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
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