if a sketched wiring diagram of the original circuit were provided?

    If I read it right a simple resistor inline to the lights will do the trick in the current form. If you go to the relay system a diode would surpress the collapsing field and a cap would act as a shock absorber.
     
  4. cfabe

    cfabe Well-Known Member

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    A clamping diode won't work and is not necessary on an AC relay, and since you'll be running this all on 120AC, you should be fine.
     
  5. agmantoo

    agmantoo agmantoo Supporter

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    What you stated will work fine. Both the light and the coil windings are resistant loads. Go for it! I would use a latching relay that way it will stay on or I would supply the feedback off a second set of contacts to latch the relay even after the motion detector ceases to detect.
     
  6. cfabe

    cfabe Well-Known Member

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    Actually a coil is an inductive load, that's why on a DC relay you need a diode to suppress the voltage spike created when it's switched off and the magnetic field collapses. But on an AC coil relay it's not necessary.
     
  7. agmantoo

    agmantoo agmantoo Supporter

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    I acknowledge that a coil is an inductor. When a motion detector circuit is viewed there is already a diode in the circuit for the coil in the relay in the detector. In this case, the detector relay will in turn control the coil to a relay rated at a higher amperage to prevent burnout. The coil in the second relay will be sensed by the contacts in the motion detector relay as a resistant load ( coil windings) and the inductance of the second relay will not have any impact IMO on the motion detector relay. That is how I viewed the proposed circuit.
     
  8. Oxankle

    Oxankle Well-Known Member

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    Damn, you guys are good. I figured someone out there would tell me what I needed to know.

    I will pull the current for the coil off the motion detector circuit; that circuit can handle the lights plus the tiny load for the relay coil. The current for the power supply will come off the relay output and will stay on as long as the motion detector lights run.

    I will set the lights for twelve minutes. If I cannot grab a shotgun and get to the barn in l2 minutes I do not deserve to own tools, saws, log splitters etc. Actually, the thieves will probably haul it the moment they hear the siren. They have to walk almost a quarter of a mile, so they will be running for their cars.

    Already planning a second set for another building but I want to be sure this works first. Once I have it working properly I will think about wiring in the extra pairs in my phone cable and having the alarms go off at the house.

    By the way; won't the relay become a latching relay if I simply run a lead from the output side to the output side of the motion detector circuit?
    Ox
     
  9. Oxankle

    Oxankle Well-Known Member

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    Bearfoot and Green Alien:

    I thought of smaller bulbs, which is the same as I'd get with a resistor, no?

    I passed on that idea because I was afraid that the power supply would still tax the circuit and I felt that I needed the lights anyway. In addition, with the relay and full power I can place one siren inside and one on the roof to scare the bejeebers out of the prowlers and alert the neighbors. I have a neighbor who, if he hears the siren, will be willing to meet them at the road with his .45. We are all sick and tired of thieves.
    Ox