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building a homemade tachometer ...

Discussion in 'Chainsaw' started by Molecule, Jan 23, 2005.

  1. Molecule

    Molecule ArboristSite Operative

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    Radio Shack (and others) has several inexpensive "Digital Multimeters" (DMMs) which can measure pulse frequencies. On sale they run about $50. That's a lot cheaper than a tach, and DMMs are always useful around the house, etc. They can measure pulse frequency from 400 on up (to 4 million hz), but the limit is the max pulse voltage, which is 10v, above which the circuitry is probably fried.

    Obviously, one would *never put the meter leads in series with the plug (10K volts ? would absolutely fry the meter), so what I did was make a loop of wire (actually 5 loops of 14ga multistrand) and then slipped the loop around the spark plug wire. so the spark plug wire goes from the ignition to the spark plug, clean like before. but wrapped around the sparkplug wire is now a loop of "pickup" wires. The DMM leads are connected to the loop.

    When idleing, the current in the spark plug wire "induces" a current in the pickup loop, which is hooked up to the DMM. It measured idle speed ok, but the frequency was very jumpy, so I need to stabilize the pulse pickups somehow. But, increase the throttle just a little, and the meter output jumps all over the place, as though it's internal circuitry can't "tag" or decide on a frequency.

    What does the pickup on a Stihl tach look like ? Maybe I could copy that.

    Any hardcore electronic hobbiests out there have any suggestions ... the money I save would mean a new pair of boots.
     
  2. sedanman

    sedanman Just cut the piano!!

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    The price of a digital multi meter rises in direct proportion to its update rate. What you are seeing is a high frequency signal as displayed on a low frequency meter.
     
  3. Ranchero50

    Ranchero50 ArboristSite Member

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    I think a better approach would be to use a good tach reading timing light like Snap-On's and power it off an automobile 12vdc battery. I'm just not sure how it would react to a 1 pulse to one rev count on a two stroke vs a 1 pulse per 2 rev counts at higher revs. Seems it would say the saw was revving 26,000rpm for 13k and may be out of the meters range of pickup.

    I was also considering using one of my spare Autometer monster tachs and hooking the green - coil wire to the cut off switch lead and seeing if it would work. The - coil lead goes to the points, but is driven off of a 12vdc power source vs the saws 4-5vac system power. I'd still have to power the tach with a battery, but it would worth it for test cuts.


    Jamie
     
  4. scottr

    scottr Addicted to ArboristSite

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    Loop

    Molecule , why don't you try a single loop instead of 5 loops . You might be inducing more voltage than necessary . Scott
     
  5. rahtreelimbs

    rahtreelimbs A.K.A Rotten Tree Limbs

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    Have a look see. I don't think it gets much cheaper.

    Bailey's sells them.

    [​IMG]



    SenDEC's shop tach is a high RPM maintenance meter. This tachometer reads up to 16,000 RPM, has maximum RPM recall and clear functions and can be programmed for up to 3 different firing patterns. This patented meter uses one simple push button to access all of these functions. Comes with screw mounting holes if you wish to attach the tachometer to a permanent surface.


    SenDEC STS-5000 Shop Tachometer
    $69.95
    Part #17122
     
  6. ol'homey

    ol'homey ArboristSite Operative

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    I have an optical tach used to measure rpm by pointing it at the prop of a model airplane. It cost $20 new and will go to 20k rpm or so I think. I have measured the rpm of a driveshaft by sticking a dot of white paper on it. If you point it at an AC light bulb it will read 3600 rpm (60hz*60sec/min). I can check the rpm of my generator by just plugging in a drop light and pointing the tach at it. I wonder if you placed one of those in line spark testers in the spark plug lead if it would pick up the flash and read the rpm of the saw? If that doesn't work I know that if you can figure how to get a small incandescent bulb to flash with the spark it will trigger the tach.
     
  7. NeTree

    NeTree Addicted to ArboristSite

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    molecule, if you were project-minded, you could build a really nice one from rad-shack components for under $20.

    The most expensive parts might cost you $5 together... the LM555 timer and the LM324 quad op-amp.

    Everything else is discrete components.
     
  8. NeTree

    NeTree Addicted to ArboristSite

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    Just a thought... try a 1N914 diode in series with your pickup leads.
     
  9. pbtree

    pbtree Addicted to ArboristSite

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    Erik, you are scaring me! Are you into electronics as well?
     
  10. NeTree

    NeTree Addicted to ArboristSite

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    Why would you think that?
     
  11. lesorubcheek

    lesorubcheek Addicted to ArboristSite

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    I've researched a bit in the past and found this project that looked like it might work on saws www.aaroncake.net/circuits/tach.htm. It would have to be modded a bit because its made with options of 4/6/8 cylinder 4 stroke engines in mind. Cool thing is its wireless. The only part that would be expensive would be the display meter. I've found some digital ones for $20.00 or so.

    I never even thought about using an optical tach for RC planes before. I've got one but had never even considered using it for a saw. Think I'll have to give it a shot.
     
  12. davethetreeman

    davethetreeman ArboristSite Lurker

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    Could you send me the schematics?
     
  13. Roanoker494

    Roanoker494 Addicted to ArboristSite

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    You have just responded to a thread that is 7 years old. Nowadays you can buy a tach at Northern Hydraulic for $35.
     
  14. REJ2

    REJ2 Firewood Hack

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    Pretty sure some of those dudes went on to star in The Big Bang Theory
     
  15. Somesawguy

    Somesawguy Addicted to ArboristSite

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  16. davethetreeman

    davethetreeman ArboristSite Lurker

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    ?
     
    Last edited: Mar 7, 2012

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