Installing a switch on a PIR alarm detector in garage

[nigel]

Hi
Recently my garage had to be rebuilt and the switch operated by a key that was fitted outside the garage has been removed. This enabled me to switch off the PIR alarm sensor in the garage as I approached in my car without going into the house and using the alarm panel to disarm the system. I could then park the car and switch back on the sensor as I left the garage before going into the house and switching the alarm off on the panel.

The builder lost the wiring diagram for the relay key switch and has also damaged it. So I thought that I would try to design a simple replacement but using a remote control relay and a project box fitted outside the garage with two LEDs (red & green) to show the status of the PIR sensor.

Please see attached photos of PIR and old key switch with relays.

I would like to know please do I connect the PIR sensor terminals in series with the relay (NO) terminal and common? Then wire in in parallel the red LED across the same relay terminals. Finally, I could connect the green LED across the relay (NC) relay (NC) terminal. I am powering the relay with an independent 12v supply via a mains adapter that I have bought. In fact I have all the components and I am trying to build up a model of my design on a breadboard. I have the relay and LEDs working fine but I am unsure of the connections to the PIR sensor.

Many thanks for your help.

 

[(*steve*)]

The first thing you need to know is if the connection to the PIR is normally open or normally closed.

You can measure the resistance cross the PIR leads that go to the alarm when it's not connected to the alarm.

• If the resistance is low, then the PIR is normally closed and your relay's NO contacts should be used short the PIR leads to the alarm panel.
• If the resistance is high, then the PIR is normally open and your relay's NC contacts should be placed in series with one (either) wire to the alarm panel.

Presumably there are both power and alarm wires going to the sensor?

 

[nigel]

The first thing you need to know is if the connection to the PIR is normally open or normally closed.

You can measure the resistance cross the PIR leads that go to the alarm when it's not connected to the alarm.

• If the resistance is low, then the PIR is normally closed and your relay's NO contacts should be used short the PIR leads to the alarm panel.
• If the resistance is high, then the PIR is normally open and your relay's NC contacts should be placed in series with one (either) wire to the alarm panel.

Presumably there are both power and alarm wires going to the sensor?

Hi

Thanks for the reply.

Yes, there are +12v and - 0v supply to the sensor, together with two tamper wires (NC) and the two sensor wires which I will try to determine their NO or NC status with the PIR beam activated/not activated. I see what you mean about this determining the relay connections. You don't think that wiring the LEDs in parallel to the sensor will affect it?

Many thanks

 

[(*steve*)]

As long as the current draw is reasonably low, you should also be able to power a LED (presumably just one at a time) from the 12V rail as well.

 

[nigel]

As long as the current draw is reasonably low, you should also be able to power a LED (presumably just one at a time) from the 12V rail as well.

The resistance is low across the sensor terminals, approx. 12 ohms, when the beam is unbroken so I take it that is NC and when the beam is broken the resistance is high and therefore NO. So I need my relay connections to make the sensor "think" that the beam is unbroken while I enter the garage i.e. NC. So I wire it accordingly: the usual position NO and connect a red LED in series with its resistor and to NC with a green LED to enter the garage. I hope I've got that the right way around !!! I f not I guess I just reverse it!!. Please note that I am powering the relay from and external 12v source so as not to potentially overload the alarm power supply which has 17 zones (PIRs and door contacts) on it anyway. There is no possibility of a current flowing through my circuit to the PIR sensor and causing any damage or will the LEDs & resistors (370 ohms) prevent this??

Thanks

 

[(*steve*)]

OK, the sensor is NC, that means opening the wire (i.e. disconnecting it) will cause the alarm to go off), whilst shorting it will prevent it from going off.

That means the relay's NO contacts should be across the two sensor wires so that when the relay is turned on, th sensor can go open all it likes, but the alarm panel won't see it.

As for the LEDs, one can simply be placed across the relay coil (with a series resistor -- and I would use 560 ohms) so it comes on when the relay is energised. As for the other one, If your relay has a separate set of NC contacts, I would use them to switch the other LED. Do not use a set of contacts which share a common connection for the sensor and the LED unless you know for sure that the power and the sensor share a common connection (and that you connect it up right).

Since there will be a LED across the relay coil, it's important to also place a reverse biased diode (like a 1N4001) across the relay coil too absorb a voltage spike that will be generated when you turn the relay off. This could rapidly destroy the LED.

LEDs and the diode across the relay contact are polarity sensitive. You can do damage if you connect them up the wrong way. Test the LED/Resistor combinations with 4.5 to 6V DC (from 3 or 4 batteries) to determine the correct polarity as 12V is enough to damage the LED.

The 1N4001 needs to be connected so that the end with the white stripe is connected to the end of the relay coil which is connected to the +ve side of the power source. Connecting it up the other way will short out your 12V.

 

[nigel]

Thanks, I think that I follow you and I understand the point about installing the diode across the coil. I was reading about protection diodes recently, if that's what you mean. Anyway, I have a 1N4001 to hand from an electronics component kit that I bought recently,cheaply on eBay containing lots of goodies!!.

I think that I'm going to try and draw out a plan of the circuit wiring that you suggested. It wont exactly be a circuit diagram!!! well not at first. I'm kinda learning on the job. Thanks for taking the trouble to help me.

 

[nigel]

Hi

I'm not sure how I can wire an LED across the coil of any particular relay since all 4 share common +/-12v supply. I have attached some details of the relay unit that I hav e.