I hope this is the right forum for this. I wanted to share my project and it’s progress with the community. This first post contains the bulk of what I’m putting together, I’ll be adding posts as I build and test this excessively-thought-out apparatus.
Background: I’m in a very dry climate, SW US. It’s typically 10-30% RH. Last season was my first real grow, and I dried in my shed in folded paper bags. Let’s just say…it didn’t do too well. It was dry within a few days and smelled of hay ever since. I tried putting some in jars early and burping, but it was too late. I want to see how close to ideal I can get (thanks engineering) with a little creativity and spare weekend time. I feel this is worth some considerable mental effort due to the role drying plays in the quality of finished product, this is probably the single most impactful process from seed-smoke.
The Concept: 50-60ish temperature insulated cabinet that exhausts when humidity reaches 60ish percent. This will be adjustable, and I could even make this ramp down over the dry cycle. Want to be able to hold a few pounds of wet bud. I’m going to publish everything (plans, schematics, PCBs, firmware) so others can build it if they want.
The Constraints: Cheap as reasonably possible, simple execution, fairly high capacity. I can’t dry in the house - space/smell/patience constraints, a tent like people use would be pushing the limits with my other half. I want to keep the unit in my shed. That means some crude thermal management.
How: I initially was thinking something like the commercial Cannatrol, and before I knew about that I had thought about using a broken fridge to house this “machine.” I’m not about to spend $1600+ dollars on this for a limited capacity like this https://cannatrol.myshopify.com/products/copy-of-cannatrol-cool-cure , but it definately gives some good ballparks I can use to estimate my own capacity.
Let’s Sort Out Some Parts:
Dry Trays: I started by looking for drying trays. The kind for baking are really cool, perforated aluminum, but $10-20 each. Aggressive brain scratching landed me on mesh bottom propagation trays (25 for $40, basically $2 each). Perfect. https://growgreenmi.com/10x20-mesh-tray? (those aren’t the exact ones I bought but close enough)
Temperature: I ran some numbers and it looks like with my Oct/Nov temperature swings, I can keep a nice temp of 50-60F (80F high, 30F low late Nov, the plot is a little low on the hot end) if I use a large water cooler pumping water through a radiator in the cabinet. I ran these numbers assuming I would build it with 2" housing insulation board. https://www.homedepot.com/p/R-Tech-2-in-x-4-ft-x-8-ft-R-7-7-Rigid-Foam-Insulation-310891/202532856 I won’t need ice or anything, the cooler will insulate the water and I’ll wind up in equilibrium around the average temperature (60ish). Fantastic. I’ll need a radiator and a cheap pond pump to execute this plan, and if my long-season sativa makes it all the way to end-of-Nov, I may need to add a small heater to keep me running through December.
Found a radiator on amazon of similar size as my trays (20" nominal width) Amazon.com for about $20. Picked up a few PC case fans with PWM control for another $10 Amazon.com Scrounged a high-capacity fan and carbon scrubber from work for $0. Great, it’s all coming together then! I see the prices have “inflated” since I purchased both of these items 
If it isn’t clear already, in order to regulate the temperature, I need to maintain some small airflow over the radiator in the cabinet. I also want to circulate the air slightly to encourage drying, without blow-drying it. I need circulation to get a decent high-accuracy humidity measurement, and I’d like to filter the air so I don’t dirty my buds. I’ll make the circulation fans adjustable speed so I can run them quick after I budwash, and slow them down once they are dry to the touch. I’ll design to accomodate a 10" x 20" furnace filter, and probably cobble together an air inlet filter from another one. Perhaps something like this (but I don’t need a 6-pack) Amazon.com
How am I going to keep the humid air in and the dry air out? Looking around the isles of Home Depot, I found dryer draft stoppers for $8. https://www.homedepot.com/p/Everbilt-4-in-Inline-Vent-Draft-Blocker-BD04HD/205046954 These close unless there is positive airflow, so I can use on on the inlet/outlet of my drying cabinet to maintain humidity unless I’m actively venting it. I think with the output/input filter boxes also attached, I’m not going to be losing much through these. They feel suprisingly decent in-isle examination. It’s also a convenient way to attach my carbon scrubber to the outlet, so my neighbors don’t smell the dankness from my shed for 2 months continuously.
Control Electronics: My favorite. A couple weeks ago, I designed the controller. I made this DIY-friendly, so large SMT parts and inexpensive hardware. It uses an Arduino Nano Every ($10), which is about the cheapest Arduino that exists. There’s also the potential to drop in a WiFi or BT-capable Ardunio. It has 4 output channels for 12VDC, or 120VAC switching. Those are for: Humdifier, Dehumidifier, Cooler, Heater. I’m thinking ahead if anyone wants to use this somewhere they will need those, I’m optimistic I’ll get away with just my exhaust fan. The 12V heat/cool outputs can be PWM’d so you (or I) can make a proportional heater/cooler if you want. Naturally, I’m thinking of using a 12V pond pump to run my coolant circulation pump and large power resistors if I need a heater, so I’d use the PWM outputs to set the flow/heat power levels. FWIW this would also work great without modification for a greenhouse controller, or tent controller.
There are two 12VDC fan output channels with PWM control. The exhuast fan runs for a programmable number of seconds and whatever speed you dial in when the humidity exceeds a certain level. I did some math based on the humidity sensor response time, volume of the chamber, my fan, my typical humidity levels, and it looks like 5s on, and 60s wait to act again. I’ll set the speed of the fan based on the ambient humidity on average, but it’ll be about 25% speed (this will drop my 60% RH to about 55% RH). It will only run the fan if it crosses whatever setpoint I enter.
If you are about to ask how I’m going to sense humidity, I’ve got ports for pre-made sensors. I picked these Gravity: SHT31-F Digital Temperature and Humidity Sensor - DFRobot They respond fairly quick but it’s going to be important to burst the fan, and wait about a minute for the humidity to stabilize before deciding another action.
I finally got the boards in from JLC PCB ($4 each) and put one together. I wrote the basic program and you can set where everything turns on or turns off, and save it to EEPROM. The fan speed adjustments work beautifully with my fans. Now I need to build the cabinet. I’m drafting it up in Fusion 360 before I buy the foam and get to cuttin!
Of course, I have spreadsheets for the calculations (thermal/humidity). Some may be interested in that, but my expectation is that won’t be hugely popular. I’d be happy to show my work if anyone wants to see it, this write-up is meant to be inviting, informal and not overly technical. That’s all I do most of the time, so I think this will do a good job of drying herb without a comprehensive understanding of all the details. I hope some of this is useful to others, I’m learning a lot here in other areas.
Please let me know what you think! And I have some extra boards if anyone wants to build one 
(specially because drying/curing has been a weak aspect of my game)
