> So you say I need 4 wires for a sensor ? 2 for VCC and GND and 2 for the
> alarm state ?

No.  I was thinking two wires total for each sensor.  If your sensors are
powered, then it would be easier to provide a third power line, but it can
be done with two lines only.

How much power do your sensors require during normal operation?  How much
when tripped?  Can you design the sensor circuit or are you stuck with
something?

> What are the most common sensors : normal-open or normal-closed ?

I don't know, but this is something you should be able to find out
yourself.


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