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Stamp tachometer problems

Discussion in 'Electronic Basics' started by steamer, Dec 22, 2009.

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  1. steamer

    steamer Guest

    --There's a site up with an excellent description of how to build a
    tach with a Basic Stamp, at: Nili/rpm/tachometer.html
    I built this circuit the other day, then had an expert pal make sure
    I hadn't done anything wrong (left out one wire!).
    Once all was set to rights I tried the three programs to see what
    would happen.
    First one: got some readings on the debug screen when waving a
    magnet back
    and forth near the sensor: a good sign, yes?
    So then I loaded the second program and got the proper responses
    from the display, so all was well there.
    After loading the third program things started to get weird; i.e.
    we were unable to get consistent readings. What I did is tape a neodymium
    magnet to the edge of a little 2" sanding disk, chuck it in a small drill
    and power it up near the sensor. It seemed to do *something* but not what
    I expected; i.e. sometimes I'd get a reading after I removed the rotating
    but not while it was spinning. I thought maybe the neodymium magnet
    was too strong and switched to a weaker one but results were just as
    I'm wondering: might there be a problem with the "flywheel"
    being too small in diameter? Don't think this could be the problem but one
    never knows.
    There were also times when the numbers displayed were almost
    random; i.e. readings being much higher than the rate at which we waved
    magnets near sensor. It was all a bit baffling.
    We're sort of in the dark here, so any clues folks can pass along
    be greatly appreciated. Thanks,
  2. ehsjr

    ehsjr Guest

    Yes, a good sign. Did you get the proper number displayed
    that matched the number of times you waved the magnet
    back and forth?
    8888 and then 2845, as per the site you mentioned?
    The motor may be affecting the sensor, or broadcasting
    electrical noise into the circuit. Try a different
    setup that does not use that drill - or any electrical
    motor, for that matter. Also, be sure to run your
    circuit from a good 9V battery - NOT a power supply.
    No. Flywheel diameter has nothing to do with it. Spacing
    between the magnet and the hall sensor is a factor, as is
    RPM at which the flywheel is moving.
    That makes it sound like the results from program one were
    not correct. You need to be able to count the number of
    waves correctly:
    "At this time you test the program moving the magnet close to the Hall
    sensor. In the debug screen will see how many times you moved the magnet
    back and fort per sec, that number than is multiplied by 60 and gives
    you pluses per minute."

    If you can't get the correct results here, then you won't get
    the correct results with program 3. I'd suspect a wiring error.

  3. steamer

    steamer Guest

    --Yes; max rpm maybe 400.
    --Good idea; will try that next.
    --Heh, yeah; still I live in hope that some day I'll be able to get
    reliable numbers for my steam engines..
  4. steamer

    steamer Guest

    --Not really; chalked it up to inconsistent speed of magnet, but...
    --Yes, just so.
    --Hmmm; well I'll point out that the drill was on a flex shaft so
    motor was well removed.
    --Re: battery vs wall wart: will try that too; thanks!
  5. Bob Masta

    Bob Masta Guest

    If you want to test your sensor setup on an
    independent tachometer, you can download my
    Daqarta app that uses your sound card for input.
    One advantage of this is that you can see the
    input waveform and play with the triggering, which
    may give clues to your Basic Stamp problem.

    If the discussion below isn't adequate, note that
    you can right-click any control to get Help about
    it. (Or ask me!)

    Click on the thin button below 'Input' on the
    toolbar to open the Input control dialog, and make
    sure Line In is set. You can connect your sensor
    to a standard sound card audio cable cable using
    mini-gator clips. The tip is Left, which is
    easiest to grab, and the sleeve is ground. Toggle
    Input on and you should see the raw waveform from
    your sensor. Adjust Input Level (more-positive
    values are more sensitive) for an unclipped

    You can play with the Trigger controls to get a
    stable display if the default isn't acceptable.
    (I usually prefer Normal mode instead of the
    default Auto.)

    Now go to the Options menu and hit Frequency
    Counter (or use CTRL+F). Click on RPM and you
    should be in business. Note that the Frequency
    Counter window borders can be dragged to resize
    it, and the readout will grow or shrink to match.
    Depending on your display, you may need to make it
    wider the first time you use it, to see the
    controls at the far right of the counter. The Cyl
    control works with RPM for use with engines, but
    it's also for reading RPM from gear teeth. You
    need to set it to *twice* the number of teeth to
    get a direct RPM reading.

    This setup should give you an independent view of
    the signal into the Basic Stamp. Daqarta's
    30-day/30-session free trial should be enough to
    debug your Stamp problem. Enjoy!

    Best regards,

    Bob Masta

    DAQARTA v5.00
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Sound Level Meter
    Frequency Counter, FREE Signal Generator
    Pitch Track, Pitch-to-MIDI
    DaqMusic - FREE MUSIC, Forever!
    (Some assembly required)
    Science (and fun!) with your sound card!
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