555 circuit failure to spark ignitor

Discussion in 'General Electronics Chat' started by samjesse, Sep 14, 2008.

  1. samjesse

    Thread Starter Senior Member

    Sep 14, 2008

    I stripped an ignition coil of a car and have it on the bench. The primary coil has the terminals p1 and p2, the secondary coil terminals being s1 and s2. I believe it is a toroidal core transformer setup.

    When I briefly touch p1 & p2 to the terminals of a 3v battery, arcing occurs between s1 & s2 which is what it suppose to do, notice it happened with out using a capacitor between p1 & p2 as it is not the case with automotive ignition systems.

    Now I made up a 555 circuit to do the “briefly touch” action of fast on/off of the primary coil current but I can not get the arcing between s1 & s2 regardless of a capacitor presence or not.
    The circuit blinks a LED so it should work fine. please notice the circuit attached, the LED and the p1&p2 get connected between the Output "pin 3" and V0 off the 555 chip, the capacitor gets connected between these points as well.

    R1 and R2 are LDR
    C1 = 47μF
    the is a capacitor between pin 5 and V0 of the 555.

    Please help me understand why I am not able to obtain spark using the 555 circuit regardless of using 12v or 3v battery.

    Thank you
  2. bertus


    Apr 5, 2008
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    One major problem you face is that it takes more current than the 555 can handle to make the spark. The spark coils I am familiar with require a current of about 7 amps to build a field sufficient to deliver an effective spark at the sparkplug.

    Yours may be different in that it requires less current, but it is almost certainly beyond the capability of a 555 timer's output.

    The 555 can control the conduction of a power FET that can handle the current requirement of the spark coil.