Arduino processing two wire alarm signal - how to?

May 14, 2013 6:34 AM

I bought this cheap wireless driveway alarm system (a battery-powered motion sensor with base that beeps when someone pulls into my driveway) and one of its features is that you can connect it to "to any security system that has a two wire alarm input". I was wondering if anyone could point me to a guide for processing that output using the Arduino? I'd love to - for example - turn on a light automatically when I pull into my driveway, or send an e-mail, or other cool stuff.
posted by exhilaration to Technology (8 answers total)

A two-wire security system probably accepts "contact closures" or "contact openings" -- basically, a switch opens or closes, and the system sees that and does something. To sense that on your Arduino, you hook it up just like any other switch. Here's a tutorial for detecting a button change on an Arduino, which is probably close to what you'll need.

It's probably worth calling an electronics-geek friend to poke around on the security system with a voltmeter, just to make sure there's nothing that might zap the Arduino, but if the thing you're connecting to the Arudino is battery powered (with less than 5-6 volts) then you're pretty safe.
posted by spacewrench at 6:53 AM on May 14, 2013 [1 favorite]

You could try checking out the tutorial on Xively.com (the new name for cosm {the new name for pachube}) for ways of interfacing with your sensor feeds.
They are all about internet of things and frictionless use of sensors.

I believe you could then use your Xively feed as an input to IFTT to have emails sent to you or cameras turn on or whatever. (This stage may not be necessary, you may be able to get Xively do that for you)

Plugging it into your Arduino should be the same as a switch as Spacewrench says.
I wouldn't expect it to blow up or anything, but it is worth finding out what kind of voltage is on the output. It's almost certainly fine, being a domestic system and battery powered, but it never hurts to check.
posted by Just this guy, y'know at 7:11 AM on May 14, 2013

Best answer: I checked the website you linked to, it is actually a three wire alarm input, not a two wire, which means it won't act exactly like a simple contact.

Check out this MAKE article. It's almost exactly what you want to do I think.
posted by Just this guy, y'know at 7:23 AM on May 14, 2013 [1 favorite]

Response by poster: You're right, there are three contacts - looks like they made a mistake on the product page.
posted by exhilaration at 7:36 AM on May 14, 2013

Best answer: From zooming in to the picture, it looks like the 3 pins out are +, -, and signal which is exactly what you want. + goes to Arduino 5v, - goes to Arduino ground and the signal goes to your favorite input pin.
posted by plinth at 8:01 AM on May 14, 2013

Plinth, which picture can you read that from? I see a three-pin connector, but I can't read any labels.

If the output pins are in fact +, -, out, you probably DON'T want to assume it's OK to hook up + to Arduino Vcc. In particular, if the sensor is battery-powered, and it supplies voltage on that pin, you shouldn't connect it to the Arduino's +5V (which, presumably, is also powered by a separate power supply).

This kind of problem is exactly what multimeters are good for, though. I doubt it's a very complicated interface.
posted by aaronbeekay at 9:54 AM on May 14, 2013

The picture with the connector on it that says to refer to your alarm system manual. One is clearly +, one is -, the other is unreadable (but will be signal).
posted by plinth at 11:26 AM on May 14, 2013

Response by poster: Here's the picture from the manual: http://i.imgur.com/XezlIgH.jpg
posted by exhilaration at 11:39 AM on May 14, 2013