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A brighter speedometer? LED display? Answered

My current speedometer uses the same type of display that you may find on a solar-activated calculator or cheap alarm clock (the ones that display black letters on a grey background). problem is, I cant see it in bright sunlight! Is there is a way to make a speedometer with my own set of 7-segments displays that use LEDs, or if I could convert my current one to using them?


In slightly older cars this was fairly easy. There's a "sender unit" which basically opens and closes a switch once per revolution of the axle; if you measure the frequency of these pulses and know the circumference of the wheel you have the speed.

These days, I think the speedometer is actually a readout from the car's computer, and the sender talks to the computer. So it may be harder to find that signal and tap into it without disturbing the car.

I'd start by getting a maintainance manual for the car -- which is a good thing to have anyway, if you plan to make any mods to the vehicle -- and look though its diagrams to find out where this signal comes from and where it goes to. Then get a meter onto it to find out what it's doing, and set up some form of high-impedence (to avoid disturbing the computer) tap. Run the output of that to a frequency meter with appropriately calibrated reference clock.

Remember, if you're going to mount this on the dashboard: Do NOT block visibility (the police get significantly upset with that), and make sure there's a dimmer control on the LED's brightness so this doesn't ruin your night vision (red LEDs may be a good choice, for just that reason).

Or... There are probably aftermarket secondary speedometers commercially available, from catalog houses such as Whitney. That would be already built, and come with installation instructions; all you'd have to do would be wire it, mount it, and possibly calibrate it. Heck, there used to be "car computers" which used this signal and a few others to dynamically calculate MPG and the like, long before cars had their own computers smart enough to report that sort of information; those too might still be available. One piece of advice when dealing with these catalogs: Always buy something specifically intended for your vehicle rather than the generic, and never buy the absolute cheapest one in the catalog because it's going to be awful.

Yes, I was planning on red LEDs, though I was aiming for something like the standard bike speedometer. They usually use a reed switch to count rotation frequency. Anything like that would be perfect. I suppose I should have specified - I am building an EV motorcycle (like that of Stryker's) and I'm trying to keep weight and cost low. If I could make a digital speedo that weighs a few ounces for a couple dollars, i would be much happier than having to resort to a $30+ 1-3 pound needle speedo. Thanks for the help so far!

As you say, the bike speedometers might be adaptable for your homebrew bike. Mine certainly goes up to 35MPH at least; I'm not sure how far above that it would read. Above a certain point, the reed switch might not have time to close unless you used a stronger magnet... and/or might not rebound fast enough to keep up with it. Optical would overcome that but would have problems with weather and possibly dirt. Have fun...

thats a good point - i found a schematic for one online but getting the parts looks tricky. it does you optics for the sensor though, so it looks like Ill be thinking of a solution for that until the parts arrive. thanks for the info

Seconded. Great info -- and I suggest checking into stuff that can read the vehicle speed via the ODB/2 port.