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Frequency counter as gas engine tach?

Discussion in 'Electronic Basics' started by Chevy454, Jan 26, 2008.

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  1. Chevy454

    Chevy454 Guest

    I have a Radio Shack frequency counter (#22-305), with two frequency
    ranges and a digital pulse mode. Its input is a 50 ohm BNC connector.

    Can someone suggest a suitable inductive pickup for this to wrap
    around a spark plug wire that will provide a signal that is not too

    My first impulse was to use a a few feet of RG-58 with a BNC at one
    and and a foot of insulated wire on the other end soldered to the
    center conductor, fitted with an alligator clip to clip to the
    exposed cable shield after wrapping the spark plug wire a few times.

    Ken C
  2. Tom Biasi

    Tom Biasi Guest

    There are many ways to pick up the impulse from the spark plug. Your way
    will grab some signal.
    You will then need to condition the signal for your counter.
    You will need a scope to look at what you are getting and shape it to the
    needs of your counter.

  3. Jamie

    Jamie Guest

    Use 1 loop of wire in a very small radius. Use a pair of diodes
    like 1N914's to clamp the input at the 2 connections. Each diode
    goes the opposite direction across the connection..
    You may also want to use a 47 ohm resistor in series with the loop
    feeding the center connections. the diodes should be clamped at the
    center connection.
    this will give you an average of 0.6 volt +/- pulse to the input of
    the counter..

    all you should need to do is just lay the loop next to the wire.

    It's a crude method but I've done a coupling like that before to
    detect RPM.
  4. The frequency of an engine at highway speed, maybe 2500 rpm, is 41.66
    Hz. The frequency of firing of a single cylinder is 1/2 of that
    (four-cycle engine), or about 20.83 Hz. If you can pick up a signal
    for all cylinders, as on the center of the distributor cap on an older
    car, you would get 125 Hz for a six-cylinder engine.
    According to some web references, the frequency counter you have has a
    lower frequency limit of 1 MHz, so it probably won't work.
    Even if your frequency counter could see it, for such low frequencies
    a period measurement might be more practical, if you invert and scale
    it for rpm.
  5. Clint Sharp

    Clint Sharp Guest

    Actually, it might not be. Depends on the ignition system in use, if
    it's a Distributorless Ignition System it could be using wasted spark
    where the coils are double ended and fire two cylinders (one firing, one
    exhaust stroke) at a time, hence it fires twice per revolution. It could
    also be multi-spark in which case you're screwed. Of course it might not
    even use an ignition coil if it's a lawn-mower engine or similar!
    On a DIS coil pack you can get a very usable signal from the cables back
    to the ECU from the coil-pack, I've used 12 turns of roughly 26 SWG ECW
    on a toroidal ferrite core using a couple of diodes and resistors to
    clamp the signal as 'Jamie' describes and had excellent results.

    The best results I've had were using a hall effect device (get one from
    a dead DC brushless fan) on one of the injector bodies feeding a simple
    comparator (LM393). Of course this will also work on the body of an
    ignition coil if the car doesn't have fuel injection or you just can't
    get at the injectors (single point injection)
    Period measurement with averaging and smoothing (you have to reach a
    compromise though if the engine can change speed quickly, think bike
    engine) is the best way to go, even if the frequency counter can measure
    that low the gate time will be a killer unless there's a PLL to lock in
    (then you have problems with jitter because internal combustion engines
    are rarely stable from one revolution to the next for a myriad of

    If the OP is just trying to measure RPM it doesn't have to be a spark
    plug signal, there may well be a top dead centre signal from the
    camshaft or crankshaft or at least a crank position sensor that outputs
    some kind of pulse train (usually an inductive sensor and a pattern
    formed into the flywheel with a missing pulse to indicate some period
    before TDC)
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