Hello,
I'm working on a speedometer project for my car, I have located the wire which supplys the original instrument cluster with the signal below, obviously the frequency changes with speed:
What is the best way of dealing this signal for an Arduino to read? It appears to be around a 150mV (300mV peak to peak) pulse. Is a basic op-amp and possibly a recifiter in order?
Any advice would be very much appreciated!
One consideration would be that we (you) don't know how much load you can put on this signal without possibly effecting something else. Because of this I would suggest you look at a comparator such as an LM339.
You could possibly get by with an op-amp as well but the LM339 is designed for this purpose and there is less internal delay in changing state. However if you have an virtually any general purpose op-amp that works with a single supply and down to 5V (or you at 3.3) ti should work find.
Hi,
neilbaker86:
I have located the wire which supplys the original instrument cluster with the signal below, obviously the frequency changes with speed:
Apologies in advance if I seem a bit skeptical, it's just that I don't think that looks like an actual signal - it looks like noise induced from some squarewave signal nearby. It is also quite a low level to be wired up to an instrument cluster.
Do you really mean speedometer and not rev. counter ?
Are there any other signals in that wiring loom that look the same, but maybe 12V or something ?
Anyhow, If you are happy that's the one... Yes, an opamp or, as has been suggested, a comparator might be ok.
You'd need to feed that signal via a resistor to a diode to clamp the -ve spike and just read the +ve one (assuming you'll be running the sensing circuit on single-rail 5V).
This LM339 Application Note might be of interest.
Yours,
TonyWilk
Hi,
When you connected the scope to the signal point did you connect the scope gnd to the car chassis?
What road speed was that at?
Thanks.. Tom... 
Hello,
Thank you all for your kind replies, yes, the ground is connected to the vehicle chassis. It is definitely the speedometer, and it is the correct wire as the speedo drops down to zero if you disconnect it. I cannot remember what speed the vehicle was going when the screenshot was taken, but the frequency (displayed at the bottom) definitely goes up and down linearly with speed. I think from memory it was about 300hz at 30mph or thereabouts.
Here is another screenshot from the scope showing one of the positive pulses with the time reduced.
neilbaker86:
Here is another screenshot from the scope showing one of the positive pulses with the time reduced.
Ok, following on the LM339 theme...
Diagram:
Done with LTspice (it's free), I've linked the LTspice file in a .zip.
Yours,
TonyWilk
pulse_comparator.zip (732 Bytes)
Hi,
Attached it is maxim chip that you may use in your project. It will take the magnetic pulse and cleaned it. Just a suggestion.
MAX9924-MAX9927.pdf (343 KB)
