triggering an alarm by cutting one wire
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=86688
Printed Date: May 16, 2024 at 8:14 PM
Topic: triggering an alarm by cutting one wire
Posted By: sburban
Subject: triggering an alarm by cutting one wire
Date Posted: December 05, 2006 at 8:58 PM
I need some help with this. I am installing an alarm in a 2005 kenworth truck with a sleeper. It has a refer box on the back with a negative trigger switch on the duoble doors, what i need to do is if anyone was to cut that wire which runs under the truck the alarm will trigger instantly. is there a way to wire a relay or something that will do this. Thanks....
Replies:
Posted By: Ween
Date Posted: December 05, 2006 at 9:30 PM
hi, you'll need to set up the switch at the door to open its contacts when the door(s) are opened. wire a resistor (4.7K ohms or so) with one side to 12 volts battery, the other to your wire coming from the box you'd like to protect. connect this wire also to the positive trigger door switch input of the alarm module. when the doors are closed and wire not cut, the ground on them will keep the postive trigger low. once the wire is cut or door opened, the resistor inline will apply a positive voltage to the door trigger input, setting off the alarm....DEI used to have a closed loop trigger module, not sure if they still make them however. hope this helps mark
Posted By: dualsport
Date Posted: December 05, 2006 at 10:51 PM
Mark's description should work if you have an input trigger that's not being used by anything else, and doesn't have an internal pulldown that would override the external 4.7k resistor being used-
In case you don't have a trigger input available for the purpose, you could just use a BS170 or some similar MOSFET transistor as the driver for a trigger. Since the MOSFET gate input is such high impedance, you could use a very high resistance for the pullup, which has an advantage of using less standby current from your battery.
Only when the wire is cut, the pullup resistor will turn on the transistor to drive your trigger.
Posted By: Ween
Date Posted: December 06, 2006 at 2:30 PM
hi...DEI 502T Closed Loop Trigger Module m
Posted By: sburban
Date Posted: December 06, 2006 at 4:06 PM
thanks much ereryone
Posted By: hotwaterwizard
Date Posted: December 09, 2006 at 10:24 AM
When the switch is in the closed position the relay will latch until the door is opened. This will trigger the alarm if it is still set. If someone cuts the wire on either side of the switch the relay will drop out and trigger the alarm. If the truck is a 24 volt system just use a 24vdc Relay. Pins 30 and 87 follow the switch action and you would put those wires in place of the existing switch wires to the alarm. Keep it simple! 
------------- John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Posted By: splaudio
Date Posted: December 09, 2006 at 8:02 PM
ultra has that wire on there higher end models but it is a + wire but with a relay u can flip the polarity...
Posted By: dualsport
Date Posted: December 10, 2006 at 12:33 AM
Using a relay would be fine for a home system or something hooked up to AC power, but it would probably cause excessive standby current drain on a car alarm system that relies on batteries. Using transistors or CMOS logic gates would be best if the alarm doesn't provide an input for the purpose, and lower cost than a relay to boot-
Posted By: hotwaterwizard
Date Posted: December 10, 2006 at 12:50 AM
We are talking about a Big Rig here. They have 4 batteries. A relay is 1/4 of 1 amp. Where is the excessive amp draw? it could run about 2 years before it draws down the batteries. No modifications to the system are needed since the relay mimics the switch.
-------------
John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Posted By: dualsport
Date Posted: December 10, 2006 at 3:48 AM
Probably could get away with it in this case, if it's not left sitting for long periods, but in general, adding another 250mA draw would be best avoided.
You probably just meant it as a figure of speech, but of course even a big rig battery setup isn't going to last anywhere near 2 years; that'd need a 4,380 amp-hour battery- more like 50 car batteries. Just in case someone reading that might assume a car setup could run the relays while sitting for 6 months-
The DEI closed loop module for sure doesn't use regular relays in the energized mode while in the standby condition, because there's not much advantage in doing so, other than maybe being a bit simpler to wire up.
Posted By: splaudio
Date Posted: December 10, 2006 at 7:13 AM
a energised relay will kill a cars battery in 2-3days...
Posted By: Ween
Date Posted: December 10, 2006 at 10:57 AM
hi, using the 4.7k ohm resistor i suggested, would add less than 3mA draw to the batteries in standby/ armed/ parked not running. insignificant in my opinion, especially given the vehicle in question. m
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