Chapter 6: Security/Automation Sensors

The book "Practical Arduino" by Jonathan Oxer and Hugh Blemings covers a range of projects and techniques from basic to advanced. [Book site] [Buy online]
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Joined:Sun Sep 15, 2013 6:49 am
Chapter 6: Security/Automation Sensors

Post by mdance » Thu Oct 03, 2013 3:55 pm


I am new to electronics and using the chapter 6 alarm system project as my first project so please excuse me if my questions or assumptions are wildly incorrect.

Will this project work with a single two wire PIR sensor (see the links below for the exact model)?

Can I make this work with a 7V battery pack supply (I don't have a 12V power supply on hand)?

With only two wires on the PIR, how do I connect it to the arduino side? In particular I am worried about how to avoid damaging the arduino with too much voltage/current.

I am not sure if my resistance values are correct but this is what I measured from the PIR:

11.50 MO
10.50 MO
9.50 MO
9.86 MO
9.60 MO
12.30 MO
10.83 MO
10.30 MO
9.98 MO

External Links:

PIR Sensor:

Alarm System Details:
Breadboard Setup

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Re: Chapter 6: Security/Automation Sensors

Post by jonoxer » Tue Oct 08, 2013 12:17 am

I have bad news and good news: that particular alarm system doesn't use the standard connections of a traditional system so it won't work as described in Practical Arduino, but there is an alternative method that may do the job quite easily.

Because that system uses a 2-wire interface for both power and data connections to PIRs, you can't just measure the resistance across the outputs like a normal sensor. The alarm panel and the sensors implement some kind of scheme for passing data over the same connections as power: I don't know anything about that particular alarm, but it could be simple AC coupling, or something equivalent to a Dallas Semiconductor 1-wire bus system.

So you'll need to ignore the existing wiring to the sensors and detect their state in a different way.

What jumps out to me is that there are status LEDs on both the sensors and the alarm panel. That means you could run (for example) 4-core (2 pair) security cable to each sensor, use one pair for the regular connections, and use the other pair to sense the state of the LED. You could put an opto-coupler input across the LED connections, and sense the output. Or you could do the same thing centrally, detecting when the zone status LEDs in the alarm panel turn on.

Which approach you take depends on how the alarm system connects to sensors. If you can link multiple sensors on the same pair of wire, you may not be able to differentiate between them at the alarm panel which may only show one LED for all the connected sensors. In that case you'd need to use spare pairs in the cable to link directly to the status LEDs inside the sensors. Or if you're really lucky and the alarm panel shows a different LED for every sensor, or for every zone (and you only have one sensor per zone) you can do it all in one place without extra wiring to the sensors.

So I don't think you're out of luck, you can still connect your alarm system to your Arduino but you'll need to use a different technique to the one shown in the book.

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