Spark lines from o'scope

[Tom Cloud]

>A bit of help from you all out there.  I attached my inductive pickup from my
>timing light to the coil wire and hooked it up to a port on the o'scope
>(Chan. A).  Got the nice spark lines and coil ocsilations like I should, but
>as RPM changed, so did the grouping of spark pulses.  
>
>Here is the request:  Is there a simple and cheap ( < $30) circuit that I
>could attach to Chan. B that would adjust for a certain about of pulses
>across the screen, then start over (like when #1 fires).  It should work from
>idle to max RPM (6000).  
>
>A triangle wave generator would give me the sweep, but at a constant
>rate...???..

to make it always trigger on cylinder #1 -- or any other -- pick off
a signal from that individual wire and take your trigger from that.
You can use either the external trigger or, more realistically, just
set the trigger to take its input from the channel that's looking
at the individual cylinder you want to use as the trigger.

to keep the sweep width constant .... hmmmm, I'd have to think on that
one.  You don't want a tri-wave ... you want a sawtooth fed into the
horizontal ("X") input -- put the 'scope into "X-Y" mode or external
on the sweep knob -- however your scope is set up.  Off the top of my
head, I'd say you need to take a tach signal (i.e. a f-v DC level)
and drive a VCF -- like an Exar 2206 or an Intersil 8038.  They
can be had for a few dollars -- they are function generator chips
that will give sine, square and tri-waves.  Been awhile since I've
used them so I don't remember if you can vary the duty cycle to get
a sawtooth.

Tom Cloud

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