i'm making an RPM reader for my gokart and i'm hoping you could help me out with some issues.
i already tried this:
but it doesn't work.
i have a starter kit and i'm hoping to use the LCD thats included ( alphanumeric LCD (16x2 characters)
i'd also like to change the reed switch to a hall sensor, do i have to watch certain characteristics when i buy the sensor?
What didn't work? A reed switch isn't going to respond if you ask it to switch too quickly. It's also not a very reliable solution. Still, the basic principle seems ok. Is the magnet close enough to the switch?
As you suggest, a Hall sensor is a better idea in the long run. You probably want one with a digital output, good rise/fall times, and unipolar switching. I've used a TLE4905 Hall sensor for speed sensing on an RC car (up to ~10,000 transitions/second), but there are plenty of other possibilities.
Do you need physical RPM, or are you just looking for engine RPM? I ask because most gokart data acquisition systems get RPM from the engine and do it with a sensor tied to engine spark. I'm fairly certain it's some kind of clamp sensor that just goes around the outside of the plug wire and doesn't actually CONNECT to the wire (and thus is some kind of current sensor, I'd guess). I'd research that angle if I wanted engine RPM.
If you are looking for axle RPM (which can be different from engine RPM since you have a clutch that slips or a gearbox, depending on your gokart), then yeah, you'll need something more like a hall effect sensor. Stingray covered that pretty well in his response.