low/no draw deadmans trigger

Hello All, myself and many others have been trying our best to hold onto our expensive offroad lights and my lightbar was recently tampered with when some punk decided he needed it more than me.

What I'm looking for is a low or no draw circuit that would trigger an alarm (my air horn) if someone cut the wires to my lights.

My current plan is to install a normally closed relay always energized so the path between my alarm (horn) and battery is open with the relay's ground wire looped up the lightbar's pigtail back to ground so next time they carelessly cut through the pigtail they'd break the ground, de-energize the relay coil and close the curcit. This would then trigger my airhorns and oem horn causing them to immediately evacuate their bowels and run away like the coward thief they are.

The problem I'm guessing I'll find is the constant energized (opened) relay will burn out quick and/or my battery will die if I don't drive the truck for a few days.

Can anyone suggest a more passive low/no draw trigger that will close a circuit, sound an alarm, if the wires are cut? Maybe something like this already exists but my search for an N/C "deadmans switch" has been null..

Also in an attempt to save my battery in the event someone triggers the alarm. Maybe I should have some type of timer on the alarm so it doesn't honk my horn for 20 minutes if I'm not close to the truck..

My plan is:
Terminal 30: Horn +
Terminal 85: 12v +
Terminal 86: Wire shrink wrapped along lighbar pigtail to Ground
Terminal 87: 12v +



Thank you for your help.

I don't think this is relevant to my question but my vehicle is a 2007 Toyota Tundra. Here's a picture for reference.

It will work but you'll drain your battery. Install a Viper alarm or R/S + Security, feed the blue trunk trigger to the lamps and set it to normally open.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

You are suggesting the alarm to act as the timer or would the alarm give me a trigger also?

I've found a post https://www.the12volt.com/installbay/forum_posts.asp?tid=115611 asking a similar question and someone suggested a MOSFET to act as the relay to eliminate battery drain. I've tried to educate myself on how a MOSFET works but don't see how to apply this in my situation. The MOSFET still appears to need voltage at the gate I.E. a circuit that is closed once the wire is cut..

Does that suggestion hold any water?

Here's another attempt at linking the post.

Here

Yes PM Oldspark or Dualsport for any ideas, they are more into that kind of thing than me. Please note that the Idiot confirmed my comments on battery drain.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Use an alarm. Use the positive trigger wire and wire as follows. At the alarm brain, connect a 10Kohm resistor from power to the positive trigger wire. Run the positive trigger wire along with the ground wire of the light bar all the way inside the light bar if possible. The ground wire will keep the wire grounded. If they cut that wire the 10K ohm resistor will make the voltage go positive on that wire which will set the alarm off. A relay will draw about 160 milliamps. The alarm and resistor will draw less current than the relay.

jwhyte1445 wrote:

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What I'm looking for is a low or no draw circuit that would trigger an alarm (my air horn) if someone cut the wires to my lights.

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Here's another approach. Crude, and not 100% reliable, but just might obtain the desired effect (sharted pants) without any battery drain unless triggered.

Use a relay:

#30 = Ground
#85 and #87 wired together = trigger input, and (-) output to horns. Use this output to trigger another relay for air horns, if a (+) output is needed.
#86 = Hot at all times.

Run a #14 wire within the harness with the other wires to the light bar. Don't connect it to anything in the light bar, just to relay terminal #85 & #87. Loom the wiring closely together by wrapping tightly with electrical tape.

The idea is that if a thief cuts the loom with cutters they will inadvertently short the trigger wire to any other wire in the harness which will be at (-) potential through the filaments, the relay will be latched on, and horn will sound until someone disconnects battery or relay. Or the battery goes dead. It should be triggered reliably with incandescent or halon lights. Reliability for being triggered will decrease with LED lights, because only the light bar ground wire will be at (-) potential, rather than every wire in the harness in the case of conventional lights.

As I said, it's crude, and will wiz off the neighbors because it will sound until the battery goes flat. I probably wouldn't use this approach either, but it would be cheap and easy to do, and would afford some peace of mind protection.

Which is all any security strategy can really do. Peace of mind that you're trying something.

A MOSFET...

An N-channel MOSFET with S to GND and D to ground the +12V connected "alarm" (siren, horn etc - or maybe a detonator?).
A resistor from G to +12V to turn it on - probably anything from 10k to 1M Ohms.   (10k means 12V/10k = 1.2mA draw; 100k = 120uA, etc.)
Also G to GND via your grounding link that is broken upon theft.

G needs to be typically 5V above S for the MOSFET (D to S) to turn on.
The grounding link keeps G at GND (0V) so it does not turn on.

You could add an optional resistor in the G to GND circuit - ie, from 100 Ohms up to one-tenth of the G to +12V resistor. That's to limit current flow in case of a D to G fault.
Maybe also a small capacitor from G to GND - ie, a 0.1uF or 0.01uF greencap etc to help guard against noise.

I’ve been re-reading this post to try to draw it in my head.    

I’ll start reading up on these components. I understand the basic uses for resistors as a “reducing valve” but I’m confused by connecting a resistor between “power” and the “positive trigger wire” .
If the positive control wire is only looking for a pulse will it still receive that pulse if my light has no power to it at the time the wire is cut?
Is the positive trigger wire making a loop to/from the light or is it actually terminating to the ground inside the bar?

Thanks you for your patience.. I thought I understood electronics until I started reading on this forum.. Ohmg…

Thank you for the suggestion. I can understand the route you're explaining but this setup will be for an LED bar unfortunately.

oldspark wrote:

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A MOSFET...

An N-channel MOSFET with S to GND and D to ground the +12V connected "alarm" (siren, horn etc - or maybe a detonator?).
A resistor from G to +12V to turn it on - probably anything from 10k to 1M Ohms.   (10k means 12V/10k = 1.2mA draw; 100k = 120uA, etc.)
Also G to GND via your grounding link that is broken upon theft.

G needs to be typically 5V above S for the MOSFET (D to S) to turn on.
The grounding link keeps G at GND (0V) so it does not turn on.

You could add an optional resistor in the G to GND circuit - ie, from 100 Ohms up to one-tenth of the G to +12V resistor. That's to limit current flow in case of a D to G fault.
Maybe also a small capacitor from G to GND - ie, a 0.1uF or 0.01uF greencap etc to help guard against noise.

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Having G tied to +12v and ground seams to be conflicting. Is the resistor some how isolating voltage +12v from the ground?

With this design is the horn (I like the detonator idea) able to be grounded for use still as the S and G terminals are only linked when the gate is active right?

I'm sorry if I'm way off..

jwhyte1445 wrote:

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Thank you for the suggestion. I can understand the route you're explaining but this setup will be for an LED bar unfortunately.

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Use speaker wire as the trigger.

One conductor is the trigger, the other to ground at the relay end. At the light bar end, insulate each conductor from the other, and from anything else. You could even use more than one run of speaker wire to increase the chances of the cutters grounding out the pair as it's cut because both conductors are in proximity to each other.

The larger the gauge, the greater the chance of the cutters making contact with both conductors simultaneously.

G does to +12V via the resistor (10k-1M) and to GND via the shorting link. No conflict.

a little outside the box....
what if you wired the lights with power to them all the time and then had a relay ground them to turrn them on?
then you could connect an alarm to sense the (powered) ground wire at the relay. that wire goes dead....an alarm senses it and yelps... maybe needs a 528T relay to sense the "change in state" of that wire.

its not as difficult as it sounds.

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yuk
quiet rural missouri, near KC.
If your system moves you physically and not emotionally, you have wasted your money.