Hello,
Is it possible to use an Arduino Diecimilla to measure the speed of my car when I am driving around?
My car (Honda CRX Del Sol '92) doesn't have ODB-II, so I can't get such diagnostics that way...
GPS is possible, but I have no idea how to work this out.
(The speed must be displayed on a LCD display later on in the project)
Thanks.
Sorry about that. Got multiple times error 500.
Duplicates are now removed.
It's certainly possible one way or the other. Does that car have electronic gauges or is there a speedo cable from the transmission to the speedometer? If you have electronic gauges then it'll be more straight forward. What you do is find the signal wire for the speedo and hook an oscilloscope to it (don't have an oscilloscope? Get one, I'll wait...) Now, you should see that the signal wire is likely a digital signal going from 0V to likely +12V. The trick is to figure out how many volts it is (because you cannot directly hook up +14V to an arduino pin!) and how many pulses per mile are being output. There is a remote possibility that this info is available somewhere. If not you'll have to sort of guess then calibrate your guess later. It could be in the ballpark of 16,000 to 64,000 pulses per mile. From there you should be able to integrate for time to come up with the current speed. Frequency to voltage chips are also available. Using one would make your job a lot easier. Then you could just hook the F2V chip output to an analog pin on the arduino and read that. You'd still need to figure out which voltage corresponds to which speed. That's easy enough. You guess then drive 60MPH on your gauge while someone else follows you. They tell you how fast you really went and you divide from there. Note that you should really be careful doing this as 120MPH and 60MPH are not the same to an officer of the law. 
If you are obviously off by a lot then fix it before you go out racing around.
If you've got a mechanical speedo then you would probably have to put a hall effect sensor somewhere (and don't forget the magnet to go with it) and read the output of that to determine speed.
Thank you for your reply!
Does that car have electronic gauges or is there a speedo cable from the transmission to the speedometer? If you have electronic gauges then it'll be more straight forward.
I asked around, and my Honda has electronic gauges.
and hook an oscilloscope to it
I don't have it...
(I even didn't know what it was by name, so I gogled it for images.) Can't this be done by a simple multimeter?
Are you sure you dont have OBD-II? Thats right around the time they started adding that.
See if you have an OBD-I port. Odds are youll have one if you dont have an OBD-II port. If it was me, thatd be the first thing Id try before adding extra hardware to find the speed.
First off, yes if you have OBD1 or 2 then that would probably be easiest. Maybe... It's not that bad to get the speed if your speedo is getting pulses.
You can't easily check the speedo signal with a multimeter. The problem is that the line level is rapidly switching between 0 and full voltage. What is your multimeter to think? Depending on how fast the wheels are spinning you might be able to see the voltage swing back and forth or it might just totally confuse the meter. However, if you have no o-scope then you still could use a multimeter to find the maximum voltage and step it down if it's too high. Then provide the signal as an input to the arduino and you can watch the signal transitions within a sketch. But it's more fun to use a frequency to voltage converter. You can even make one yourself out of discrete components. Google is your friend.
The speed pulse signal is at the outside pin of a grey, 3 pin connector. This connector is in the engine compartment on the driver side, on the gear box.
With the key in run position you should see battery voltage on one pin and ground (0v) on another, the 3rd pin will pulse from 0v to 5v as the wheels rotate.
The speed pulse wire colour is listed as Yellow/white.. it should be the same at the ECU and at the gauge cluster- but knowing Honda it may be different- I have seen White/Yellow used as well on the CRX's. :-?
Hope this helps