I have a fairly extensive sprinkler system in which I have used all of the available wires. I've added new landscaping that requires irrigation, but without available wires to use for the valve control, I need to come up with a creative solution (I don't want to dig another 100' of trench). My control box has plenty of available zones, it's just a matter of moving the electrons out to the valves. One thing working in my favor is that these systems only ever power one "zone" at a time.
There are some expensive solutions for multiplexing that use one wire pair (http://www.smarthome.com/3139/DuWa-8ZEXT-Irrigander-8-2-Pro-Irrigation-Sprinkler-System-Zone-Expander/p.aspx) but I think something simpler will work for me.
I want to make a little box that sits on the end of the existing pair of wires and toggles between two or more solenoids each time power is applied to the wire. This will work since the solenoids are normally closed, and only stay open while power is applied. I'll program the controller to hit the same zone twice (or more) to toggle the power to each solenoid in turn.
So, to the electronics question: What do I need to think about to make this work. I'm a rank amateur. I think I need:
- Some kind of "memory" to know which solenoid to turn on next
- Relays to route the 24VAC to the right solenoid
- Parasitic power for whatever logic is needed
- Copious use of diodes to prevent magic smoke escaping
I'm doing well to get an MSP430 to light LEDs, so some tips on how to make this work would be very greatly appreciated.