Depending on the processor, he may be able to use a PD contact that would operate from an normal coil. A word to the wise though, if your using a mechanical switch you might get a bounce, causing the PD to double "click".
In directsoft Edit mode, hit F7 then click on the contact tab, then click on Differential.
Edit: the PD contact is only avialable on the 450 cpu. All others will need a PD coil, then use a contact from that coil.