My own goal was to build something that would break things down enough that once transferred to my worm bin, the compost won't draw flies. This will allow me to move the worms inside during the winter without an entourage of flying insects.
Agitation helps add oxygen to the mix and mechanically break things down while allowing moisture to escape, and high temperatures (we'll be shooting for 140F) are ideal for the voracious thermophilic bacteria which take over naturally when outdoor compost piles build up enough internal heat on their own. As a bonus, flies stay away from things at that temperature.
Doesn't this waste energy? It does use some electricity, but if you're a city dweller who can use this and a worm bin instead of curbside yard waste collection, then disposing of kitchen scraps this way saves energy! I'm still working on getting 1.21 gigawatts back out of it though....
Sound complicated? We're just replacing the computerized controller in another appliance that holds, heats, and agitates things: a bread maker!
Step 1: What You'll Need
- a bread maker ($5-$25). I used a Zojirushi BBCC-V20 that I found at a thrift store; I'd highly recommend this brand because of the way it's clearly designed and labeled internally, as you'll see later.
- a microcontroller ($0-$30) with at minimum several digital I/O lines. At least one analog-to-digital input and one PWM output is preferable. I used a Basic Stamp II (only digital I/O), but I highly recommend using something newer and easier to program such as an Arduino USB Board ($30 at SparkFun)
- (optional) an LCD ($0-$20), 2x16 character or so, so that Mr. Compost can tell you what he's doing
- miscellaneous resistors and connecting wire
-a coat hanger for upgrading the bread machine's stirrer/kneader into more of an auger that will thoroughly turn the compost
My total cost: $25 for a bread maker; everything else was laying around. If you have to buy everything, you should still be able to do this for under $50.
For a fun comparison, the NatureMill Plus Edition is $299 - but keep in mind that Mr. Compost doesn't replicate all the same functionality.
- a soldering iron and solder
- wire strippers/cutters and pliers
- screw drivers
- a logic probe or voltmeter (preferred)
- vinyl (electric) tape and a sharpie
- a breadboard is very helpful for assembling intermediate test circuits
- a computer for developing the programming for your microcontroller
- a drill or dremel tool and small drill bit to make holes for coat hanger wire (5/64" worked for me)
- a beer bottle to help shape the coat hanger
- comfortable going beyond "Warranty Void If Broken" stickers
- solid understanding of circuits & some experience reverse-engineering circuit boards
- comfortable working around live circuits with potentially lethal voltages
- experience soldering, especially in tight places
- basic understanding of PID feedback control using Pulse-Width Modulation (PWM)
- whatever programming language is required by your microcontroller
This is a fairly technical project. I've focused on covering all the high-level aspects that are important while also selectively including details that will save you time, hopefully without being too long-winded.