I would greatly appreciate someone to help me out with this design. What I am doing is controlling a Lionel train with a simple triac/diac (actually a Quadrac) type control for a fan motor at 120 VAC. I can wire the control on the primary side of my 120 VAC to 18 VAC transformer and it works fine to control the train. My problem is I don't want it on the primary side I want it on the secondary 18 VAC side. I can run multiple trains from this transformer and just need individual controllers. The breakover voltage of the diac is too high for this lower voltage. I must start from scratch with the control and don't know if I need zero crossing detection or not or if I will need a opto-isolator or not. In my small mind this shouldn't take much of a circuit but my background isn't electronics (have alittle knowledge), it's in control wiring. Could someone post a schematic or two on how to accomplish this control. Thank you.