Some of you may remember (but probably not) about a year ago I posted a project to make a replacement siren/alarm for my Crossfire. It was an entirely non-commercial project for my own amusement. I have been using the original 6VX version that used the light flash relay for the lock/unlock function for the last year. This is the relay that flashes the lights when locking and unlocking. I used a fog light connection for the theft/panic alarm signal. I was never really satisfied with this as it is rather crude and not using the functionality that actuated the actual Bosch siren/alarm. After much study and help from folks on the forum I/we determined that the original Bosch siren used the ISO9141 (K-line) communication protocol. Much thanks to Erwin Oegema and Klay Watson for their help in finding and decoding the protocol.
Well, I finally have a working prototype of the alarm that works quite well. There are five basic functions, 1. Lock, 2. Unlock, 3. Theft Alarm, 4. Panic Alarm, 5. System off/standby. All of the functions broadcast a different message based on the status. The messages use text to voice computer translation and sound effects from royalty free websites. These are recorded to an SD card and replayed by an Arduino Pro Mini controlled MP3 player. The amp is a single chip TDA7379 20Watt bridged stereo amp driven by a DC boost controller to raise the 12-14VDC of the car to 20VDC at about 3Amps to achieve the 20watt output.
The Arduino code is written in C++ using Microsoft Visual Studio with the PlatformIO extension.
The PCB was designed using EasyEDA from JCBPCB. I made a mistake on the 7V6 version of this board and replacement boards have been ordered. I swapped the source and drain connections on the high side P-channel MOSFET. You can see the kludge repair in the top view picture. The MOSFET turns the DC boost convertor and the LM7805 5VDC regulator on when the amp and MP3 player are needed. These are turned off and the Arduino is put to sleep to conserve power when the alarm is inactive.
There are various chips available to interface the K-line to microcontrollers. I went with a DIY approach to signal shift the 12+VDC K-line to a 5VDC signal usable by the Arduino. The K-line drive the LED side of a PC817 opto coupler and the transistor side is connected to the Receive port (Rx) of the Arduino. This signal is also tied to pin 2 (interrupt 0) of the Arduino and is used as an external hardware interrupt to wake the Arduino when data is received. Idle current is < 2ma so it will not drain the cars battery.
Each function has the volume set to a level appropriate to the need. The theft and panic alarm are VERY LOUD and really annoying. The Arduino wakes when serial data from the K-line is received. This is decoded and the function requested by the PSE (Pneumatic System Equipment) is performed.
On the 7V7 board I have added an LCD screen port to facilitate any future needs for troubleshooting. There were also some component placement changes to facilitate easier assembly.
The project has been quite a learning experience and fun. I could have bought a replacement siren for MUCH LESS than I have spent in $$’s and time making my own, but…
If anyone is interested in discussing this project, you can message me here at this group. I am not offering this as a completed alarm kit or any other commercial product. I will make any files available to others that feel that self-abuse is a valid form of entertainment! As always, use of any of this information is at your own risk. There is no warranty of use or fitness implied or expressed.

 

Well done!

 

Shardin
Where would this new replacement be placed within the Crossfire, since there is no enclosure like the OEM siren?
Curious....
Dennis
DTMenace

 

I will be using a plastic enclosure and plan on putting the unit in the cowl space where the original alarm resided. The board in the picture is only 4" (100mm) X 2{ (50mm).

SIDE VIEW OF ENCLOSURE

 

Hello. I apologize in advance for my English. I'm from Brazil and I own a Mercedes ML350 that uses the same alarm siren. I faced the same problem that Crossfire owners faced. I read your posts about it and also adopted a similar solution using the signal from the turn signal lights. But I was very interested in part 2, regarding the decoding of the K-line protocol. I would be very grateful if you could share your knowledge and the documentation of this project, especially the K-line protocol codes. I just joined the forum and I don't quite understand how it works, and this is the right channel. Thank you in advance and I apologize for any inconvenience.