Electronic engineers ! Help !!!!! (1 Viewer)

[Jaws]

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 !!!!

 

[hilldweller]

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 !!!!

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.

 

[Jaws]

Good solution
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.

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[hilldweller]

Good solution
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.

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 ?