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Thread: Sharing PIR

  1. #1
    Automated Home Jr Member
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    Default Sharing PIR

    OK, probably something of a dull question to those who know the answer but... can you share the same PIR sensor with an alarm system and Cortex? I am reonvating a house and inherited a fully functioning alarm system with it. As it is here already I figured I would retain it. Only problem is I don't want to install a second load of pet friendly PIR in the same rooms, as the PIR are already there it would make more sense to use them. However I can't get my head round how much resistance wiring a connection to an idratek module would contribute to the alarm system, ie would it cause a problem and confuse the system into going off all the time? Would wiring in series be better or parrallel? any advice appreciated.... I am wary of experimenting atm as I don't want to trigger the alarm, the next door neighbours have already moaned about several false alarms from the fire alarm system (had the brigade round and everything!) Cheers, Doug

  2. #2
    Automated Home Legend Karam's Avatar
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    It is not straightforward. The alarm system PIRs will have a voltage across them supplied by the alarm system, so they can't simply be wired in parallel with the IDRATEK digital inputs. Depending on the alarm system one possibility might be optoisolators in series with the sense loop, with the isolator output driving a digital input on the IDRATEK system. You'd have to work out polarity, check the normal sense current is acceptable for use and that the voltage drop of the isolator LED would not cause a problem. Another possibility is the use of an intermediate double pole relay which is energised by the PIR (coil connected in series with PIR and voltage source) and then one set of contacts of the relay feeding into the alarm sense loop and the other set into IDRATEK.

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    Moderator toscal's Avatar
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    Some PIRs have two contact options, a normally closed one and a normally open one. So it may be possible to connect the one that the alarm doesn't use to IDRATEK digital input and just program accordingly.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.
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    Thanks both for the replies, I figured it wasn't straightforward! I will have to check the pir and see how many contacts there are, otherwise I will be on the lookout for something suitable. Another thought has just occurred, what about taking some sort of signal from the onboard LED driver? If the led lights on the pir whenever presence is detected perhaps something as simple as a photodiode set up (thinking power meter monitoring here) would work and not interfere with the alarm wiring at all. I think I will have to experiment with that one. Doug

  5. #5
    Automated Home Legend chris_j_hunter's Avatar
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    IIRC, toscal was keen on the Optex SeQuad PIRs - two relay outputs, too, IIRC

    Chris

  6. #6
    Moderator toscal's Avatar
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    Here is a press release on the Optex http://www.home-security-action.co.u...detectors.html
    and Optex europe can be found here http://www.optexeurope.com
    I have a copy of the SeQuad manual somewhere. See if I can find it.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.
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    Automated Home Legend chris_j_hunter's Avatar
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    manual available by registering with Optex & then downloading ... the two contacts (relays) seem to be one for central area, one for outer areas, with the device intended to be corner-mounted - the assumption being the relays will be connected (*) & unwanted visitors will enter via openings in the walls & then move into the room ...

    (*) in parallel, presumeably - the contacts being NC - so it takes both to trigger ...
    Last edited by chris_j_hunter; 7th March 2010 at 05:44 PM. Reason: correction

  8. #8
    Automated Home Sr Member mcockerell's Avatar
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    Quote Originally Posted by Karam View Post
    It is not straightforward. The alarm system PIRs will have a voltage across them supplied by the alarm system, so they can't simply be wired in parallel with the IDRATEK digital inputs. Depending on the alarm system one possibility might be optoisolators in series with the sense loop, with the isolator output driving a digital input on the IDRATEK system. You'd have to work out polarity, check the normal sense current is acceptable for use and that the voltage drop of the isolator LED would not cause a problem.
    It may be possible to use the method that I descrbe in this thread:

    http://www.automatedhome.co.uk/vbull...ead.php?t=2674

    The input threshold current for the HCPL-3700 is typically 2.5mA; there is also a 3760 device which has a lower input threshold current of just 1.25mA.
    It may be possible to wire one of these into each of your PIRs.

    Hope this helps.

    Martin

  9. #9
    Automated Home Legend Karam's Avatar
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    Indeed the HPCLs might be a good option, I had hesitated to suggest using isolators in parallel with the sense loops since I'm not sure what sort of currents constitute an alarm condition on 3rd party security systems (also different types such as those that use resistors to sense fault conditions). Maybe someone with more wide ranging experience in that area can comment or maybe it has to be done on a trial and error basis with the specific alarm system.

  10. #10
    Moderator Gumby's Avatar
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    One thing to consider - there is a functional difference between PIRs for alarms and PIRs for automation.

    Most Alarm PIRs are deliberately de-sensitised to reduce or prevent false alarms. However, for the most responsive HA you want the PIRs as responsive as possible, Cortex filters in software when using them for alarm functions.

    Although the sensitivity is switchable in many 3rd party PIRs, if you whack it up to max then you increase the chances of your burglar alarm false alarming.

    With regard to pet insensitivity it obviously depends on your lifestyle and whether you shut your pets in particular rooms at night or when the system is armed, but is pet insensitivity actually critical in most rooms (both for HA and alarm function)? Perhaps there is a different way of getting the same effect.

    I have pet insensitive sensors on my normal alarm system, and they are so insensitive that they don't detect me half of the time (and I'm not the smallest of targets), especially at night when I forget to disarm and then set the alarm off - and yet they were over-sensitive enough to trigger from my medium sized dog and really upset the poor creature who had to suffer from the internal siren until I got home.
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