[Gmecm] Interfacing HEI module to Pertronix pickup assembly

[David Allen]

  Hey all.  I have to convert a distributor from points to ECM-controlled 
HEI ignition.  The distributor can not be modified to use a magnetic pickup 
coil.  It can, however be modified to use a Pertronix Ignitor pickup.
  The Pertronix pickup provides a grounded signal suitable to drive an 
ignition coil directly.  I have to use this signal (firing event on rising 
edge of square wave pulses) to operate an HEI module.  What sort of 
interface circuit will work here?  I have a few ideas but I bet this has 
already been done by someone here on this list.
  The Pertronix circuit, when used with a 100R pull-up resistor, produces a 
square wave that is lowest at 0.75 v and highest at battery voltage. The HEI 
module expects an isolated magnetic pickip coil.  I have an idea of using an 
op-amp comparitor circuit that switches at 1/2 battery voltage, driving a 
small isolation transformer that will give the HEI module it's signal on an 
isolated circuit.
  I will draw a diagram later and post it.  Thanks for any input!
Later,
David