Can someone tell me why you should have a door contact if the area is covered with a PIR.

I was just going to use PIR sensors as I hate seeing cables anywhere and that is obviously a problem with using door contacts.

If you dont have a door contact on your entry door thenthe burglar will have the same time as you do to disable the alarm system.

Pete

PIR doesn't tell you if the door is open or closed.

Depends where you're going to use them. It's a good idea to use them on entry\exit doors as a minimum so the PIR's can still be on instant alarm. If someone chucks a brick through the window they've still got the entry time to do something whereas if the door contact is the entry zone the PIR will trigger a full alarm instantly.

But if I am coming in, surely even with a door contact I need a delay to disable the alarm. How does this differ from a delay on a pir.

The Burglar doesnt normally come through the door (in fact only 3% of burglaries are through the front door and thats mostly flats) .

So if you have a door contact on the front door and a pir in the hall, when the burglar comes in through the window the pir sets the alarm off imediately.

Alternately when you open the door there is a thirty second delay (approx).

If you dont have a contact you both have thirty seconds (approx) from the pir.

Pete

and door contact is more convenient because it starts entry delay exactly same time every time you open the door. PIR might let you come in for a yard before it detects you and it might vary. Surely it will detect you moving but not exactly same time every day. Door contact however always reacts at the same time.

If you dont have a door contact on your entry door thenthe burglar will have the same time as you do to disable the alarm system.

Pete

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but without the code!!+ unless you have window in the hall way the only route into the place would be via a pir progd as instant alarm..alarmimg windows and doors is more of a reminder than detection as 9 times out of 10 the door/window area is covered by trap protection anyway..

What I was planning on doing is putting a PIR in the hallway facing the front door with a delay to allow me access to dissable the alarm. There is no window in the hallway which would allow access. The only other access points would all be protected with PIRs on zones that would trigger without a delay. So I guess in this instance there is no real requirement for a door contact.

So if I understand correctly any delay set on a door contact would override a pir detection. So if I were to come throught the door and cross the path of a PIR set to immediately trigger the fact that the door contact has been broken envokes a 30 second delay irrespective of any PIR's.

I can now see why door contacts would be used, but not necessarily in this instance.

How would a burglar dissable the alarm without the code anyway, dont the tamper circuits protect against this ?

Edited January 29, 200521 yr by Chewie

What I was planning on doing is putting a PIR in the hallway facing the front door with a delay to allow me access to dissable the alarm.  There is no window in the hallway which would allow access.  The only other access points would all be protected with PIRs on zones that would trigger without a delay. So I guess in this instance there is no real requirement for a door contact.

How would a burglar dissable the alarm without the code anyway, dont the tamper circuits protect against this ?

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just watch for the post thru the letterbox..you gotta watch those royal mail lot