On Tue, 29 May 2012 15:58:48 +0100, John Rumm
wrote:

On 29/05/2012 14:04, Jim K wrote:
On Tue, 29 May 2012 13:46:54 +0100, John Rumm
wrote:

On 28/05/2012 23:32, Jim K wrote:
On Mon, 28 May 2012 22:25:15 +0100, Dave Liquorice
wrote:

On Mon, 28 May 2012 20:00:48 +0100, Jim K wrote:

low voltage PIR? what from where?

As used with intruder alarm systems. Many to choose from...

including ones rated for exterior usage?

a normal 240v PIR and some sort of a relay?

Some 240v PIRs operate relays rather than being solid state. Wether
the relay contacts are "volt free" is another matter...

precisement - which "some" are those though?

Something like:

http://www.screwfix.com/p/140-standalone-pir/11291

Is certainly relay based. One side is wired to mains live to provide a
switched mains output as its "normal" output. However isolating that
should not be too difficult. (I can't remember if its wired literally,
or if you would need to cut a PCB trace)

thanks john , out of curiosity - how do you know?

I have a couple fitted here. They click when they activate ;-)

i'll happily acquire one and delve in with me meter, standby..... ;)


For reliable "person" detection, then using a pair with some external
logic to detect sequence triggers rather than simple single triggers
is a way that many intruder alarms minimize false triggers.

would such logic be available for an electro numpty like me to buy
reasonably pricedly?

Should doable if you don't mind knocking up something on a bit of
veroboard. (obviously posh ardunio type solutions would also work - but
probably overkill)!

In fact, you could probably use one with no modification, but use its
live out to provide the live in to the relay on the second one. Then use
its live out to drive an extertal relay to give you your no volt
contacts for your trigger. That would take advantage of the timer
mechanism built into them to reset your trigger as well. So the first
would act as an "activation trigger" - it goes live - set to hold on for
the shortest delay available (few secs), that then enables the output
from the second one


mmm intellisting - can you point me to a suitable relay plse?

Also in practical terms how would the 2 PIRs be "aimed" etc to achieve the
reduction in false positives as compared to say a single PIR?

each PIR pointing across the target zone with a gap between? if so how
would that catch persons moving in both L-R and R-L directions?? (i.e. if
PIR 2's function relied on PIR 1 being triggered first, then wouldn;t that
only work "one way"?)

I imagined taping up a lone PIR's detector lens (by trial & error) such
that a vertical "slit" of detection zone was left to trigger the IP cam to
take a snap whilst the subject was mid-frame in the cam's field of view....

Cheers
Jim K