trailer alarm

Hi guys. Happy New Year! I am interested if anyone has any ideas with regards to alarming an enclosed trailer. The tow vehicle has a Code-Alarm which has a -ve instant trigger. I would like to have it wired through the trailer plug so that when removed (or triggered through another trigger on the trailer) the towing vehicles alarm sounds. The only way I can think of is through a relay but I don't like this idea as I don't want the relay to be energised for most of it's life. Being energised whilst "in an alarmed state" is o.k. I.E. siren blaring but not when just armed. Any ideas? Thanks, dts.

You could wire in a "phono" plug like they do on electic scooters, as long a plug is in curciut is open, when removed connection grounds.  Someone taking the trailer would't think twice about it's functioin, just pull and run.   Or worse mount a motion sensor around plug area.

Yep - just break a link. And I reckon from your -ve instant trigger, a (say) 10k resistor to GND (at the alarm box?) AND to the "breakable" link. The +12V trailer trigger (should) be normally closed and in series with the breakable link to the alarm input. If the existing trailer trigger opens OR the breakable tow-away link opens, the alarm's -ve instant trigger is no longer +12V... the resistor pulls it down to GND (ie, -ve going). The link could be a spare trailer-plug pin. A grounded "other trigger on the trailer" requires inversion....

hi, and dei has the 502t...old piece...closed loop trigger. less standby current than the 10K pull-up resistor if i recall. mark

It's the alarm that determines a suitable pull-up size for the resistor (ie, if it has an input impedance, the pull-up should be no more than 10x that impedance (Ohms). 10x means the alarm input will see 10/11ths of the voltage - ie, "high" +12V will be 10/11 x 12V = 10.9V (9% down from +V=12V). I just chose 10k as a reasonable leak (1.2mA) while probably suiting the alarm input. [Some require 1k - IMO too low (12mA IMO is too high)!] Or maybe the input has to drop below (say) 3V to be recognized as a "low" (or -ve going) in which case it can be 4x the input impedance. [ It's simply a voltage divider where Vinput = +V times (alarm input impedance) divided by (pull-up resistance plus alarm input impedance). ] If that leakage is a problem, use a FET. Then the pull up can be 1M Ohm (for 12uA leakage)or higher...

Thanks guys. Yeah I completely forgot about the 502t. Used one years ago with success. I will also experiment with the resistor but need to work out what the best value is. It's going on a Code-Alarm SRT-9000. Has anyone had much to do with these? I have installed 6-7 of them and found them o.k. They apparently changed something as the earlier ones didn't get the range quoted (distance). Not sure how to tell if this is the newer one or not. Thanks again for your suggestions.