If you're anything like me, people are always writing down your license plate for no good reason. I've given some thought to building a rotating license plate mount that would make Q proud. Here's what I've come up, to date. A few caveats are in order. First, putting something like this on a car would be completely illegal, so I wouldn't recommend it and I'll accept no responsibility if you do so. Second, I'm sure there are far more elegant ways to build this thing and I'd love to hear your ideas. I haven't found any other instructions, which is what originally drove me to think this thing up. Third, I won't be giving any measurements because the dimensions will be dictated by a number of factors that will differ from person to person. It'll take some measuring on your part and a little trial and error, I'm sure. Okay, let's kick this mule.
Step 1: Making the Triangular End Plates
You're going to start with a pair of triangular boards. I would recommend plywood. DO NOT use steel. These must be equilateral triangles. Arced cuts are taken out of each corner, as shown.
Step 2: Flanges and Spreader
Mount flanges, like those shown, on both of your triangular pieces. If they make something like this out of PVC, I would recommend that, otherwise, maybe aluminum. NOT STEEL. A corresponding, non-steel spreader pipe is fit permanently between the two flanges.
Step 3: Axle Installation
Drill holes through the center of both triangular end pieces. insert a length of all-thread through the holes and spreader.
Step 4: Add Mounting Brackets
Come up with some 90-degree mounting brackets. Preferably plastic, but probably aluminum. NOT STEEL. Bold these onto the exposed ends of your all-thread axle. Do not over-tighten.
Step 5: Pump Some Iron into the Mix
Cut three pieces of round stock IRON to length and mate these to the notches in your triangular end plates. Use nails or glue or something.
Step 6: Add a Back Plate
Make a simple plate that will ultimately connect your contraption to the car's rear bumper.
Step 7: Affix Magnet
Now you're going to get a super-strong magnet. Permanently affix this to the center of your back plate. In the illustration, the magnet is yellow, but yours doesn't have to be yellow if you don't want. Make sure you have clearance between the magnet and the iron bars as the device rotates.
Step 8: Actuator Mount
Mount a 90-degree bracket on top of your back plate, as shown.
Step 9: Procure Actuator
The best idea I can come up with is an electric car antenna. I'm sure someone out there has a better idea. Something faster and more powerful would be ideal. Something between the antenna and, say, the pneumatic murder weapon from No Country for Old Men.
Step 10: Install the Actuator
Now you're going to mount the antenna in the bracket. Cut the thing off so that when it's fully extended, it doesn't go lower than the lowest point at which one of your iron bars travels during rotation. Once the antenna is mounted and cut to size, retract it all the way. In its retracted state, it should be up out of the way, allowing the push-lawnmower-looking business to rotate freely. Now go take a nap. You deserve it. I'll wait here.
Step 11: Urban Safari
Now go for a drive. Look for other vehicles that are very similar to yours. Find two that are the same make, model, color, and general age. Write down their license plate numbers.
Step 12: A Little Forgery Never Hurt Anyone
Now swing by your local swap meet. Not the old school, clean out the garage type swap meet, but the indoor swap mart full of Gucci bags and velvet Tupac blankets swap meet. Find the guy that makes custom vanity "license plates" that are plastic but look damn near like the real thing, complete with 3D digits. Give him the plate numbers you jotted down. Go chat with the Ginsu salesman while your plates are being made.
Step 13: Mechanism Shroud
You're going to want to hide the mechanism as much as possible and also protect it from the elements. Build some protective shrouds around everything.
Step 14: Mount the Plates
Now mount your car's license plate, along with your two swap meet curios. Then it's time to make sure everything works.
When one of the three plates is vertical (i.e., aiming rearward as all good plates should), it will be held in this position by the attraction between the magnet and the nearest iron bar. When the antenna is actuated, it should extend downward, making contact with the upward-facing license plate. Hopefully, the pressure will overcome the magnetic attraction and thereafter rotate the combine-thresher-looking part. The antenna then retracts, allowing the rotation to continue until the next iron bar is "locked" in the magnet's grip.
Here lies one of the biggest potential problems, which I haven't been able to address yet. Once the antenna rotates the device to the point where the vertical plate is now facing toward the front of the vehicle, what will cause further rotation in the right direction? If the actuator was faster and more powerful, it could pop the thing into gear, using momentum but not so much that the momentum would overcome the magnetism as the next iron bar came abreast of the magnet. I don't know. Ideas?
Step 15: Face Plates and Electrical Hook-up
We're pretty much done. Just fashion face plates to hide the mechanism from the front (or is it 'rear'?). I'd make this removable in case you need to access the compartment, which you'll probably have to do for any number of reasons.
Now run the wiring to a convenient spot, using a push-button activator switch. If all goes well, you should be able to sit in the driver's seat and change license plates at will.