Intro: A Vehicle Backing Alarm With a Self-Reseting Disable Button
At one point I was in charge of maintaining the safety of a fleet of large, unwieldy vehicles. I thought that putting backing 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; 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 backing alarm, but it's a small price to pay for maintaining pedestrian safety, my sanity, and the gratitude of our neighbors.
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 backing 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.
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.
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 people find this helpful, please comment with any improvements or questions!