Hey everyone, I just got my arduino in today. I have played with them a decent bit controlling motors and servos and such. I actually programmed a few senior level mechanical engineering projects (not as impressive as it may seem at first) with them for my classmates. Anyway, I do not have a lot of electronics experience, but i have a general idea in my head. I want to make a jetski tachometer for a two stroke engine. If I wrap a coil of wire around one of the spark plug wires on the thing and use that as an inductor, will that give me a certain voltage back through that coil that I can read quickly with the arduino? I want to capture how many times i get a voltage per minute, and turn that into RPMs, which should be a one to one ratio on a two stroke, I believe. If you all don't mind, I would appreciate any help i can get. I do not know how to wire up a LCD screen for the future, but for now i can use my computer to read the output. Thanks!
EDIT: This is on an Arduino UNO. Sorry that I didnt clarify at first.
You plan seems fairly good in theory. Here are a few potential issues to be aware of.
You could easily get voltage spikes well in excess of what the inputs are rated for, so make sure to add clamping diodes to limit this. Feel free to reply back and myself or someone else can show you how to do this if you don't know.
The spikes may be very fast and hard to read. If they can pull a digital input all the way up you could use that instead of an analog input for added speed, but you may find that you still are sampling to slowly to trigger on the pulses. You can fix this by adding some capacitance to slow the spike down, and useing interrupts on a digital line to count the pulses in the code instead of sampling. If that still doesn't work you could add some other electronics to reduce the pulses to a square wave that the Arduino could read without issue, but its probably not worth taking that road unless you have to.
As far as the LCD is concerned, pick one you like, search the internet, and reply with questions. In general they are quite user friendly.