Electronic engineers ! Help !!!!!

Discussion in 'Tech/Mech General' started by Jaws, Nov 2, 2009.

  1. Jaws

    Jaws Funster Life Member

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    I want to add a PIR to an alarm system I have.. Only problem is all modern PIR's are supplied with 'normally closed' connections and I need one that is normally open.
    Yes, I could use a relay but that would mean it would be held open all the time and draw current.. This is in a battery powered situation and the extra current draw would not be welcome !

    So I got to thinking.. What about a thyristor ?

    Only problem is, my bloody brain will not function and I am damned if I can figure out a way to use one ..
    Basically I can hard wire either the +ve or -ve feed and have whichever is left going through the switched line.
    I want it to have a feed to the -ve rail when there is no voltage applied to it..

    So come on you clever buggers out there.. help !!!!
     
  2. hilldweller

    hilldweller Funster Life Member

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    Take one NPN transistor, say BC183 and one 47K resistor.

    Connect Collector to your switched line.
    Connect Emitter to Ground/0V/Negative.
    Connect base to 47K resistor
    Connect 47K resistor to +12V ( ish )
    Connect base to PIR relay
    Connect PIR relay to Ground/0V/Negative.

    When relay is OC the transistor is switched on and Collector is connected to Emitter. Collector will measure about 0.2V and can pass 100mA.
     
  3. Jaws

    Jaws Funster Life Member

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    Good solution :Smile:
    Will give it a try .. I am hoping the detector circuit in the alarm its self is not overly sensative and will not trigger with such a small pass voltage, but then I guess I can possibly sink it with a 100Ohm or similar ...
    I shall give it a go as soon as I can get out to the thing and report back.
     
  4. hilldweller

    hilldweller Funster Life Member

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    I thought later, fit the transistor in the alarm if possible thus making it closed loop to the box. It would take effort to make a circuit trip on 0.2V, a simple transistor input requires 0.7V and it's probably CMOS which switches at V/2, in your case 6V. You need a good margin so it's not triggered by mains noise or lightning.

    Have I got this wrong ? Does the PIR have a pair of isolated contacts ? If so with just a resistor ( provided you don't need too small a resistance to switch your alarm ) you can pull the alarm up or down depending on where you wire the relay. Do you even need a resistor ?
     
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