First - this project is totally open source, and is available for usage by anyone who cares to do so in the manner of their choosing.  I'll be posting the code when I get it written, and that will also be open source.

OK, a few details. There are a few parts to this puzzle. The first is the physical side - i.e. the structure of the kiln. The second is the electrical side - i.e. the controls. The final part is running/operating the kiln. I'll devote a section to each.

**STRUCTURE**

If you go to Home Depot, Costco, or any other 'Big Box', chances are you've seen pallet rack. It's made up of vertical standards with long beams in between and wire racks between the beams to put things on. What I discovered is that there is a healthy market for this stuff; used or new, buying or selling. In many cases, a pallet rack merchant will buy the racks from an entire warehouse that is being cleared out, and chances are, there are a few standards that aren't as pristine as they'd like them to be. If you can get one to answer the phone (not as easy as you'd think), a vendor will probably sell you a few standards for cheap if you're not stuck on looks. I was able to pick up two standards that had been twisted at the top as well as some new beams and a few wire racks. I drove them home, and promptly set to work. I cut the standards apart entirely with my sawzall and removed the cross braces. I also cut off the tops (I didn't need that part) so the standards were the height I wanted them to be. My end goal for this kiln was to have the wood drying underneath, and have a work bench on the top. I then shortened the cross braces (they were set for 42" deep and I wanted 30" based on my shop size) and bolted them back to the standards. Putting it all together (the wire racks had to be cut as well), and I had the structure in place.

In the pictures, you'll see that I then cut 1/2" insulation boards to contain the space. I used several rolls of aluminum tape to seal gaps in the insulation. This is the structure - modified pallet rack for the exoskeleton and insulation board for the skin. I'll also add some sheet metal to protect the insulation from getting banged up on the front and sides.

**ELECTRICAL/CONTROL**

I am not ashamed to admit that I leaned heavily on the Arduino ecosystem to make this work. I am in no way an electrical engineer, and using an Arduino is stretching my capabilities as it is. Apology (or lack thereof) aside, my objectives for the kiln control system were:

1 - control of a dehumidifier (on/off)

2 - control of a fan (on/off)

3 - reading temperature and humidity inside the kiln

4 - logging data for temp and humidity

5 - displaying status on an LCD

6 – setting a target humidity level inside the kiln

7 - ideally posting status to the web

Based on these requirements, I utilized the following:

1 – An Arduino Duemilanove

2 – An Ethernet shield

3 – An LCD shield

4 – A DHT22 temp/humidity sensor

5 – A DS1302 Real Time Clock

6 – A relay board

7 – Three momentary buttons

8 – Jumper wires, some multi-conductor cable, other electronics stuff

The only real challenge associated with this setup was the number of pins required to make it work. Given that I hadn’t ruled out connecting a computer to the Arduino and reading data that way, I was down to 18 pins. If you add up the required number of pins to make things work, you arrive at the fact that I need 21 pins available. To get around that, I wired up the momentary buttons into a single analog pin and used resistors to make allow the board to differentiate which button was being pushed. Other than that minor improvisation, everything else was connected via jumpers and turned on. You might note that I didn’t stack the Ethernet and LCD shields together. That happened for three reasons. First, I like seeing the blinking lights on each board. Second, both of the boards want to use pin 4; to get around this, I ran one of the jumpers from the Arduino pin 3 into the LCD input on pin 4 and called out pin 3 in the code for the LCD. The third reason I didn’t do this is because the LCD shield doesn’t have pins on the top to run out to other components, and I didn’t want to go through a long soldering process to change that.

Regarding the enclosure; I wanted to be able to see the components, not just the LCD screen. At the moment, my enclosure is just two sheets of acrylic separated by spacers. At some point, I’ll have to add sides to it, as my workshop gets really dusty. However, right now, I’m living the dream with my components open to the breeze.

**OPERATION**

First, some wood drying background. When you initially cut down a tree, up to half of the weight of the wood is water (and sometimes more). That kind of wood can’t be used for furniture; it has to be dried prior to its use. The end target is between 5% and 10% of the weight of the wood to be from water. The way you dry the wood is to put it in a dry environment (duh). However, if the air surrounding the wood is too dry, the water in the outside of the wood piece will be drawn out more quickly than the water from the center of the wood can move to the outside. This causes all kinds of bad things in the dried wood, including residual tension, cracks on the outside the wood (checking) or cracks on the inside the wood (honeycombing). In order to get the Goldilocks scenario where the air outside is just dry enough to allow water to escape slowly from the wood, but not too slowly, you monitory and adjust the temperature and humidity. This desired control is the reason why the kiln is insulated and sealed. In my case, I won’t be controlling the temperature, just the humidity. The dehumidifier will add some heat to the kiln, but hopefully it won’t be a lot. Some of the drying schedules want you to have the kiln at over 150 degrees in the final steps; if mine gets that hot, I’ve got a major problem on my hand. I’m hoping I won’t ever get over 100 degrees. This should extend the drying times, but I’m just not comfortable having such a large thermal mass in my basement at high temperatures.

Part of the reason I’m using an Arduino for this project is that the drying process needs to be monitored and controlled. In a closed environment, the moisture in the wood will come to a given equilibrium with the air around it. What I’ll look for is to have the air in the kiln at a given humidity level that allows the wood to have the internal water evaporating, but not too quickly. As an example, if the air in the kiln is maintained at 80% relative humidity, the wood in the kiln will reach an equilibrium moisture content. If the air is then maintained at 79% relative humidity, some of the water in the wood will evaporate out. The objective is to bring the wood down to 6% to 8% moisture content as quickly as possible, without incurring the defects I mentioned above.

In order to do this, I’ll use my temp/humidity sensor combined with the dehumidifier. The logic will be set up to determine if the air humidity is greater than the set point humidity. If that’s the case AND if the humidity isn’t dropping too fast, the dehumidifier will be turned on. The point is, even if the air humidity is higher than I want it to be, if the air humidity has been dropped recently, I’m going to not turn on the dehumidifier. While this is easy to describe, I’m not sure exactly how it will play out; I don’t know how to determine what ‘too fast’ drying rates will be, so I’ll probably either ruin a few loads of lumber or I’ll be too conservative and take longer than I need to. We’ll see which side we end up on.