DIY Intruder Alarm -how many PIRS per zone

dogdig

Just replaced 20 year old PIRS with Activ8 PIRs going to a Pyronix Paragon Plus control panel.
Originally there were 7 PIRS in series on a 6 way cable on one Zone on the old system and these worked well.
On the replacement I cannot now set the panel with the 7 new PIRS in series. It looks as though the relay contact resistance (about 16 ohms per PIR) in the modern PIRS is unsetting the panel as the overall loop resistance with wiring comes to about 120 ohms in this circuit.
Some rejigging of the wiring will be needed - a major undertaking if carpets, floorboards etc have to be removed for access. Before I get into this how many PIRS per zone do installers recommend please.
And is there an alternative to extra wiring or cheating by redeploying tamper wires etc. As it is a domestic situation I could accept the lack of tampers on this particular circuit.
Just an idea - but has anyone used a 5v reed relay in a similar circuit with the latching voltage (current limited to about 20 mA via resistor) running through the PIR relay contacts with the switch contacts of the relay going to the control panel in place of the original direct connection to the PIR loop. I dont know how much current the relay contacts in PIRs will stand but I fancy it will be quite a bit more than 20 mA.
Hindsight is wonderful, I know and it would have been great to have used 8 way cable originally, etc. But I am where I am - stuck at the mo - and would welcome any bright ideas as to how to proceed - thanks!

Jnealon

As a start you could split up those pirs in to 4 zones using the existing cable and wiring the pirs in dual eol. Then run a new cable to the other 3

KoolKid

There should always be only one PiR per zone..

dogdig

Thanks lads - ideas are most welcome!

The relay contact resistance on the scrapped PIRS was 1 ohm or less and so the original loop worked well until old age took its toll.

With tongue firmly implanted in cheek, I suspect the change of spec to 16 ohms relay contact resistance must be a conspiracy between the PIR people, wiring and panel manufacturers to sell more equipment

But I reckon for now the best solution could well be to use your solution jnealon - but I still wonder about the feasibility of a reed relay - unconventional - but is it a practical possibilty? The relays in the PIRs are almost certainly solid state - dunno if these would cause any problems - but the back emf in the reed relay should be cancelled by a damping diode.

derek_g34

As Koolkid said, one per zone. Otherwise, if and when you have a false alarm, you wont know which detector has activated. Also, wiring the detector's into seperate zones will give you more programming options in relation to exit entry, access and part guard.

dogdig

Yeah - but as I explained in previous posts one wire per PIR is simply not possible even though this may be the modern ideal.
My question is a plea for help as to how to update an old system with the minimum of disruption. If there is no reference information to show how many PIRS can fairly safely put one one Zone I will have to experiment further. From what I have seen so far I guess it is 2-3. If it is, I can get away without extensive rewiring.
If the most really is one PIR I will have to think about disconnecting the problem loop from the Control Panel and using this loop to switch a 555 or transistor driving a 5v reed relay. The reed relay contacts would go to the Panel to take the place of the old loop. The reed relay contact resistance is only an ohm or two so the Control Panel should then be happy and it has enough take off points to power such a low consumption device.
I hoped others might have gone down this path and could pass on the benefit of their experience.
Sadly we are where we are - and the present system would be seriously difficult if not impossible to rewire to put separate loop to each PIR.
In the good old days there was virtually no limit to the number of PIRS on a loop as the contacts were mechanical relays with very low contact resistance. However these could be somewhat unreliable.
The modern SSRs are all well and good but the "relay" contact resistance at about 16 ohms is at the root of my problem. When the original system was installed about 20 years ago these fiendish inventions in PIRS were simply unheard of.
So, thanks everyone for your input - I will give the problem some more thought over the next day or two.
:cool:

Jnealon

OK. Here's what I would suggest if I was doing it

• Sell the low end pyronix stuff on ebay and get a HKC panel with the ID pirs. This will allow you to put them all one the one loop
• Get a europlex signet, wire up four pirs using dual eol and use wireless pirs for the remaining few

On the pyronix data sheet it will tell you at what resistance the circuit will go o/c. This varies but could be around 2k

dogdig

Yes - specs say o/c at 2k ohms - so in theory the loop should work. It doesnt though - so I fancy my panel is now out of spec - but what the heck - I think I will have to upgrade and may as well in view of the present problem.
Jnealon - my thanks for your input - and also to the others who gave the problem some consideration.