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Topic: Home alarm Monitoring - 4 core cable from Alarm to Arduino to transmit (Read 7233 times) previous topic - next topic


Hi ,

( Am a Beginner so forgive any unknowns!)

I have a home Alarm system with 8 zones ( so 16 points to monitor - on alarm will go NC to NO )

Want to see what best option is to connect my alarm to my PC ( via Arduino obviously )

I have some cable already going from upstairs to downstairs that is 4 core and was thinking that there must be some form of interface I can use to bring the 16 points via interface over 4 core?

Believe that RS485 somehow might be my option - but no idea where to start?  ( Wireless/Wired Network not an option at the moment - budget!)



Hmmm... Where to start? (It will give you a much better result if you split things up into smaller questions... As I am the first to jump in here, I'm going to suggest to the community that we use THIS thread to talk about....

Reading remote PIRs.. the signals, the cables.

Suggest, when you are ready, that you launch new questions in relevant sections of forum, e.g....

a) Monitoring things seen by Arduino with PC
b) What's RS485 good for? How to use?

(You probably don't need RS-485 in this... probably. Especially if you are building your own system, accessing the PIRs directly, not having to "respect" "things" being done by an existing controller panel.)

So... to the question of reading remote PIRs....

You say you have an alarm system. This is already installed, working, has it's own central panel? Or do you just know that you have these PIRs around the house, and some wires coming to a central point, and you want to use them with a control panel of your own devising?

If you have an existing panel, and you want it to continue working, then we have one discussion.
If you're talking about using the PIRs with your own, Arduino, control panel, we have a different discussion.


Ah ha yes- my complete apologies! I advise the same myself in other forums i participate... doh!

To give some further info:

  • Alarm Panel & all sensors in Place- Its got 8 zones and so has 16 connections for wires. My intention was to "piggy back" on each connection and therefore have the ability to "read" the output via Arduino also. )

  • I physically havent got an Arduino but its due in the mail today!

  • Ive been looking at a basic sketch that reads inputs (NC to NO on trigger - like mine ) and can "post to twitter"

I posted in the Networking Forum because my "main" query was how to get the inputs to "speak" to Arduino over 4 core cable. I had seen another question relating to RS485.

So thanking you in advance for any input :) ( pardon the pun!)


Ok! Here we go...

a) Be careful. Don't fry/ compromise your "big" panel.

b) Not to be picky, just trying to help, but....

You speak of NO and NC... they stand for "normally open" and "normally closed".

For a USUAL... and than may not be what you have... INTRUDER circuit, the sensors, typically PIRs have relays (or semi-conductor equivalents) which are NC... in other words, essentually a SPST momentary (doorbell like) switch which is CLOSED (conducts) when NO intruder is seen. And it is as if the burglar helpfully goes over and "presses the doorbell" to announce his presence.

The nice thing about this design is that you can connect multiple sensors, in series, and if any operate OR the burglar cuts a wire (without first creating a short across the cut, with a jumper cable, as we have seen in films) then the system "sees" an intruder. Or if raccoons in your roof space chew through the wire, as actually happened to me. For this sort of thing you have to design for the improbable, and design for maintenence and ongoing testing.

(Fire circuits, for good reasons USUALLY work the other way 'round. Sensors are NO... normally open. When there's heat or smoke, the contacts CLOSE. So these are wired in parallel. There are tricks (end of line resistors... or zener diodes...) to "supervise" these lines, too, so that troublesome raccoons don't kill your system without your knowledge... but supervision in this case is harder to do with digital circuits. Happily the Arduino has 6 analog inputs.)

So... that's a bit about NO/ NC....

c) "16 inputs"... nope. I doubt it. 8 inputs and 8x a connection to ground or Vcc.

Hmmm... unless your panel is very clever/ flexible.

The wires from your sensors: Two from each sensor (or string of sensors)?

Call the two wires from one sensor/string "A" and "B"...

Do A and B connect to the two connection points for their zone?

If yes, my guess MAY be right.

If no, then one, let's call it "A", goes to a common point where all the other A's go, and B goes to one of the two... "NO" and "NC" inputs for the zone? If so, your panel is clever/ flexible. It is set up so that any zone can be for intruder type detectors (NC) OR for fire type detectors (NO).


IN ANY CASE!!! If you are sure, sure, sure that your sensors/ detectors are JUST (equivalent to) the simple "doorbell" type switches discussed above, AND sure that you want to take the chance of frying Arduino and alarm panel, then you can probably piggyback off the signals without "issues".

"Post to twitter..." is this the main thing?

Start like this:

Put an opto-isolator on your alarm system's bell.


One of the many great things about using opto-isolators is that they ISOLATE... the hook up is simple, and worst (likely) scenario is that you fry the $2 isolator. They also provide "level shifting"... if you have, say, a 12v bell and you want to "connect" it to a 5v Arduino, an opto-isolator is the way to go, regardless of the other issues.

Then connect your Arduino to the output of the optoisolator, and the Arduino "knows" when the alarm has been tripped, and can... if you can handle that side of things!... tweet you!

While you are waiting for the opto-isolator to arrive, just hook up a REAL doorbell switch (or equivalent) to where the opto-isolator feed to the Arduino will go, and get to work on the "tweet me" part of this.

Optoisoloators available from the usual suspects...


.. will help you with hobbyist friendly ranges and info.

Hope that "rings your bell".

Oh. That reminds me...

How about setting up a SECOND alarm system alongside your current one?


And finally... The four wires to your PIRs (I assume your system uses PIRS?)... PROBABLY two to supply 12v (+ve and ground) and two to bring back the room empty/intruder seen message in the form of "these wires are shorted together"/"these wires are connected to nothing".



Sorry for my vagueness.. What I meant was *all* sensors are NC and obviously go open on Trigger so I need to detect this.
There are "physically" 16 inputs ( 2 wires per zone , 6 wires per PIR for example ( 2 tamper , 2 power , 2 sensor ) ) , I do assume myself that 1 is common. ( In some of the cases because I was short on Zones I have 2 PIRS on same zone wired in series )

Thankfully no racoons :-) ( Am in Ireland..)

I've setup a 3rd party SMS unit so I'm notified on Trigger of alarm / panic but I would love to report to a private Twitter account that I can follow ( have seen some projects here and on Instructables website also )

Opto - Isolator sounds great - need to dig more on this.. Also love the link on the Arduino alarm system - where is the "Thanks Button" here :)

So - My "core" issue is ( and you've mentioned 6 inputs ..) how to "report" status of the 8 zones via 4 core cable to Arduino ? Im continuing to dig so don't feel like you need to take too much time out , I appreciate your time already , thanks a million!



There are "physically" 16 inputs....

... I infer you are talking about the terminal strip inside the alarm panel.

8 zones, 16 terminals. Probably one input per zone + 8 terminals all connected to the panel's ground rail. Not the only possible scenario, though, so proceed with that assumption at your own risk.

6 wires per PIR for example ( 2 tamper , 2 power , 2 sensor )

... Assuming tamper switches OPEN when tampre detected, the simple thing to do is to include the tampers in the "sensor" loop. Not "perfect", but "works". (Better, of course, is to monitor tamper separately... and have that configured to ring bell if the tamper switch opens at any time. Of course, that means you need extra wires, and you need a "bypass" somewhere, so you can work on the system without going crazy from the bell.)

How can I "report" status of the 8 zones via 4 core cable to Arduino?

What? You want to carry the state of 8 zones from main alarm panel to Arduino over 4 core cable? Why not put Arudino AT the alarm panel, use 9 shortish wires (8 signals, one common ground)? And then, if necessary, one cable to where-ever the Twitter signal goes out from?

If you really want to send the state of 8 zones more than about 4 feet, some serial solution is probably the answer... but are you going to use two Arduinos? One at each end of the 4 core cable, a sender and a receiver? Otherwise which one are you going to do without, and how? If you would be happy accept a little lack of reliability, there is a way to do without the "sender" Arduino. You would always know that SOME zone had been tripped, and probably know which... but it might mis-read once in a while. Details later, if you ARE talking about separating panel and tweeting device.

Serial is no big deal... but is much more work than 9 wires. And, as I said earlier, none of this is as hard as getting something to automatically post tweets. Perhaps best to pin down how you are going to pin down tweets, and then do what it takes to feed your tweeter. Or just get a big dog, a woofer, and then you won't need a burglar alarm.

(Racoons are well worth the occasional nuisance!)


Thanks again TKBYD. You've answered my question really. 2 Arduinos more than likely required.

Yes , I do want to report zone status of any of 8 zones over 4 core to Twitter ( i have the various code & steps read a few times so this should'nt be a problem , I intend on testing proof of concept this week ) via Arduino :)

My PC is upstairs , I suppose I could have Arduino beside alarm and get an Ethernet shield and go that route. I wanted to investigate my options because the 4 core cables is there already.

But from your thread it does seem like I need 2 units to send/receive.

Thanks v much and have a good one!


Ah! So the machine that will send the tweets is your PC? You want to leave it on all the time to do that?

If THAT's the set-up, then put an Arduino by the alarm panel, and send the what the inputs are up to the PC in a serial signal.

For Arduino-to-PC serial linking see...


For the limited data you want to send, you can probably just use TTL level signals, at a "slow" speed. (9600 baud) Use a checksum.


Hi TK ,

For now I will be leaving on but future plan is to use an ethernet shield or wireless option.

Thanks a million for link - will start reading , great site btw  - is that yours?

Have a great one!

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