Creating a Tachometer Using a PICAXE 28X1.

Discussion in 'The Projects Forum' started by Jack Bourne, Apr 21, 2009.

  1. Jack Bourne

    Thread Starter Active Member

    Apr 30, 2008
    Hi, I am going to build a gokart this summer and I would like to build a tachomemter for it.
    I know you get a lot of this saying that people want to build tachs but bear with me.
    I know the engine I am using does not have any data as its a simple engine.
    How would I go about building one?
    My main idea is to:
    Attach a magnet to the drive shaft and use a hall effect sensor to record pules or even a reed switch. The sensor is actually cheaper.
    I can forsee problems with this method due to the moving parts.
    Can you give me any advice.
    Forgot this forum was around.
    Thank you.
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    If it is a single cylinder, magneto driven ignition, a sense wire next to the spark wire (Not Connected) will get 1-2 volta of induced voltage in my experience.

    I've oonly done this to check for spark, and not interface with a uC, but the appropriate isolation and clamping of the signal to logic levels,it could be worth a try. Much cheaper and easier to change into a different kart that way as well.

    look for schematics online for a timing lite to see the isolation needed.
  3. bundick

    Active Member

    Dec 19, 2007
    Neither the Hall effect or the reed switch's are fast enough to be used as a tach.
    THat idea of catching a pulse off the Ignition is best.

    I've never done this but its an interesting idea.

    How about making one loop around the spark plug wire, with a pretty well insulated wire of small guage? (What the hell, use any guage you want to)
    Ground one end of your Tap (robber) and use the other end to input to a counter assembly of some sort.

    Test the application on your car right now. Make a robber, ground one end, and attach a small incandescent light bulb to the other end of the robber, ground the bulb and see what happens.

    By the time you get back to us, I'll know because I'm headed to the barn right now to locate all that stuff for a test of my own.
  4. Jack Bourne

    Thread Starter Active Member

    Apr 30, 2008
  5. bundick

    Active Member

    Dec 19, 2007
    Maybe I'm wrong about the H-E Switch.

    I'm not wrong about the Reed switches though.
    We started using magnets and reed switches when we made our first Weather system.
    They dont last long in the atmosphere, and are way to slow to catch a fast wind driven propeller.

    Assume several gates associated with an analog meter. Like a Frequency to voltage converter.
    If they are all 5us gates and an enabling term for a total of 10 gates, you've lost 50 us to the driver on that one pulse.
    So how many us is the H-E going to see a 3500 Rpm?
    Now that I look at this, it dont matter, because the Meter cannot react fast enough to notice a missing pulse or two.

    What I call a 'robber' is just what I said about the wire wound one turn around the spark plug wire.
    That effectivly is a counter of the Ignition pulses. At 3500 Rpm you'd get 3500 pulses.

    By robbing those pulses, you may be able to feed that into an amplifier/Meter movment and get a higher reading for more pulses. Like an AC volt meter.
    I think in a counter like a tach, the 800-1200 pulses per Minute are kinda slow to the meter needle might be bouncy or erratic at low Rpm's.

    I think I remember something about those counters working better at higher Rpms.