Create circuit powers on when wire cut.

Thread Starter


Joined Feb 4, 2017
Hello. So couple days ago for the second time in as many months I've had someone steel the lightbars/drive lights off my vehicle.

Snuck up and snipped the wire and unbolted them.

So now im looking for ways to make it hard for them.

Is it possible to wire up a loud so that if a wire is cut the horn goes off?


Joined Mar 14, 2008
You could if you added an extra pair of wires in the cable to act as a loop, carrying a very small current.
If the wire was cut, the current would stop which could be detected to sound an alarm.

But how about just mangling the bolt threads with a hammer so they couldn't be removed?


Joined Feb 20, 2016
Here is a simple circuit of one of the many ways it can be done.

When the ARM switch is on, power is supplied to the alarm horn relay but it will not sound as TR2 is held off by it's bias being shorted out by the light..
If the light is cut off, this short goes away and TR2 will switch the relay on and the horn will sound.
It will also sound if you turn on the light if the ARM switch is on so acts as a reminder to turn the lights off.
When the ignition is on, TR1 turns on shorting TR2 base to ground so stopping ti from sounding when the car is running.
There are a lot of other ways to do it but I hope this helps.
Probably a 10K resistor from TR2 base to ground would be a good idea to add, and if you want 2 lamps to work independently, add the 2 x 1K resistors as above, but include a diode in each feed to TR2 base with the cathodes to the base. But that would require the wires to come back from the inside of the lights to ensure they were cut as each light was removed.
As it is, the Alarm On LED will probably glow when the lights are turned on. Quite a bit of refinement could be done to this.

If your lights are LEDs, this will probably not work as the lights will not pull to 0V when off, but you could experiment and measure the voltage on the light cable with the lights connected and install a series zener diode to drop that volts so TR2 will not turn on, but will still if the light is removed.
Last edited: