Alarm sensor circuit question

My home alarm has door sensors as shown in the attached pic. If I disconnect these sensors from the alarm panel, how could I monitor them using Arduino inputs? Maybe connect each one to an analog input and trigger on specific values? Any advice will be greatly appreciated.

Beware of pop ups on the picture.

Weedpharma

Are you trying to have these sensors work as inputs for the arduino so that you can take control over your home alarm system? You probably could use digital pins for this because its either breached or not breached.

You will need to find out how much voltage is coming out of the trigger and drop it down to 5v so you dont blow your board.

I am guessing that the 2k is to make the system able to detect any interference with the circuit, ie to short the wires before breaking the circuit.

If that is the function then you could either build a set of comparators to detect any one breach then use digitla input to the Ar, or multiplex the analogues into one Ar analogue pin.

typical home alarms are 12VDC
typical sensors are normally closed. that means 12VDC is present when 'safe' or in the safe position.
when you break the circuit, open the door, move past a motion sensor. break the glass on a glass break sensor, or cut the wire, the circuit opens and that indicates an alarm.

all you need to do is to create a voltage divider for that circuit, say 10k over 7k and you will have an value you can sense of 5v when the sensor is safe.

as you can see in the nasty picture with pop-up add (use imgur or some such) there are multiple sensors in series and one switch on the loop. when all series are closed, you get a value based on the 2k resistor. when any one BUT the parallel switch is open, you get nothing.

weedpharma:
Beware of pop ups on the picture.

Weedpharma

What? Don't you want to know how "You Can Earn Big Money In A Snap"? :smiley:

When I encounter pop ups, I loose interest.

Weedpharma

weedpharma:
When I encounter pop ups, I loose interest.

Weedpharma

Yeah, me too.
Much better to just attach an image, then after posting, copy the attachment's link, edit the post and paste the URL into tags using "Insert an image". Then these forums host the image, rather than another site.
(Duane Degn posted a thread on this a few weeks ago, but it's already buried.)

OldSteve:
Yeah, me too.
Much better to just attach an image, then after posting, copy the attachment's link, edit the post and paste the URL into tags using "Insert an image". Then these forums host the image, rather than another site.
(Duane Degn posted a thread on this a few weeks ago, but it's already buried.)

when I have to do that much work, I loose interest.

dave-in-nj:
when I have to do that much work, I loose interest.

It's no more work than uploading to another site, copying the URL, then inserting the URL into a post using "Insert an Image".
The only difference is that the forum hosts the image.

OldSteve:
It's no more work than uploading to another site, copying the URL, then inserting the URL into a post using "Insert an Image".
The only difference is that the forum hosts the image.

i drag to my imgur.com free hosting site.
copy url and paste.
then i always have my pics available.
and this site hosts the image

dave-in-nj:
then i always have my pics available.

They're always available here too, in >Profile >Show Posts >Attachments.
I just have that page bookmarked.
Horses for courses. I was just pointing out another alternative to using image sites with annoying popups. We've both shown there's no need for that.

Originally I was wondering if I could completely disconnect the sensor circuits from the alarm panel and connect them to the Arduino board. But if I read the above comments correctly I should connect the Arduino inputs in parallel with the sensors, still connected and powered by the panel... is that right? The door sensors are acting like they're normally open - 5.5v with door closed and ~13v with door open. dave-in-nj's reply sounds the easiest, a voltage divider across the 13vdc.

DO NOT CONNECT the pins directly to the Arduino while connected to the panel.

The voltage is too high.

Weedpharma