Rear Window LED Display Gives Other Drivers A Piece Of Your Mind


[Gagandeep] was sick and tired of discourteous drivers on the highway, so he decided that he would put together a display to let them know what he thought of their poor driving skills. He planned on putting the display up in the rear window of his car, so he had to ensure that it did not obstruct his view while driving.

He decided that an LED matrix would be the best way for displaying images and text while on the go, so he got busy constructing a 40×16 mesh grid for his rear window. Using a wooden template to get the spacing and positioning just right, he spent several days soldering the 600+ LEDs to one another. He used 74HC595 shift registers to manage the LEDs in groups of 5 columns, while an ATmega AT89C51 was tasked with generating the text and images to be displayed. All of the ICs were deadbugged in place, helping achieve [Gagandeep’s] desire of keeping his view unobstructed.

While we’re not well-versed on the legality of such a display, it looks great when animated. There are plenty of pictures of the grid in various stages of construction as well as videos of it in action in his Picasa album, so be sure to check them out. If you are looking for code or Eagle files, you can find those here.

72 thoughts on “Rear Window LED Display Gives Other Drivers A Piece Of Your Mind

  1. Could someone look up the legality of car decorations? I’ve wondered why more people don’t put things on their cars. It seems like an area ripe for hacking, but maybe there is a law about what you can light up or display.

    1. Laws regarding lights on cars are usually very strict, but of course vary in different countries or state to state.

      Where I live, almost anything but “normal lights” (tails, turn signals, sidemarkers, etc.) are illegal to use while on a road. Anything at all blue, and anything red with the capability to strobe is illegal to even have installed without a permit.

      If you want to decorate or hack up your car, lights are not the way to go.

      1. That’s pretty much what I was told. I asked the local police district commander if my red, rear-facing, 7-segment LED display was OK. He said as long as it didn’t flash, and didn’t obstruct my view.

        It said “Bush Countdown” and displayed the DAYS:HOURS:MINUTES to Bush’s departure from office on Jan 20, 2009.

        Got a lot of horn honks, thumbs up, and smiles. Not one middle-finger.

        1. While Australia does forbid a few more things than you may be used to, it also has excellent consumer protection laws, because lawmakers aren’t owned by corporations.

          Australia also does not require you to buy care insurance, unlike some other overprotective-parent country I can think of.

    2. I once wondered about the legality of something, so what I did was call the local police station, the first person wasn’t sure and got another one who gave me the info, they had no trouble assisting.
      Not that I advise everybody to call (and you should obviously NOT use 911/112/999 emergency numbers) but I thought I’d share the concept, the cops are just people doing a civil service job.
      There might be different responses depending on region though.

      1. oh and as a side note, if you really want to know what a law is, dont contact the police, they will tell you what they want you to know, A) look up the law in the books, like i did B) call an attorney C)Google search D) ask the question on an ask site but be very specific.

    3. The law at least where i am on any extra lights on a vehicle, ie, interior, exterior, underglow, the bulb must not be physically seen from a distance of 15ft from the vehicle, and the lights must not be flashing or strobing while the vehicle is in motion on a legal roadway. There are no restrictions on colors, blue, red, white, purple, yellow, green. Again this is in my area. I have been ticketed alot for exterior lights and underglow that are blue, but ive gotten every single one dismissed on the basis and facts of the law.

  2. I’ve considered doing this for years, but have always had concerns of increasing road-rage. I would have to choose my messages to be polite. Instead of, “Get off my ass!!” I would have to phrase it, “Please back off a little, you are making me uncomfortable.” And that’s just too wordy. :)

    I’d rather do ascii art to show them what I think of them. On second thought, maybe it’s not a good idea for me to do this. :)

    1. Exactly. I always figured I’d end up getting shot after electronically telling off some idiot.

      Better I think is to have a small, directional EMP canon that you can use to disable their car. They’ll never know what hit them. ;-)

      [yes, I’m kidding and I know that would be illegal but this HAD is mostly about fantasy anyway]

      1. EMP canon… now we are talking!!
        I would use one on any car that races by my street or that shakes my house with their loud music.
        And it could be fully automated… microphone for sound and radar for speed, connected to an Arduino with tracking capability…

        My gears are spinning, going to visit eBay for parts needed …

      2. I’ve had the thought of the EMP cannon for several year now. I bet people would pay good money for something like that, but as stated before, the legality of it would be an issue.

        As for the LED display, I’ve been wanting to put an off the shelf scrolling marquee in my car. I would link it to my android and use text-to-speach for input so everything stays hands free while driving.

      3. EMP Canon – I WANT TWO!

        One for the front of my truck, to disable the a$$hat that just cut me off and one for the back of my truck to disable the car that has decided to kiss my tail pipe.

        Where I live I think the lights would be illegal

        White – Facing forward (Emergency Vehicles)
        Red / Blue – Police
        Red – Fire
        Blue – Volunteer Fireman
        Green – Voulnteer EMT
        Yellow – Caution (Tow Truck, workers oversized…)

        Rear facing Red might be OK as well as forward facing Yellow. Anything else could get your car impounded (depending on the officer involved)

        Ah well the EMP cannon would be more fun anyway. Then I could just flip them off as I drive away, no threat of Road Rage retaliation.

      4. I had the same idea on the EMP cannon.
        But one thing I thought could be an issue is you could EMP you’re self too if some of the RF pulse got reflected back.

        Maybe use it in an older car with no electronic controls or shield everything.

        It also would be very illegal.

    2. I drive around with a bubble gun in my car, periodically blowing bubbles out my driver’s side window. This makes sitting in traffic more fun. If you’re at a stop with someone ahead of you and you have a tail wind, you can puzzle the driver in front of you when suddenly bubbles go past their car.

      This will also get a Lexus right off your ass cuz they DO NOT want bubbles on their car! HA!

      BTW… I also have a scrolling LED marquee which currently reads, “I’m not drunk, ….. I’m listening to Carolyn Wonderland and I’m dancing.”
      Neither of these things have ever gotten me pulled over. {Knock wood}

  3. he says “currently it is showing hard code text but i have plans to update it from mobile via bluetooth, so that i can provide feedback to the following cars.”

    So he’s going to respond to bad driving by pulling out his phone and typing messages on the highway? :/

    1. what i have in mind is ill create a application on mobile with 1 click button to display custome text. this mobile will be fixed on my dashboard… so just one touch and there you go.. hehehe

  4. Even with the open mesh, I think I’d be annoyed with the LEDs in my back window. I’d probably opt instead for SMD LEDs wired together with fine magnet wire. I did that a few weeks back with a single LED (0603 package, 40AWG), and I couldn’t even see it from 10 feet away. Of course, it was a beast to solder freeform like that…

    But very cool idea, and something I’d LOVE to do myself. (“GET OFF MY TAIL!” or perhaps one on my front grill, displaying mirror-image to the guy in front of me “MOVE OVER!”)

    1. You are very right mohori.. I did thought on the same lines before starting the project. but cost factor and some availabilty factor forced me to use square LEDs. My next attemnpt will be using SMD.. :)

    1. Perhaps it should say soothing things like “If you back off a little, you’ll arrive at the same time, but more relaxed.” or “Hey, man, it’s not like you’re racing against the clock because you have a bomb collar on, but maybe you do, either way you should back off and listen to Classic FM oh shit did you just explode, dude?”

  5. That display would cause far more accidents than someone making you nervous driving… Seriously if someone is already driving like crap, do you think it is a good idea to distract them?

  6. Wasn’t that long ago that an enraged driver decided I pulled out to closely in front of him… so he pulled up next to me on the highway and RAMMED my car. Just like the frikin’ movies.

    As a victim of a road rage I can see this is a terrible idea.

  7. Since that back window is slanted, he should really consider using that to his advantage. That way, if he gets pulled over, he can just say, “Hey, it’s just laying on the floor, sorry”.

    Think Heads Up Display.

    May or may not work out well, probably too late as far as soldering the LEDs go though.

    But yeah, around here, you’d just tick people off even more, and put your life at risk.

  8. I would definitely get into trouble with this.
    I would constantly be pulling in front of cars with Obumma stickers and putting up the message.

    “How About that Change”

    Or better yet, the people with the Obama Biden stickers. A long but great line.

    “I get the Obama fascination but Biden, Really???, Biden?”

  9. This is really neat! I did something similar in my car PC project:

    I can answer the question of legality, at least as far as Ontario is concerned. Many states have similar language.

    Generally, the road law defines the type, quantity, and color of light that you are allowed to use. Usually you are allowed up to four forward-facing white lights, and some rear-facing red lights. Usually blue and green are not legal for use at any time. Flashing white or red is not allowed, except for a rear flashing red turn signal.

    Key point: Usually, amber (orange) is allowed in all configurations. You can have it front and rear, flashing or not (sometimes called ‘intermittent’). That’s why my LED message sign is an amber-only sign. Most tow trucks and other companies that aren’t emergency services use flashing amber lights.

    Some states don’t allow it, I happen to know that Vermont does not allow amber flashing lights except for turn signals.

    Hope this helps!

    1. A sheriff actually did that to me once. It was a Chevy Silverado and the bed light was actually a high intensity flood light. He passed me on a back highway and I didn’t get my brights off quick enough and he turned those suckers on, absolutely blinding and stunning me for a moment. Fun at 60mph on a road with no shoulder.

  10. Small scale of this project already exists as a consumer product. Drivemocion

    I used this in the past and it was hilarious to see people’s reaction. When someone let me in at a merger for example I showed them “Thank You!” message etc.

  11. This would be illegal in Germany. Only the police has the right to use lightend displays. Over here just flashing the blue lights isn’t a signal for the driver to pull over. The police often uses rear mounted LED displays to do that.
    So I guess it’s also illegal in other European countrys, but there are diffences especially when it comes to the police’s rights.

  12. In Ohio all I can think of is that it’s blue/red, and anything that blinks is not allowed.

    Exceptions to turn signals which you are legally obligated to have and to use, emergency vehicles, and maintenance vehicles, such as construction and snow plows.

    I don’t think this sign would be legal, but some people do need to back off.

  13. I always put Christmas lights on my car when the season comes around, I haven’t had any problems with that.

    I feel that with this, as long as you don’t use it directed towards a cop, you’ll be all set :P

  14. A lot of drivers piss me off, but I’d never be so presumptuous to think I was a driver who was without fault (I’ve put a car through a wall at 50mph I can presume nothing). While I now think I’m a more sane car operator I’d still appreciate someone telling me I’m too close (for their liking, though I usually keep a minimum of a car length back). Having said that, a giant display such as this would immediately get my back up and give me road rage.

    A less obtrusive method may be in order, but until someone invents “stay back”, “it’s definitely safe to pass” and “Jesus Christ you left that braking manoeuvre waaay to damn late” lights, this may be the best solution.

  15. i’ve long been wanting some manner of easily communicating to the 18-wheeler i’m passing that he’s dragging a chain, throwing a great many number of sparks in all directions, and at great risk of starting a fire since it hasnt rained in nearly a year… am i the only one that would use this power for good?

  16. How about mechanical signs that rotate up with servos? Use a counter weight on tge sign to keep seevo load low, or attempt to make a double sign per servo (rotate left or right). Use reflective tape to spell out messages on a black mesh background. Keep it simple. THANKS. NICE JOB. BUBYE NOW. PICTURE OF A MIDDLE FINGER. HONK 4 A FINGER! BACK OFF.

    You could even do a scroll type of system, that zipped quickly between messages on a continuous roll of black mesh with a brushed motor and some sensors, or a stepper motor.

  17. I’d love something like this on my car, only yesterday I was trying to tell a Ford Focus driver that both her brake lights were out (I only knew she was braking from the few remaining LED lights in the strip above the rear windshield), I had to wait for 3 miles for her to pull up at the lights to let her know.

    Here in the UK the law isn’t clear until you look into it with a magnifying glass, I got bored one day and did so and this is how I’ve come to interpret the law, these apply to a civilian vehicle that is in motion on a public highway, the below covers light emitted from a bulb, LED and/or “retro reflective sticker” (the stickers you see on police car body panels), all lights are static unless stated;

    * The only flashing lights permitted on a car are turn signals (or as we brits call them, indicators) and a green flashing light indicating ABS in use
    * Red lights may only be visible from the rear of the car
    * Orange lights can be used wherever you like
    * The only lights visible from the rear of the car must be red, orange or white work/reverse/license plate lights

    The above only covers direct emissions of light (not shadows / reflections from the road), so what does this mean you can get away with?

    * Orange, blue, green, white, yellow, purple light anywhere on the car so long as the SOURCE of light isn’t visible from the rear and providing the lights do not dazzle people
    * You can legally have non-flashing neons under your car providing the source of light isn’t visible (so bolted to the chassis underneath) and any neons on the rear of the vehicle are either red or orange
    * You can legally have non-flashing neons visible to the naked eye on the vehicle so long as they red on the rear, orange on the sides and white at the front, so long as they are not bright enough to dazzle people

    As a side note, there is unfortunately a culture here of using HID lights as headlamps, most are imports from the EU / US designed for Right Hand drive vehicles (so they send more light to the right), however when fitted to Left Hand drive vehicles here in the UK they tend to dazzle other drivers, what’s annoying is police generally seem to turn a blind eye to it (geddit?)

    Anywho, I digress, as to the legality of this sign? If the LEDs were red or orange and the display didn’t flash (probably get away with a slow fade in), it should be legal. You might get pulled by the police due to their ignorance and it being a grey area so it could help your case if you had a copy of the “The Road Vehicles Lighting Regulations” printed out kept in your glovebox for reference if this happens :)

  18. I like the idea of a guy crashing into a pole because he was typing “please slow down” :D

    I for one would NEVER use this because I’m quite sure I would use it badly. I get people mad just talking at them. They would litteraly run over me.

    But (semi) seriously ; About those (perfectly sound IMO) laws regarding the use of lights on a car, What about using the offender’s headlight as source ? Using a matrix of tiny mirrors..?

  19. I found that when the power on LED goes very low they actually give a nice subdued effect almost more ambient coloring than light but still discernible, so you could bypass laws in your area that way, just make it subtle.

    Another idea is to make a net like this project but put it under a diffuser that hides that there are LED, but you could still get nice decorative effects instead of messages.

    Or use IR LED and make ‘special’ messages for the road cameras ;)

    Pretty nicely done how he made the grid incidentally, that’s how you get results, some dedication in the right areas.

  20. I’m making a 16×2 char display (96×16 leds) but when I have that many columns, the duty cycle goes down the drain and the LEDs are VERY dim, even with NO resistor.

    I’m not the greatest at understanding ASM. What did he set his clock cycle delay to?

    1. Hi mike

      I have used 100 Hz duty cycle for 16 rows.
      that is each row (40 LED together ) is blinking for (1/100) sec, then next row. this is acchived by timer interupt. :)
      More over I have overdrivern the LED with 5 V insteat of 3V.

      let me know know if you need more help on this.

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