More Inductive Pickup

[Bruce Bowling]

~ 
~ > I was told that an inductive pickup (from a timing light) will
~ > produce enough current to flash an LED.  I have not tried this yet,
~ > however, I am wondering if the pulse width from a spark is long
~ > enough to "see".  What is an "average" length of a spark?
~ > 
~ 
~ I found that the voltage was enough to light a LED most of the time.  
~ I used a wave shaping circuit (AM radio detector like) and then through
~ a opto-coupler.  This worked for some motorbikes and not others.  Something
~ to do with lower compression I think.
~ 
Well, last night I hooked an LED across the inductive pickup probe
(as per Wayne Glasser), and hooked it up to a plug wire, and it works,
at least for a stock GM HEI.  It was dark outside, and one can see the faint
pulses from the LED, which got brighter when the car was revved up,
I think helping my eye persistance.  What was interesting to note was
at idle, every once in a while, the light flashed a "tad" brighter
for one pulse, and this corresponded to a RPM sputter (I have a somewhat
long duration cam installed).  

The opto-coupler is a nice idea in that it adds another layer of
isolation (separate from the pickup itself) for the logic-level
circuit.  The light was very faint on a garden-variety LED, and I 
do not know if the LED in an opto-coupler will conduct the
phototransistor very hard.  But there are opto units with Darlington
phototransistors, and even some with Schmidtt trigger outputs.

Of course, hooking this rig up to an MSD ignition would count
every spark pulse (something like 20/spark event at idle) and
screw up any counting logic circuit.  One fix is a retriggerable
monostable, like a 555, with a one-shot period slightly longer than 
period between MSD pulses.

I now can play...

- Bruce




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               Bruce A. Bowling
  Staff Scientist - Instrumentation and Controls
 The Continuous Electron Beam Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
                bowling at cebaf.gov  
        http://devserve.cebaf.gov/~bowling
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