Is there any good way to make a completly automated watermelon/pumpkin farm in vanilla Minecraft without commands?

First you want to make this or something like it:

Then you'll want to add a lever like this:

After that just use the lever then use it again. After that you'll have a fully automatic farm. Have Fun =D

Ok so Have an observer facing the stalk have a piston facing the place where it will grow have the piston next to the observer, horizantal place a block behing the piston and the observer put a redstone dust on top of each voila! just build a hopper collection system and ur golden

If you wire the circuit without observers to make the melon a conductor, completing the redstone flow, then you can have a piston to crush it then go into a hopper minecart. Voila!

Example: BR R

When the melon grows: BRMR

Then the piston crushes the melon since the circuit is completed.

Key:

B: redstone block

R: repeater

M: melon

RR
XX
OP
S
DD

R=Redstone dust, X=Solid Block, O=Observer (face pointing down), P=Piston (also facing down), S=Stem (Watermelon or Pumpkin), D=Dirt. This is a sideview, and is tileable.

This works under the premise that the vine itself updates when a watermelon/pumpkin is produced. To make it fully automatic, run a hopper minecart underneath the dirt to suck up drops, and place glass around the front so the drops don't fall somewhere they shouldn't.

you need to have a platform with observers and pistons above a pumpkin patch, with a minecart collection system underneath. this is alot more efficient then using things like water streams.

heres a video on how to make one