Caterham tacho with Zetec setup?

[John Gaines]

I have a lumenition ecu with a zetec wasted spark coil set up. The problem is that the original tacho uses 3 wires, one of which would have been on the - side of the coil to trigger it. The new setup has a Lumenition 460 ecu with two ignition amplifiers and the original zetec coil. The lumenition has a tacho take off but this is a positive feed. I seem to be lost with what to do , is it a case of purchasing a different tacho or is there a way around this?

Some one must have come across this before?

Regards, John.