It's nothing fancy, and it can all be breadboarded very easily. But sometimes it's just nice to have it all in one neat little package, so here it is. It's capable of switching three 24V AC lines (think furnace, air conditioner, and blower fan). Simply screw the hot wire into the terminal labeled HOT, the three lines you want to control into 1, 2 and 3 on the DRIVEN side, and the low voltage control signals into 1, 2 and 3 on the DRIVER side. You'll also need a ground wire going from the micro to the GND terminal. I included an extra empty terminal next to HOT in case your wiring includes the common wire of the 24V supply (mine doesn't). It does absolutely nothing, just there to keep the wire from feeling left out and causing potential mischief. There's space for 3 optional LED's for some nice visual feedback of what's switched and what's not. And that's it! Pretty simple, really.
And the design files:
For the software side of this, you could try using my simple script from here, but I'll admit it's not very user friendly (or code efficient). For much better software complete with browser interface, check out Wyatt Winters Rubustat made using this board.
For sale in the Makeatronics Store.