PIR Sensor Setting Off For No Obvious Reason

[paul_g_787]

Hey everyone.

My house alarm keeps setting off randomly. 3am, 4.30am, 5.15am, 4.15am and 5.30am. Every time the alarm reads the same PIR sensor as triggering the alarm.

There are six connections on my PIR sensor.
12V and GND from the backup battery in the main alarm unit.
Alarm loop in/out (this is the loop triggered by the PIR sensor)
Tamper Loop In/out. (This is the loop triggered by a micro-switch that is activated when the cover of the PIR is removed).

Firstly I of course checked the voltage at the sensor which is 13.7V. 
So I cleaned the sensor, the reflector and the housing inside and out. But it still set off the next night.

The next day I did a good sweep for spiders and also covered the sensor with some thick card using gaffa-tape to secure it.
I tested that it would not detect through the card by dancing about in front of the sensor, and also holding a lit cigarette lighter in front, confirming the card was blocking IR from getting through. I could not set it off. So went to bed and it set off again in the middle of the night!!!

So the third night I disconnected the two "alarm loop" wired and shorted them together using a WAGO connector. This time the alarm did not set off!! Hooray so it is definitely something to do with this sensor and not the wiring.

So the fourth night I connected the "alarm loop" wired back to the sensor and left the cardboard in place. Guess what! it set off again!

So it definitely seems to be a faulty sensor unit to me. The question is how shall I proceed in diagnosing it?

I have taken out the PCB and measured the 3 electrolytic capacitors with my BM786 and my MESR-100 ESR meter. These all check out ok.
I cannot see any obvious bad joints or corrosion of any kind.

I have attached some photos of both sides of the PCB and the connection diagram from inside the cover.

Any advice what to check?

I am not sure what the large metal canned component is. Made by MEDER electronic.
I also am not sue what the IC is either KM16-02E. I had a look for a datasheet but couldn't find one as I don't know what the IC actually is.

Edit: The sensormodel is an Aritech EV125E

[paul_g_787]

Alright so had a good poke around on the circuit. everything I can test seems to be in order. Diodes, resistors, capacitors. and all connections seem good.

So for tonight's test I have swapped the sensor PCB with one from another room. I will see if it trips tonight and if the same PCB trips or not.

[Gregg]

Passive infrared alarm sensors are not desirable due to the number of false alarms they cause.  I personally only use the dual tech sensors that combine passive infrared and microwave, both of which have to be triggered before an alarm condition is set by the device.  I have never had a false trigger with 10 dual technology sensors that I have installed in my shop and friend's shops whereas the passive IR only sensors were a constant problem.

[floobydust]

I've seen PIR sensors false-trigger from the usual insects as well as unexpected IR sources (car headlights) shining on surfaces as they for example do a U-turn. 
Make sure the field-of-view doesn't include the street through or near a window.

Lately it's RFI from Smartmeters, they TX high power and it triggers PIR lights as well. I would guess this is your problem. I think the JFET in the sensor somehow amplifies it perhaps through the crystal window. Residential Smartmeters seem to be a mesh network in the 900MHz ISM band but I have a water meter that blasts 433MHz periodically.

You can try an experiment, cover the housing in tinfoil. No IR or RF should get in...

[paul_g_787]

I don't think it is the fact the sensors are PIR sensors is the issue. We have 12 of them that are all original to the house (1995) and we have lived here since 2006 and to my memory we only ever had 3 false alarms (all three due to spiders). Maybe they are not the best but they have proved very reliable in this case.

Only this one particular sensor is having the issue, it has only happened the last week and this sensor is the only sensor on this zone. So I am guessing most likely there is a fault with either the sensor, the zone, or maybe (but least likely) the wiring.

As I said before, tonight I have swapped the hall sensor with a different sensor from another zone (same model). I will see if I get a false alarm again tonight and I will see if it is the other sensor in the same location or the same sensor in the different location that triggers (if any).

If the sensor is the issue the other zone will get the false alarm.
If it is the zone, the wiring or some RFI hitting this one particular spot in the house then I would likely expect the same zone to get a false alarm tonight.

If the issue is RFI I guess I could just cover the inside of the housing of the sensor with foil and connect the foil via a wire to the power supply GND terminal. Thoughts?

[paul_g_787]

Alright so I just got woke up again so thought I would post now as I am up. The alarm was set off and I have now confirmed that it is the sensor which is faulty as after swapping the suspect sensor with another one in a different zone, the same sensor triggered a false alarm in the other zone!

So now how shall I troubleshoot the sensor?

I still am not sure on what the large silver component is. Maybe a relay switch?
And also I am still searching for a data sheet for the IC.

I am off back to sleep now. Goodnight! 

[fzabkar]

I still am not sure on what the large silver component is. Maybe a relay switch?

I suspect it is a 6V reed relay:

https://au.rs-online.com/web/p/reed-relays/3491803
https://www.tme.eu/en/details/np121c902500250/reed-electromagnetic-relays/meder/np12-1c90-2500-250/

https://www.tme.eu/Document/91a2071e33ec78bbbec4e4401ed4ea7f/np-metal.pdf

[paul_g_787]

Ah excellent. I did suspect it was a relay and this has confirmed it! Thanks!

I just tested the relay and it is working fine. it switches a 33Ω resistor in series with the alarm loop and when the sensor is triggered it goes open circuit and breaks the loop. This appears to work fine.
Powering both sensors up on my bench supply and waving my hand in front to set them off showed they are both switching the relay fine and the alarm loop terminals both measure 33Ω when the relay is closed and OL when it is open.

I have just compared the faulty sensor PCB with the working sensor PCB.
I tested every capacitor (ESR and capacitance), resistor and diode in the circuit and also the transistor and relay switch.
Then I repeated the same tests on the known-working sensor and all these components are measuring the same on both PCBs.

The two parts I was unable to test were the IR sensor, and the IC.

The only part I had a problem with was the micro switch.
On the good sensor when pressed it closes the circuit between the two tamper pins and this reads 0.002Ω but on the faulty sensor it measures between 51Ω 348Ω, and it is unstable and fluctuates about even if I press the switch hard.
Could this potentially cause the Alarm loop to trip in some way even though it is the switch on the tamper loop? 

I tried using switch cleaner and contact cleaner but this made no difference so I have temporarily shorted the terminals of the switch with a short piece of wire so it is simulating the switch being permanently closed.
I guess I will see if it sets the alarm off again.

[paul_g_787]

Well bypassing the tamper switch made absolutely no difference. The sensor is still setting off the alarm loop.

I tried swapping the sensor with another in a third zone and the same sensor set off again so I am absolutely convinced the sensor is faulty, however as to what the fault is I still do not know.

I have been looking at some replacements but they are all different in the fact that the resistor in series with the two alarm loop terminals is a different value.
My current sensors are 33Ω but some are 56Ω, 4.7K etc.

Does this make a difference?

[floobydust]

I would say the sensor is 27 years old and for the time and hassle, just replace it.
I did have one with small spiders set up shop underneath the PIR sensor and IC. You can get corrosion from flux, bad vias, tin whiskers. The pyrosensor a noisy JFET leak as well.

Your alarm panel should specify the loop resistance for the open/shorted loop detection - if you are using that. Typically 4k7-5k6 depending on the panel and 2k2 for fire loop for example. These are usually seen as an external resistor connected at the sensor's terminal block and also the feature programmed/enabled in the panel as well.
The 33R is typical for power feed and the tamper switch also has a low value current-limiting (smoke) resistor. Their value does not matter.

Just get a new PIR that runs off say 8-16VDC, they are pretty much commodity items.