A Vehicle Back Up Alarm With a Self-Resetting Disable Button




About: I'll tinker with anything, but I'm often to be found repairing, improving, and restoring things. I'm always looking to develop new skills (anyone near Boston want to teach me welding?), but am currently best...

At one point I was in charge of maintaining the safety of a fleet of large, unwieldy vehicles. I thought that putting back up alarms on the trucks would increase everyone's safety during maneuvering in the parking lot, but I hated that darn beeping!

I considered having a simple switch to turn the alarms off, but I was pretty sure that no one would ever remember to turn them back on.

To solve this problem, I created a system where (as usual) the alarm begins sounding when the vehicle is placed into reverse, but there is a disable button installed on the dashboard. The operator can press the button to silence the alarm (assuming the operator has a spotter); however, the system will reset once the vehicle is taken out of reverse, so that the alarm will sound again the next time the vehicle is shifted into reverse.

The system requires a bit more work to install than a standard back up alarm, but it's a small price to pay for maintaining pedestrian safety, my sanity, and the gratitude of our neighbors.

A critical safety note: If a vehicle has poor rearward visibility (which is the most common reason for installing a back up alarm in the first place), then drivers should only disable the back up alarm when they have a spotter to stand behind the vehicle and ensure pedestrian safety. Turning off a back up alarm and backing up blindly is irresponsible and dangerous, so don't do it! (Also, if you work for a business, OSHA requires back up alarms to be used on all vehicles lacking rearward visibility unless a spotter is being used.)

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Step 1: Gather Your Materials

This is a pretty simple project. You will need:

~ some wire
~ a momentary push-button switch (normally open)
~ a Single Pole Double Throw (SPDT) relay
~ a back up alarm (as discussed below, consider getting a white-noise, self-adjusting alarm)
~ some electrical tape, liquid tape, or other all-weather insulator (remember, parts of this are going to be under a truck!)
~ some cable (i.e. "zip") ties couldn't hurt

You'll also need a drill to create a hole in the dashboard for your switch, and you may need trim tools, screwdrivers and whatever else it takes to get behind the panels in your particular vehicle.

About the type of back up alarm:

If you are buying a back up alarm to do this project, consider getting a white noise alarm instead of the older beeping style of alarm. The white noise alarms are less obnoxious, but they are also becoming standard on many job sites. This is because pulses of white noise are easier to hear in a noisy environment and easier for a listener to quickly directionalize, but they do not carry for hundreds of feet and annoy the neighbors like beeping can.

While you're shopping for back up alarms, you might also consider getting an alarm that is also self-adjusting, meaning that it senses the ambient noise and determines how loudly it should sound in order to be heard. These self-adjusting alarms can be a major improvement over the ones that are always too loud (except on the noisiest job sites), or the ones that always seem to be too quiet.

Step 2: The Schematic

Here's the fun part. As shown in the schematic, you hook up a +12V source in the cab to your momentary switch, and then run a wire to the back bumper where your alarm and relay will be mounted. I potted the relay to the back of the alarm with epoxy, just to make everything super weather-proof.

The way that this system works is the following. When the vehicle is shifted into reverse, the 12V from the backing lights passes through the "normally closed" side of the relay and powers the alarm. If the button is pressed, it energizes the relay, which flips the current coming from the backing lights over to the "normally open" side of the relay, where it goes into a loop, holding the relay open and preventing the alarm from receiving power. Once the vehicle is shifted out of reverse, the relay is no longer powered and resets to its "normally closed" position, ready for the next round of beeping. (If this is unclear, see the comments for a slightly more expanded explanation.)

There's a somewhat amusing side-effect to this setup. If you press the button when the vehicle is not in reverse, the relay will be energized, flipping it over to the "normally open" position and allowing the current coming from the button in the cab to flow through the relay and "upstream" to the backing lights, turning them on. This behavior could be eliminated by placing a diode (basically a one-way gate) in the circuit as I annotated on the image, but I personally find it useful to be able to flash my backing lights at the touch of a button. It seems to be an excellent way to impart to the person tailgating you that you would like them to kindly remove their grill from your tailpipe. It's certainly safer and more polite than the lovely Bostonian tradition of the "brake check."

I hope that you find this Instructable helpful. Please comment with any improvements or questions!

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    11 Discussions


    Question 2 months ago on Introduction

    I want to be able to “toggle off” my back up alarm in consideration of my immediate neighbors when I leave for work. But I don’t trust myself to remember to always turn it back on when I leave my driveway. Seeking a way to shutoff back up alarm but simultaneously create an irritating buzzer in cab when it is disabled. Any solutions?

    5 answers

    Answer 2 months ago

    Your concern is essentially the reason that I wrote this instructable.

    In this design, there's no buzzer in the cabin to remind you to turn the back up alarm back on. Instead, the system remembers for you.

    Because of the circuit I built, the system is self-resetting. The back up alarm will sound every single time you shift into reverse, but will go silent once you tap the button to silence the alarm. You can't forget to turn the alarm back on because it turns itself back on (well, resets itself to a "ready" state) as soon as you shift out of reverse.

    (If you're worried about not being able to turn the alarm all the way off, I find that if you hold the button while you're shifting into reverse, the alarm either won't sound at all or will only sound for a fraction of a second.)

    If anything is still unclear after rereading the instructable (also see my answer to the other question in the comments), please feel free to follow up!


    Reply 2 months ago

    Thank you for responding. How can I get one of your modules to try out? Would it come with instructions as to what circuit to interrupt?
    Thanks, Russ


    Reply 2 months ago

    This isn't a product I've built, it's just something I made for myself and then wrote instructions to help other people build the same if they want to. (That's pretty much what all of Instructables.com is.)

    All the build instructions are on this page. You can build the necessary circuit using the materials listed in Step 1. Just read the instructable a few times, look up any unfamiliar terms, and ask if something really doesn't make sense. There's no complex circuitry here, just five wires, a button, a relay, and a commercially made back up alarm. These are all things you can easily buy online or maybe in a local electronics shop.

    Even if you have zero electronics experience, I think you should be able to build the circuit illustrated in the schematic with reasonable ease. (The schematic follows most of the usual schematic symbol conventions, but is also designed to be easily understood even by those with minimal familiarity with schematics.)

    Let me know if I can help, and have fun!


    4 months ago

    I want to install this in our RV (for obvious "campground etiquette" reasons).

    But, I am not electrical diagram inclined, so please help me understand your diagram.

    Does the single wire coming back from the momentary switch get split to both the coil AND the NO connections? Wouldn't that mean that 12V power would be entering the relay from both the back-up light source AND the switch?

    1 reply

    Reply 4 months ago

    Yes, you're reading the schematic correctly. The reason for the extra complexity is that the plan uses a momentary button rather than an on/off switch. This makes it impossible to forget to turn the backing alarm back on again. (This is important, because with any vehicle that lacks visibility out the back, you should only ever turn the backing alarm off when you have a spotter behind the vehicle who is directing you and watching for pedestrians. Otherwise, you're just asking for injuries, lawsuits, and, if you're an employer, OSHA citations.)

    Here's how it works when you use the system:

    1. You push (and release) the momentary button from the driver's seat, which energizes the coil and pulls the relay's switch over to the open position.
    2. Once the relay reaches the open position, the power from your backing lights is able to flow though the relay's switch and then around the loop of wire into the relay's coil, which keeps the switch in that position, thus continuously powering the coil (even though you have released the momentary button) and keeping the backing beeper disconnected.
    3. This condition (relay coil energized by the backing lights and beeper disconnected) will remain in effect until you shift the vehicle out of reverse and the backing lights turn off, at which point the coil is no longer powered, so the relay returns to its default position.
    4. Now that your backing lights are turned off and the relay has reset, the beeper will come on again the next time you shift into reverse (unless you press the button).

    Does that make sense?

    (Also, I'm in the process of adding some new content about backing alarms, specifically comments about the newer and safer white-noise alarms and self-adjusting alarms, so you might consider looking at my Instructable again in the next day or two.)

    I hope this helps!


    Reply 2 years ago

    You could. You would build the schematic as shown except for the part with the switch and 12V source in the cab.

    You would then power it from a consistent 12V source near the rear bumper (if you can find one) instead of running a wire to the cab. Finally, you would install a remote momentary switch between the device and the power source and install the remote control in the cab.

    Really, though, it seems like that would be a less reliable solution that might require batteries and could look pretty unprofessional, all in the name of avoiding running one wire from front to back. I would strongly suggest just running the wire.

    Getting behind a few interior panels to run a wire isn't too tough in a car (tip: follow the wires from the taillights and see how they get back to the front), but if you really don't want to deal with it, you can always run the wire under the vehicle. Just be sure to attach it somewhere that will keep it away from road debris.

    mr fat

    3 years ago

    great design, simple and smart.