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Three ideas for a "liftgate alarm" for P-U

So I recently bought a beat-to-hell 1988 Dodge Dakota for a work truck.

While I doubt anyone will steal the truck itself, I am living in a moderately high-crime area, and want to avoid becoming proof of the adage, “everything I own is wet or stolen.”

So, on the presumption that anyone who wants something out of the bed will first lower the liftgate, I want to rig an alarm that sets off a LOUD horn if the gate is lowered. Additionally, I want to do this DIY, with cheap/salvaged components.

Three ideas:

1. Lowering the gate closes a mercury switch (garbage-picked from an old-style thermostat), which powers the horn.

2. Lowering the gate moves a magnet away from a magnetic relay, closing it and powering the horn.

3. Lowering the gate breaks an electrical contact to a relay, allowing a (normally-closed) switch to close, powering the horn.

Any ideas on practicability of any/all of the three?


Bryan (meanjoe75fan)

And actually (considering the health issues re: mercury), change 1. to “Lowering the gate causes a pinball to travel to the end of an enclosed PVC pipe, completing a circut and powering the horn.”

I got an after market locking tailgate handle with a key for one of my trucks. No camper top I take it? Sounds like you could make any of them work, the mercury sounds the easiest as long as it is out of harms way.

If there’s no cap or cover, why would a thief take the time to lower the tailgate? Most tailgates make some noise as they are opened, which is not good if your stealing something. They’ll just reach over the side or the gate and take what they want.

If you want an alarm I like the mercury switch idea best. I’ve had older cars that had mercury switches for the trunk lights. As long as you protect it from damage it should be safe.

I envision you disabling this alarm after you set it off yourself a few times. My suggestion is; don’t leave anything valuable in the bed.

I agree mc,
In thinking about it the wires in the mercury switch may not be adequate for the job, plan b, 2 bucks on ebay LOUD 90db PIERCING SOUND


Easy. Plunger switch.

Second the vote for a plunger switch.
Simple circuit ground on the horn hot wire. A switch like all the door jamb, dome light switches used to be.
Then you’ll need to hide a disconnect switch for yourself to not blow the alarm.

This reminds me of the stories heard when CB’s were popular. While I am not suggesting this, there is the story about the guys who rigged the coaxial cable connector at the back of the radio with treble hooks and when the vehicle owners came out to find the radio still in the vehicle they would in turn find dribbles of blood on the transmission hump.

Thanks, all.

The plunger switch certainly sounds simpler and more effective than anything I suggested…sometimes it takes someone else to apply the KISS principle, I guess!

The reason for a gate alarm is what I wanted to put in the bed is my motorcycle, possibly along with other, heavy (100#+) stuff, where the logical means of removal would be via lowering the gate. (The wheel would be cable-locked, requiring dragging it out, but still…)

What you’re seeking is called a “normally on” pushbutton. It’d be disengaged when compressed and engaged when released. You can get one at any hardware store. One common use is to energize your brakelights when the brake pedal lever is pushed in.

And every time you put the tailgate down to load or unload the truck the alarm will sound?

How about a simple latch & hasp with a good padlock?

Or a cap with opaque sides and rear and a dog-barking recording that’s activated when someone fools with the rear door?