Introduction: Solar Powered Compost Drum System

Who’s really that diligent about turning their compost regularly? Let’s face it, we probably don’t turn it as much as we should (I know I don’t) so I was interested in setting up an automated (and green) way to accomplish this.

For those just getting started into composting (I’m still fairly new to composting), the key to getting it right is having the right ratio of ingredients (green vs brown), having mass (more the better as the middle of the pile can heat up and break down) and introducing oxygen into the pile for the microbes to thrive (there are of course more factors the more you research, but I've found these to be basics that worked). This instructable solves this last one – turning the barrel to add oxygen.

I’ll first take you through my 2 failures (learn from my mistakes) then to my successful third attempt.

Note – I’m not an engineer or have a technical background (I’m in sales). I just had an idea and learned as I went so please excuse any areas in this instructable that are a little vague when it comes to mechanical science. :)

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Step 1: Concept #1 - Wind Turbine (Failed)

My first idea was using the wind to rotate my compost. I was focusing on the idea of building and using a vertical axis wind turbine (VAWT). I actually stumbled upon another fellow instructable by kdunner. He created a wind powered Compost here: I thought that looked great so decided to make one myself.

As you can see in the pictures, I cut out each layer on my cnc, glued them up, shaped them and sealed them (easier said than done). The idea was to attach this wind turbine to a gear motor to turn my compost barrel. Unfortunately, I came upon some sad news when I tested the wind turbine in my backyard with no load other than the weight itself. Apparently, my wood design was simply too heavy to turn in a light breeze and not efficient at what I was trying to accomplish. Maybe if I used lighter material or 3D printed the turbine it would have worked, but in the end, I just made some lawn art for windy days. :/

After more thoughts of this method, there was also the fact that I wanted my compost turned once daily. Apparently turning it too much can cause the pile not to heat up properly which is counter intuitive of what I'm trying to accomplish. There might be stretches of days where I don't have any wind or days when it's too much. These inconsistencies also led me to scrap this idea.

Step 2: Concept #2 - Wooden Gears (Failed)

After doing some research it seemed very clear that solar power was the more efficient than wind power. With that new knowledge, my second concept was to use a gear motor hooked up directly to a battery which would be powered by a solar panel (this idea was halfway there). I already had a small gear motor and to assist with the additional compost weight I needed to add additional gears to provide additional torque (more force to move the load). Here’s where someone with an engineering background could tag in but you’re stuck with me. I will say this educational video of gears from the early 90’s was extremely helpful in understanding the concept behind gear ratios. Basically, the higher the gear ratio, the more you can convert high RPMs into torque and move a heavier load.

I decided to build the additional gears myself. I found and purchased a gear template generator program from and designed my gears to fit on the end of a compost drum. Due to how the motor and drum was positioned, I went with a 90-degree angle. Looking back, I shouldn’t have done that design as it allowed too much side to side movement and the gear teeth would get off track. When that happened, the amount of force would crack my wooden gears. Because it wasn’t completely reliable, I ended up scraping this idea. If you have any ideas what I could use the wooden gears for I’m all ears! (They look cool!)

Step 3: Concept #3 - Bike Chain and Sprocket (Success)

For my third attempt I decided to use the chain and sprocket off an old bike someone was throwing out (believe it or not it took me a while to find one for free). The chain and sprockets would connect the compost drum and gear motor, powered by a solar panel and battery.

Step 4: Obtain or Make Your Compost Drum

I did it both ways since I was trying various ideas. I built my drum compost barrel by reading a lot of instructables on here and the web. I attached a lot of pictures with brief descriptions per photo showing the process I went through to make the 55 gal compost drum and stand.

I also scored a pre-manufactured compost drum for cheap and figured I could use two compost drums (one cooks while I fill the other with kitchen scraps).The pre-manufactured compost drum I purchased is this one. (Note - I took out the divider on this particular model to allow more mass and so that the pile can heat up more).

Step 5: Obtain Your Chain and Sprocket

I found a free bike that someone was throwing away and figured it was perfect for this project. The frame itself was cracked (and not worth saving or giving away) so I took a grinder to it to get the chain and sprockets off.

Step 6: Attach Bike Sprocket to Compost Drum

Next, you need to adhere the sprocket firmly to the compost drum. It’s very important that the sprocket is aligned directly in the middle. My particular compost barrel has a bar going directly through the middle which gave me the dead center. Knowing that, I used a bi-metal hole saw bit to cut the same sized hole in the sprocket and bolted it to the compost drum.

Step 7: Modify Gear Motor to Accept the Bike Chain / Build Motor Housing

I was able to take apart my bike’s chain derailleur and use the idler pulley. Attaching the idler pulley to the gear motor became a sort of Frankenstein given my limited parts available to secure the pulley to the gear motor rod (it would have helped if the rod was threaded or if I could have put threads on it myself with a tap and die).

The small dc motor on my gear motor was broken so I ridged up another motor I had to it. Again, more of a Frankenstein look given the parts I had readily available.

I believe the gear motor itself has a gear ratio over 6000:1. The high the ratio the more it’ll turn the high speed RPMs from the motor to torque. This tiny motor packs over 50 in-lb. I then had to build a box to protect it from the elements.

Step 8: Setup Solar Panel

I had to do some research before proceeding with setting up my solar panel. Again, there are great resources on Instructables with other’s projects that give great tips.

The basics of my setup involve a solar charger, the solar panel, 12 volt battery (salvaged old car battery) and a timer (items found in the supply list).

I built my solar panel stand out of leftover scrap wood in the shop (nothing fancy here). The point was to have the panel angled at the sun while securing it off the ground. I mounted the panel to one of my compost stands and feed the wires into my shed where the battery, timer and solar charger were housed.From here I setup the timer to kick on once daily.

Step 9: Let the Compost Cook!

From here it's time to start adding to your compost and letting mother nature due its thing. The end result will be organically rich compost for your garden and flower beds plus the added fact you're adding less to landfills. :)

The posted video shows my completed drum full of compost from the past few weeks. It’s time to leave it alone and let it cook! The video is at actual speed and the drum turns at about 2 RPM. It takes about 6 minutes to do a full rotation.

If you like what you see, please be sure to vote for me in the Backyard Contest! Thank you for looking!

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