Background Info: I have just expanded my already huge sprinkler system (16+ zones). The old timer only controlled 12 zones so I added an additional timer to control the new valves/solenoid. Each timer needs to output 24VAC to the main valve solenoid and one respective valve (without the main valve turned on, there will be no water reaching my other valves). So the old timer controls 12 zones (from before) + Main Valve & the new timer controls 4 (new zones) + Main Valve. The main valve must be turned on to let water flow to the manifold where the other valves are. With one timer, life was simple. 2 leads (24VAC polarity doesn't matter) went to the main valve and the timer would turn it on at the beginning of the watering cycle (it would also turn on each zone one at a time to cover all of the backyard). Now with two timers, both timers will need to turn on this main valve solenoid. At most times (ideally), only one timer will be supplying power to the main valve. However I want to play it safe (in case both timers decide to turn on at the same time) and feed 24V to the same main valve. I need to isolate the two sources. Soo...I have two 24VAC sources and need one output to the solenoid of the main valve. If either one of the sources is active/triggered by any of the timers, output should shoot 24VAC. Sprinkler Valve Solenoid shouldn't take more than 2 amps Now I was thinking of using a couple of diodes but remembering back from college, I think they are effective for DC circuits (and only half way block AC current). Please help me create a diode/relay schematic to put this together or if there are any other suggestions.