Swapping Speedometer Needle For LEDs

[Ah2002] didn’t like the shaky needle in his car’s speedometer so he replaced it with a ring of LEDs. The old speedometer had a cable which rotated along with the gearbox for mechanical speed measurement. By connecting the stepper motor from a printer instead of this cable, a voltage is generated that fluctuates with the speed of the car. The fluctuation is linear so a given voltage measurement can be directly associated with one particular speed. By using a trimpot to calibrate the input voltage, [Ah2002] connected the signal to an LM3914 dot/bar display driver. These can be chained together, lighting a string of outputs based on the single voltage input. The result is the board seen above, which was covered with a printed paper graph in the final assembly.

Judging from the video after the break, we’d bet there was some distracted driving during the calibration process. The driver appears to be holding the video recorder, and since a cellphone GPS was used during calibration we wonder if [Ah2002] was adjusting the trimpot, looking at the GPS, and driving all at once. It’s a fairly awesome hack, but do be careful when you’re working on something like this.


44 thoughts on “Swapping Speedometer Needle For LEDs

  1. Definitely like it, although I think I;’d go with red. Looks better (imo) and better for your night vision. When you want to give your car a more custom look doing a mod like this is a great alternative to replacing the whole assembly and usually losing your cars actual mileage.

  2. Not to mention it looks like there’s snow on the ground, and he at least hit 60MPH while running a camera (100km/h or mph, couldn’t tell). It’s just reckless.

    Also, I’m pretty sure that at least in the US it’s quite illegal to mess with your speedometer aside from replacing it with something approved.

  3. @Darkrocker

    Unfortunately it states that he replaced the cable mechanism with a stepper motor so he no longer has the cable running to the odometer.

    Still a neat hack though. :-)

  4. Nice hack… I wonder if he was pulled over for excessive speed the excuse “But Sir I didn’t know I was speeding, my LM3914 was just out of calibration!” would work?

  5. Looks like it could make the Kessel Run in less than twelve parsecs.

    No clue what the actual rate was, but it wasn’t 100kph or mph according to the snail-esque moving road stripe.

  6. great until you have to service the bulbs. I had to service my dash bulbs at 6 years having them constantly on with the headlights. So based on that, he’s gonna be changing the low mph’ers more often than the high ones…so, I would add a small pulse or something not noticeable to the eye, to clip the power on the lower MPHs that are almost always going to light up.

  7. @itwork4me – small LEDs have running times on the order of a decade of time /on/. If he’s got them running properly, then even if he’s driving at 20mph that’s a million-mile fixup issue…

  8. Optical or Hall effect encoding for the speedometer cable would be a better idea. Ideally Hall effect. This is how wheel speed sensors work since they are unaffected by dirt and grime and are essentially friction free.

    I hope that stepper does not catch fire on a long trip, but from the sound in the video, his car seems to be making a lot of noises as it is already.

  9. Optical encoding of the speedometer cable would be unstable, or very difficult to hold rock solid. Hall effect could not work with the cable, so the cable would need to be removed and the sensor mounted in the transmission or the wheel, also it wouldn’t generate a voltage to power the leds. How about a dc or stepper motor attached to the cable inside the speedo so replacement is easy.

  10. why not put a small piece of metal on the cable needle and have it complete a circuit to turn on the led’s instead? I must have over simplified this, because for some reason no one has mentioned it as an idea…

  11. I kinda recall a LED dash in the 80s 90s that was a digital kmh/mph readout with a series of boxes for rpm. the rpm boxes would start in the bottom left corner and then grow upwards in a angle until it hit the top edge and then continue rightwards.

    I think a whole lot of car companies experimented with digital readouts, but went back to dials for some reason.

  12. “Great until you have to service the bulbs…”

    Hello time traveler! Welcome to 2010, where we have these awesome little things called LIGHT EMITTING DIODES!!

    While visiting this temporal region, please feel free to read up on the delights of these tiny, solid state wonders so that you might bring news of them back where/whence you came!

    Oh, and try the fish…

  13. You can estimate your mileage for a sale if you have a non-functioning odometer. i have a 70s jeep that hasn’t had a working speedometer or odometer since long before i have owned it. it becomes illegal if you have intent to deceive.

    yes its legal to change out your speedometer. i doubt this mod would be legal in most US states…but there may be a way to have it calibrated and checked to make it legal. but to me it looks like he still has the bouncing needle issue (higher numbers are flashing)

    and just fyi…most bouncing needle issues involving a cable driven speedometer can be fixed with some lube, 90% of the time that’s the issue.

    also there are hall sensors available for mods like this for as little as $30.

  14. Ever see an LED on a keychain burn out? I replaced the LED twice from a cheap keychain. Those LEDs are probably manufactured in china or pacoima…so really let me know when the first burns out. I give you 160000 miles or six years…keep me posted. Regardless, cool until yer pulled over.

  15. Service the bulbs after 6 years of having the headlights on? Must be an American car…my Toyota Camry is past 7 years…I also drive with my headlights on (pre-DRL vehicle) and the dashboard lights are fine.

  16. Looks great! With rgb leds he could could display more than just the current speed: average speed, or coupled with a modern satnav even the speed-limit or rather the exceedance of it in red. Would require more logic of course and would need some robust design with the drivers psychology in mind.

  17. Itwork4me, dude, your crappy keychain burns out cause its a crappy key chain with a led connected direct to the battery and letting the current ride as hard as it can

    using a simple concept called current limiting, while using it you can get insane amounts of time out of led’s, for example my alarm clock, which has been on 24/7 since 1989

    do us a favor and go lean some (very) basics before running your flap

  18. The legality of it is kind of a grey area.

    The speedometer part is not illegal to modify, (You actually don’t need a speedometer to be legal) but messing with the odometer is the illegal part.

    You really begin to question things in 4×4 ing.

    I have a jeep that was lifted by the PO and larger tires were put on. The speedometer gear was never changes so the speedometer and odometer are actually 17.9% slow!

    On top of all that when speedo-s get old they tend to either bounce (like what the author said) or stop working all together.

    If you check out your title there is a box on there that says odometer reading exceeded limits or odometer reading inaccurate. Check this and your good to go.

    Only issue with not having an odometer is your guessing when to change your oil!

  19. @lwatcdr
    Yes, the support circuitry would be very similar, however the issue with the optical encoder becoming unstable would be from dirt/grease interfering with the optics, and slop in the movement of the cable. Hall effect would still be vulnerable to the slop in the cable, but wouldn’t have the grease issue. As previously mentioned, it would still be better to relocate the sensor directly to the transmission or driveshaft, as this would reduce the slop issue.

  20. Here’s another idea:

    Why use the stepper as a tach/generator to do an analog speed to V translation?

    The stepper should produce a sinusoid when it’s turning. You could count those pulses the same way you could count the pulses from an optical sensor or a hall effect device.

  21. I think the hall effect would be the best bet. The shaft is already steel and would be virtually infoulable. I would also then integrate speed to give an odometer reading specifically for purposes of maintenance and keeping track of fuel mileage.

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