This combined pumpkin and melon farms is designed for fast growth rate in small size with automatic harvesting.
Due to its high density, there is not enough space to place pumpkin/melon detection and harvest circuit for every growing space, so the farm's harvesting is to be driven by clock.
The farm can be extended in 2 horizontal dimensions and can be stacked every 8 blocks. Layer 1 and 9 can be merged in stacked farm.
The first floor consists of farmlands and dirts in checkerboard pattern (in layer 2) with alternating rows of pumpkin stems and melon stems on farmlands (in layer 3). The fruits will be pushed by piston from above. Piston in layer 5 are place on air and dirt. (The picture also show piston on air and water column but it is unnecessary.) Between the pistons in layer 4 are jack o'lanterns. They can support redstone dust and redstone repeater while providing light to stem under them.
Hoppers (in layer 1) under farmlands will collect the items. They are connected by rows of hopper, one hopper row for every 3 block rows.
Harvesting signal are sent in layer 5 above piston. This redstone circuit layout is designed for lowest height as first objective and low redstone cost second. There is one main trunk consists of opaque_block-repeater-opaque_block-redstone_dust- , then the signal is transmitted to primaray branches every 4 blocks. Primary branches start with redstone dust and repeater alternatingly and has opaque_block-repeater-opaque_block-redstone_dust- pattern. Primary branches has secondary branch on their sides every 4 blocks. Each secondary branch has 1 redstone dust that helps power piston and opaque block 2 block away from primary branch.
The farm can operate and can be stacked with just layer 1-5, but the addition of layer 6-8 helps improve yield per volume.
The second floor (layer 6-8) has sticky pistons under dirt which received signal from below them.
The second floor has less growing spaces than first floor because hoppers under farmlands need to be connected, thus less spaces are available for piston under dirt. But its height is only 3, so its yield per volume is still higher than that of layer 1-5.
Non-harvesting rows in layer 7-8 is filled with potatoes on farmland. Farmlands help stems on these rows to have a little better growth rate.
Redstone pulse length should be 2 redstone ticks for proper operation of sticky pistons.
Repeater clock shown in picture triggers every 82 redstone ticks. This clock is chosen for simplicity and is made of renewable material.
Hopper-based clocks are more compact but they require redstone comparator which is non-renewable.
Nice job making layers #1 and #9 mesh!
Kudos also on harvesting the alternate (potato containing) layers via upward facing pistons to double the efficiency of the RS signal distribution mesh.
I'd be inclined to replace the clock with one with a longer period than 82 RS ticks and am less concerned with making the clock components completely renewable (and with fitting it within the footprint) that you seem to be, but these are clearly cases of YMMV.
I'd be interested in knowing how this design compares in terms of both output per hour per cubic volume and created lag with ilmango's "efficient melon/pumpkin farm" [link is https://www.youtube.com/watch?v=x0tmAUKHDgU ] which uses hopper minecart pickup and is correspondingly less compact vertically.
At a guess, the design shown here would give higher yields per volume occupied, but the large number of hoppers might produce more lag than the minecart system. (From what I can see the piston usage is about one per harvest space in both and harvesting system in karyonix's design should at least partly offset the large number of hoppers. )
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
Here is harvesting unit design for checkerboard farm which use minecart to collect pumpkin and melon slices instead of hopper. Its stacking height is 5 block per floor. It probably has a little lower yield per volume.
1 Jack-o'lantern on stem.
1 Observer (sensing side up, signal side down) on jack-o'lantern.
1 Detector rail on observer.
4 pistons (facing down) around jack-o'lantern under observer.
Arrange plus sign shape units over farmland and stem layer. Fill space in observer layer with opaque blocks. It become full surface that can support a layer of rail.