Inductive Pickup Circuit

[Bruce Bowling]

I am making a tabletop ignition system for a high school
demostrator, and I wish to demonstrate how an inductive
pickup works.  The ignition system is nothing more than 
an HEI distributor driven by a motor, and 4 spark plugs.
My goal is to make a negative transition logic pulse
from the pickup to drive a counter circuit.

I have an inductive pickup from an old timing light.  It
seems nothing more than a ferrite core with a coil 
(about 20 turns).  I tried to scope the signal output,
but my scope is acting screwy lately and had great trouble
syncing (my somewhat long-duration cam in my Jag did not
help matters). I *think* that the signal output is in the
order of 100 millivolts.

Ripping apart the timing light and following the circuit
around concluded that the pulses from the pickup are
fed into a gate of an SCR (through a 100 ohm resistor,
and 0.002 micro cap to ground).  The SCR is in parallel
with the primary of the xenon trigger coil / 0.1 micro
cap.  Apparantly, the SCR dumps the charge in the cap
through the coil to create the HV trigger.

I guess I could duplicate the circuit and tap off of
the cap and feed this into a hysteresis buffer of some sort.

Does anyone out there know of a better trigger scheme?  Maybe
something even more reliable (inductive-pickup timing lights
occasionally go whack-o and fail to flash)?

- Bruce

--
-----------------------------------------------------
<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-----------------------------------------------------
               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
-----------------------------------------------------
<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-----------------------------------------------------