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Spark Plug Wire Tachometer Conversion To Magnetic — Parallax Forums

Spark Plug Wire Tachometer Conversion To Magnetic

JymBoeJymBoe Posts: 4
edited 2014-02-01 16:04 in Accessories
I have a tachometer using a wire wrapped around a spark plug wire as input, is there a way to easily convert the input to a magnetic pickup?

Comments

  • kwinnkwinn Posts: 8,697
    edited 2013-12-21 12:06
    Probably no real easy way to do it, but the level of difficulty depends on the engine. How many cylinders does the engine have, and are there any accessible gears or pulleys that can be used to mount a magnet or sensor?

    What kind of circuit is used to measure the RPM. Something that can be reprogrammed or something that is fixed and needs a specific number of pulses per revolution?
  • JymBoeJymBoe Posts: 4
    edited 2013-12-21 13:07
    This will be used on a mini lathe, no spark plugs, hence the conversion. My thought was to affix a magnet to the main shaft. The tachometer is a cheap battery powered device only displaying RPM and total hours operated. It can be adjusted for the number of pulses per revolution.
  • kwinnkwinn Posts: 8,697
    edited 2013-12-21 14:08
    In that case it could be relatively simple. Either a magnet could be fastened to the end of the shaft, or if there is a toothed gear on the shaft it could be used to count teeth per revolution.
  • JymBoeJymBoe Posts: 4
    edited 2013-12-21 15:14
    What I need to know is how to best position the tachometer input wire to receive a pulse when the magnet passes by. The wire normally would expect a pulse capacitively coupled by the spark plug wire but, in this case, the wire would get a pulse induced by the magnetic field. I don't know how the two types of pulses would compare regarding amplitude.
  • kwinnkwinn Posts: 8,697
    edited 2013-12-22 11:06
    JymBoe wrote: »
    What I need to know is how to best position the tachometer input wire to receive a pulse when the magnet passes by. The wire normally would expect a pulse capacitively coupled by the spark plug wire but, in this case, the wire would get a pulse induced by the magnetic field. I don't know how the two types of pulses would compare regarding amplitude.

    You would need more than a piece of wire near the magnet to get a useable signal. I was thinking you intended to use a hall effect sensor or pickup coil to produce the signal from the magnet. A pickup coil might produce a signal that had enough amplitude to drive the current tachometer input circuit, but a hall sensor would probably require some modifications to the input circuit. Can you post any information you have for the tachometer?
  • JymBoeJymBoe Posts: 4
    edited 2013-12-23 14:22
    A pickup coil is what I had in mind, just not sure how it should be oriented to get max signal. The tachometer is a couple of ICs, components and a display, not something I have the tools to diddle with. I think that I will have to concentrate on feeding a pulse to the input wire as it exists if I'm to use the tachometer that I have.
  • kwinnkwinn Posts: 8,697
    edited 2013-12-24 21:19
    A coil from a small relay might work with the core pointed at the magnet pole. A small BLDC would also work, and would not need a magnet on the shaft since it has one internally.
  • HughHugh Posts: 362
    edited 2014-01-29 05:11
    I have just started measuring engine r.p.m. by wrapping a few turns of wire around the sparkplug lead and connecting this 'coil' to the anode of an an LED. The LED cathode is grounded to the chassis. Each spark sends a pulse of light down a length of TOSLINK to an IR receiver with a Propeller detecting and counting the time between pulses. A 3V3 zener protects the LED.
  • kwinnkwinn Posts: 8,697
    edited 2014-01-30 18:27
    The spark plug wire induces a current in the wire wrapped around it so you do not really need the zener. If that is all it takes to produce light output from a led then connecting the coil to the led in an optoisolator along with a resistor to complete the circuit on the transistor side of the opto might be all you need to produce a useable output signal. Worth trying it out at least.
  • HughHugh Posts: 362
    edited 2014-01-30 23:39
    kwinn wrote: »
    The spark plug wire induces a current in the wire wrapped around it so you do not really need the zener. If that is all it takes to produce light output from a led then connecting the coil to the led in an optoisolator along with a resistor to complete the circuit on the transistor side of the opto might be all you need to produce a useable output signal. Worth trying it out at least.

    I talked myself out of doing that - partly because I wanted to be able to see how far the pulse of light were getting (!) and partly because it is a noisy environment and even with shielding I might be picking up the odd pulse from somewhere. Might use it in the Mk II version.
  • kwinnkwinn Posts: 8,697
    edited 2014-02-01 16:04
    The nice thing about using an optoisolator led as the detector is it's low impedance. It usually requires several mA of current to produce a detectable output signal on the transistor side, which makes it relatively immune to most forms of electrical noise. I use optoisolators and 10-20mA drive for signals in noisy environments or areas with ground loop problems. The combination of high noise immunity and electrical isolation is great.
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