Need help designing a circuit

Discussion in 'The Projects Forum' started by oswaldonfire, Jun 21, 2010.

  1. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010

    I am trying to design an electronic ignition delay device for an engine. I am going to be basing the timing of the spark off of the rotation of a shaft. I was thinking to do this either optically or by using a hall effect sensor.

    My question is this: Once I get a signal coming off of the rotation of the shaft (sending an impulse for each rotation), how do I turn that into a spark? I was thinking of maybe using that signal to drive a FET to switch on and off a high voltage power source at the appropriate times. Does this sound feasible? If so, can you give me any pointers on where to start?

    Thanks in advance,

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    What is the engine and what is the application?
  3. SgtWookie


    Jul 17, 2007
    I suggest that Hall-effect sensors are the way to go. Optointerruptors can get fouled quickly, rendering them useless.

    I suggest using two Hall-effect sensors; one that outputs a single pulse somewhere around 45° before top dead center, and a second sensor that pulses each time a flywheel gear tooth passes.

    With those inputs, compared to a couple of fixed-speed counters, you will be able to determine from a starting reference point over time, what the actual crank angle is, what the RPM is, and whether the engine speed is increasing, decreasing, or constant.

    This is an application that literally screams for a microcontroller. Trying to do this using logic ICs would be agonizing.

    Generating a spark from the derived data really isn't a beginner's topic. Unless you've been doing electronics projects quite regularly for a few years, you would be better off to stick with an off-the-shelf solution.
  4. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
    Beenthere, The engine is a small 2-stroke weed-eater engine. I need to delay the ignition timing by about 18 degrees, because I will not be running it on gasoline.

    SgtWookie, I was thinking of a much simpler system. I was just going to position a hall-effect sensor where the spark should be, at approximately 10 degrees past TDC. Then I was going to take the pulsing from the sensor and interface it with a microcontroller, that would then output a 'fire' signal whenever it recieved a pulse from the hall-effect sensor. My question was more of how would I take a pulse from, say, the microcontroller, and turn it into a high voltage spark? Do I need to use some sort of voltage amplifier? Or maybe a continuous high voltage source that the microcontroller switches on or off with a relay or transistor?

    Hope my question is a little clearer now.