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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Spark plug Inductive pickup.
Message-ID: <1991May11.191724.9745@milton.u.washington.edu>
Date: 11 May 91 19:17:24 GMT
References: <1991May10.164429.4146@ducvax.auburn.edu>
Organization: University of Washington, Seattle
Lines: 24

In article <1991May10.164429.4146@ducvax.auburn.edu> wdperry@ducvax.auburn.edu writes:
>
>I need to build a device, using an inductive pickup like a timing light, 
>that will give a single clean pulse each time the spark plug fires. Pulse
>needs to be at least 3.5 volts.  It must be used on the spark plug 
>side of the ignition not on the coil side.

	The problem is not so much lack of signal as it is the 
waveform (lotsa jaggies).  So, use a one-shot (monostable multivibrator,
like a 74HC123).  The first transition will start the output pulse,
and until the pulse is finished, NO OTHER TRANSITIONS on the line
will have any effect.  For something like a spark plug, there is
a fairly slow firing rate (7000 RPM on a four cylinder engine
would only be 17 milliseconds, and more cylinders or slower
revs would be longer time).  Just set the monostable time constant
to 10 ms, and you'll get clean pulses from dirty input.
	Of course, you should be sure that the transformer
secondary is loaded with a low impedance (so the spark won't be slowed
by the transformer).  Best way to do that is with a diode-clamped
transistor inverter; when the transformer output is (+), the
base-emitter diode is forward biased; when it is (-), the clamp
diode (connected from base to emitter) is forward biased.  
	
	John Whitmore