flip flops to drive ignition

[Chris Conlon]

At 04:28 PM 1/15/01 +0800, Dan Zorde wrote:

>How about using the ignition outputs to drive a couple of Flip Flops, then
>you could tie opposite cylinders to the Q and Q(not) outputs, thus every
>time the ignition driver fires it toggles the FF to alternatively drive the
>coils.  You'd just need to figure out some way to overcome this 1/10 sec
>auto fire Orin was talking about.

A friend of mine has a system that works along these lines.

You need some kind of crank or cam position signal. If you only want to
do wasted spark (on his 4 cyl at least), crank position would be enough.
If you want to do coil per/on plug, you need cam position.

His system taps into the stock dizzy reluctor. In this case (Toyota
SW20) there are signals for TDC compression on #1 and #4. There is a
simple flip flop counter circuit that figures out what cylinder is
next. It free runs, based on ignition pulses, and the dizzy signals
essentially reset it to a known state.

The stock Toyota ignition driver runs off a logic level signal, +5
to charge the coil, drop to 0v to fire. I *think* at one point he
was using 3rd gen RX-7 coil packs, which are apparently waste fire
but with the same type of drive signal. Supposedly they have a
pretty hot spark too.

>coils.  You'd just need to figure out some way to overcome this 1/10 sec
>auto fire Orin was talking about.

The simple version of his circuit had this potential issue too, but in
his case it did not seem like the RX-7 coil packs objected to having
the trigger held high that long. I'm sure that power dissipation went
up undesirably at lower rpms though. One easy fix to this is to AND the
factory ignition driver sigal with each flip flop output, or use a
multiplexer. This way the coil pack will only charge when the ECU wants
it to, not over a whole 1/4 (or whatever) rotation.

Another issue he ran into at one point was conditioning the reluctor
signals. This caused some hassles. Also since the reluctors were still
needed by the stock ECU, it was important that the circuit didn't
interfere with that operation. 2 parts that may be helpful: the LM1815
(Nat Semi), which is an IC designed to deal with reluctor signals, and
MSD's p/n 8209 ignition stabilizer, which is probably not much more
than an LM1815 in a box, with a digital output.

   Chris C.

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