This is the automatic melon and pumpkin farm set up I've built on the new SMP server I play on (play.ozonecraft.com, part of the OzoneCraft Network). It's fully automatic running off a daylight sensor but also has a switch on each side for manual activation and deactivation (the server rules require an off switch).

Since this server uses claim block and you either have to donate or stay logged in to gain more space to build, I designed them vertically. This also allows for a nice vantage point at the top which you can access from 2 ladders positioned in the corners.

The wiring is very basic, each tower has 2 torch ladder columns that each control half of the pistons. Every 20mins a quick pulse is sent to the base of each tower from a monostable circuit causing the torch ladders signal to reverse and pulse all the pistons, breaking the melons and pumpkins into a central water shoot. The water acts as am item catcher/carrier and a soil fertilizer. The drops get carried downward to hoppers where they are deposited into chests.

The schematic file includes both a skinned and non skinned version of the farm so if you just want to look at the wiring, you can do so.

Additional Notes

You could save some resources by modifying the monostable circuit to only use 2 repeators and 1 pistons as well as replacing the redstone lamps with glowstone saving you redstone torches and redstone.

I chose to make the monostable circuit 1 wide and symmetrical in the design, that's why it's in the center with double resources instead of off to the side. I also preview lamps over glowstone for aesthetic reasons.