External PIR to be active for only 4 hours per day

[Bazza]

I'm looking to set up an external infrared PIR with a 24 hour timer so that it will operate an alarm between the hours of midnight and 4am each day. The alarm will be powered through a relay connected to the PIR.The idea of the relay is to set a time limit for the duration that the alarm will produce noise.

One problem I have is that if I put the mains driven timer at the the beginning of the chain it will power up PIR and this will make it activate its output briefly and so set of the alarm. It seems that every PIR send spower to its output side whenever it is first powered up. Wth th etimer powering up the PIR every night at midnight , this means the PIR will kick in briefly and trigger the relay which will trigger the alarm sounder.

I'm very much a an amateur DIY'er so would appreciate suggestions on how best to go about designing the system.

[Andy Burns]

Your neighbours are going to love you

The maximum an alarm can sound for is 20 minutes.

[GB]

Could you keep the PIR on 24/7 but put the timer in the relay/sounder part of the circuit?

[John Rumm]

Does your PIR just have a mains switched live, or does it have a pair of "no volt" contacts?

If it has no volt contacts, then leave the power supplied to the PIR all the time, but connectone side of the contacts via your external relay. Hence it will only be able to provide an output when the relay is activated, even though the PIR will respond and switch at other times.

If the PIR only has a internally wired switched live, then you may need an additional relay and delay circuit to disable its output for a period of time after power on.

[alan_m]

PIRs used in burglar alarms often have a counter. They need to be triggered a number of times in quick succession before they trigger the alarm. This is to prevent false alarms. I'm not sure that PIRs designed for external lights do this.

Is the alarm noise maker going to be internal or external to your property. If external you don't want false alarms to annoy you and your neighbours. Possible just a minute or two is enough to wake everyone.

How are you going to stop the local fox, bat, moth or cat from triggering your PIR?

[Rob Morley]

You can use a relay with its solenoid fed by the PIR, and its normally- open contacts switching on the timer output. When the PIR triggers it activates the relay and switches its normally open contacts to closed, but the alarm will only sound if the timer is also supplying power to the switched contacts.

[Bazza]

Dont worry, I'm only looking to have the alarm operate for around 10 seconds.

[Bazza]

The PIR is a STEINEL IS 1 motion detector which does not have "no volt" contacts.

[Bazza]

Super, I think I've just about got that that. I'm going to experiment today. If I've got it right the relay is connected permanently to the output from the timer and the output from the PIR is the signal that triggers the relay that is set up to close the NO contacts for the duration time as set in the relay. So far all my relay work has been with 12v minature jobs so to keep this simple, I guess I would need to find a programmable dital LED realy that works on mains voltage?

[John Rumm]

Looking at the wiring diagram for that:

It looks like it only has a switched live out...

Could you power the PIR full time, and then connect its switched live trigger output to the one of the NO contacts of a relay, and the other back to the alarm trigger input. The coil of the relay being driven by the timer. That way the PIR won't generate a false trigger at power up, and any triggers it does generate will only make it as far as the alarm when the output relay is enabled by the timer.

[Bazza]

Hi John, Yes it looks like powering the PIR full time is the way to go and then taking the output from the PIR as the trigger to kick off the relay that takes power from the timer. The relay should be a "time off" after say 10 seconds from the trigger.

That being the case, I need to find a relay that will take a 220v trigger and have an adjustable duration before switching off. I came across this, but there's no confirmation yet that it will take a 22v trigger input.

[John Rumm]

I think you might be over complicating it... :-) The timer drives the relay, not the PIR. The contacts on the relay just "gate" the output of the PIR. So when timer is "off" it does not matter what the PIR does - its output can't reach the alarm.

[Rob Morley]

You could do it that way, but is that less complicated? I had the PIR powering the relay and switching the timer output, just so the relay isn't activated for four hours every night.

[Bazza]

[Bazza]

I got it set up after receiving some bits as follows>>>

1. Constant mains supply to PIR and 24 hour timer with.
2. Solid state mains relay takes power from timer and switches output from PIR.
3. Output from first relay switches a 12v DC supply that connects to timer board.
4. Timer board supplies instant "On" 12v DC to sounder and cuts off after 10 seconds.

[Rob Morley]

That ought to do it. If, as someone suggested, you get lots of spurious alarms then you can make a Mk2 using an Arduino or similar to filter detection events.