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Hi, I'm interested in adding some harware to my SRT8 that's triggered when the factory alarm goes off. Is there a trigger point in the wiring anywhere that changes state when the alarm goes off? I thought of hooking up to the horn but it will also then trigger every time I use the horn etc. I was looking to see if there was a simple CAN interface module that would do this but couldn't seem to find one that wasn't virtually a new alarm system.
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I don't believe anyone's done a detailed workup on anything like this, but you may find this website useful if you want to tackle something yourself: https://techauthorityonlinedemo.extra.chrysler.com/service/repair/wiring/view/classic.htm

Enter your year, make, and model and it will give you wire colors for pretty much every module in the car. It's still going to require some effort on your part to figure out what is hot when, but it's a step in the right direction.
 

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Thanks for the reply. I'll take a look at the link and see if I can figure anything out. I'm guessing that it's all going to be in CAN logic somewhere but you never know...
 

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Why not use a relay that activates a new circuit to perform whatever task you want whenever the alarm goes off? Just find something in the alarm wiring that pushes out a signal when it is activated and feed the relay.
 

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Hi, that was my first approach but I can't find anything unique that's triggered. The horn pulses and the lights flash but they're also switched on at other times too. I was hoping there may be a signal wire to connect to that indicated an alarm activation state.
 

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Hi, that was my first approach but I can't find anything unique that's triggered. The horn pulses and the lights flash but they're also switched on at other times too. I was hoping there may be a signal wire to connect to that indicated an alarm activation state.
You just need to find something that comes out of the alarm wiring that puts off a signal when activated. I just wired up a relay for my fan fuse block off of my LC8i Remote wire and it works great.

You just need to find that signal wire. Nothing like a wiring puzzle to figure out. Have you tried a local stereo store to see what they have to say?
 

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Thinking it through a little more, I would guess that the alarm is CAN based and would send a digital communication signal to the modules that the alarm sets off. If that is the case it might not be possible to distinguish between a regular horn honk and an alarm horn honk (for instance) without going into the CAN network and looking for activation signals. If this line of thinking is correct you might have better luck on a site like canhack.org. They do CANBUS sniffing and signal intercept and someone there might be able to at least give you more conclusive answers about how it works.

A small word of warning though. If you're not familiar with coding or digital embedded systems then trying to work around the CAN system can be a tall order. I gave up on a project that involved CAN interfacing because it looked too time consuming.

A possible work around might be to deactivate the EVIC setting that makes the horn honk at the same time the lights flash when you unlock the car. Then use a discrete digital AND gate to give you a trigger when both the horn and lights are going. No need for CAN interfacing that way. I can get into that more if you want. I still don't know where those wires are, but I can help with the necessary circuitry.

A relay is going to be too slow (most likely) for an alarm style circuit. It has to magnetize and click over, which can have a few seconds of delay depending on the voltage of the trigger. A Bipolar Junction Transistor amplifier circuit might be a better way to go since it would have zero delay, but it won't be capable of powering a 40A circuit (like a relay would) without a beefy heatsink. More like 2-10A tops depending on the BJT.
 

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Discussion Starter #9
Thinking it through a little more, I would guess that the alarm is CAN based and would send a digital communication signal to the modules that the alarm sets off. If that is the case it might not be possible to distinguish between a regular horn honk and an alarm horn honk (for instance) without going into the CAN network and looking for activation signals. If this line of thinking is correct you might have better luck on a site like canhack.org. They do CANBUS sniffing and signal intercept and someone there might be able to at least give you more conclusive answers about how it works.

A small word of warning though. If you're not familiar with coding or digital embedded systems then trying to work around the CAN system can be a tall order. I gave up on a project that involved CAN interfacing because it looked too time consuming.

A possible work around might be to deactivate the EVIC setting that makes the horn honk at the same time the lights flash when you unlock the car. Then use a discrete digital AND gate to give you a trigger when both the horn and lights are going. No need for CAN interfacing that way. I can get into that more if you want. I still don't know where those wires are, but I can help with the necessary circuitry.

A relay is going to be too slow (most likely) for an alarm style circuit. It has to magnetize and click over, which can have a few seconds of delay depending on the voltage of the trigger. A Bipolar Junction Transistor amplifier circuit might be a better way to go since it would have zero delay, but it won't be capable of powering a 40A circuit (like a relay would) without a beefy heatsink. More like 2-10A tops depending on the BJT.

I was afraid that it was probably all CAN based but hoping for an easy trigger point somewhere. I'd thought about logic gates (thanks for your offer of design help) but it probably means multiple connections to correctly detect just an alarm state. From what I can see looking through the circuit diagrams, there is no alarm circuit as such. Different sensors are monitored and when one of them is activated lights are told to flash etc. but it looks like this control logic is all built into the computer.
 
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