re [Gmecm] Interfacing HEI module to Pertronix pickup assembly

[Robert W Hughes]

> 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.

The HEI triggers on a negative-going edge at about 1.25v. See this for a 
picture of the waveforms:

http://bhughes.lonestarfieros.org/HEITriggerWaveform.jpg
-- 
Robert W. Hughes (Bob)
BackYard Engineering
29:40.237N, 95:28.726W or perhaps 30:55.265N, 95:20.590W
Houston, Texas "The city with too much Oxygen"
rwhughe at oplink.net