Magnetic pickup pulse - help ?

Discussion in 'The Projects Forum' started by Man_in_UK, Jun 27, 2008.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I dont understand the pulse I am getting out of the magnetic pickup sensor on the ignition system of an engine.

    I need to understand exactly what part of the pulse triggers the electronic ignition unit so I can build a circuit that emulates it. I put a scope across the pickup to see what is there. The pickup is ungrounded at about 100ohm across two leads, the two leads from this pickup go into the CDI unit that measures about 3k across the two leads with one lead going to ground.

    My first thought was that the CDI uses ground as zero volts and the pickup coil would generate about 1v positive pulse as a trigger .... not so.

    The scope trace shows the pulse to be made of a negative slope - a pause - then a positive slope, each of the 3 sections some 500us long.

    I checked this input pulse against the output of the CDI to find that it is the negative slope that triggers the spark.


    I can build a device that sends a voltage as a trigger but do not have a clue about building one that generates a -2v. HELP please.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It's likely an inductive pickup. When the magnetic lines of flux pass through the coil, the polarity of the voltage across the coil changes.

    This would be easy to change to a positive-going pulse using an NPN transistor, a capacitor and a few resistors. See the attached for one possibility.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Although the pickup produces a negative pulse, the input to the CDI probably does not need to go negative to trigger the unit. There will be current and voltage limiting on the input of the CDI to prevent a negative pulse (and excessive voltage) from reaching the electronics.
     
  4. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    So you think I will be OK to stick with a positive trigger and see where it goes ?
     
  5. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    Im not really concerned with converting this pulse to a positive, I am more worried about creating a similar pulse to do the same job. ALTHOUGH, your circuit could come in handy. How would the values change to run it a 5v ?
     
    Last edited: Jun 28, 2008
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, ok. I misread your original post.

    If you wanted to use the circuit I drew up on 5v, then reduce R2 to 39k, and R3 to 390 Ohms.
    R2 keeps the transistor biased close to where it will turn on. R3 limits the collector current. R1 limits the base current. C1 blocks the DC level of the input signal, but passes the effects of the voltage level changes.

    On simulating the signal:
    Do you have a drill press?
    You could build a small magneto using a 1/4" machine screw, 1/4" lockwasher, a 1/4" nut, a fender washer, a couple of Neodynm magnets, some epoxy (J-B Weld is preferred), a flat head carpenter's nail, some magnet wire, a bit of tape, a drill press, a drill press vise and C-clamps to hold the vise to the drill press table.

    Epoxy the magnets near the edge of the fender washer, opposite from each other but on the same side. Allow the epoxy to fully cure. Insert the machine screw throught the center hole of the fender washer from the side the magnets are epoxied to, add the lockwasher, nut, and tighten the nut. Chuck the threads of the screw in your drill press.

    Take the nail and wind perhaps 50 turns of magnet wire on (your mileage will vary), as close to the head as you can without having any sticking out above the head.. Hold the turns on with a bit of tape, with the two ends of the coil taped away from the head. Clamp the nail vertically, head up, in a drill press vise.

    Place the vice on your drill press table. Adjust the position and height of the nail's head so that it is centered under one of the magnets you glued to the fender washer, as close as you can get it to the magnet without touching. Clamp the vise to the drill press table using C-clamps.

    Ensure that your drill press is set to the lowest speed.
    CAUTION: if the magnets are not securely attached, or they contact the head of the nail, they may fly off at high speed and cause damage. WEAR EYE PROTECTION.

    When the drill press is running, you should see a signal across the coil that is not unlike the input to your CDI ignition.

    You can decrease the signal level if necessary by increasing the space between the nail head and the magnets, or removing turns of wire from the nail.
    You can increase the signal level by increasing the number of turns of wire on the nail. You could also amplify the signal using a circuit similar to the one I originally posted, with a 10uF capacitor on the output to remove the DC level.
     
    Last edited: Jun 28, 2008
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I think it will probably trigger on a falling edge i.e from +V to 0V.

    If a negative pulse is required and you have a source of positive (0V to +V) pulses, as long as your CDI and pulse source don't share a power-supply or ground connection, you could simply connect the output of the pulse source the opposite way round. The CDI input will then go negative with respect to ground but put a few hundred Ohms between the two to limit the current.
     
    Last edited: Jun 28, 2008
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