I have been wanting to make a temperature controlled container to proof both my sourdough starter as well as the initial proof of any loaf. Perfect time to build an automated heating box, using PID algorithms and some sort of AC switching device.
The basic concept was to have an insulated container of some sort, with a small resistive load to generate heat on demand. Cooling wouldn’t be an issue at the scale and temperatures required, so a simple PID based heater should be sufficient.
Research
Over the course of a couple months of procrastination and “planning”, I had ample time to research the different ways to accomplish these basic goals.
Heater
For the temperatures desired for this project and the thermal mass involved, a simple 60 Watt light bulb should provide more than enough heat to regulate the container within the desired temperature bands.
After almost a full day of falling down the rabbit hole that is reading about PID theory and control design in general, I figured it would be worth it to try and go for a full PID based heater control system over a simple thermometer with hysteris. While the latter would obviously be vastly simpler, the former offered a much larger challenge, and all of the interesting concepts that I could play with. Plus there is a shiny looking library from the author of the above PID article series, which will make this a whole lot easier.
When looking into the best way to switch the AC load, I wanted to find the simplest, most efficient method possible, but mostly I wanted to play around with all the technologies and learn what would be the best choice for each particular step.
- Mechanical Relay
- Dedicated SSR
- Triac with zero crossing transistor
Insulating Container
Finding the easiest insulator to contain the heat in the proofer box was a battle between what would be best and what I had on hand. What I had on hand obviously won for the prototype, leading to the initial build being constructed in a zip up insulated bag from our monthly meat delivery service.
Ideally, one of those disposable Styrofoam coolers would be the best structure to build this entire project, with the insulating properties, the size, and the ability to mount controller circuitry.
Controller
Most of my experience in the micro controller world is working with Aruinos and clones, so that was an easy choice. Next up was figuring out how often I wanted to poll reporting data and how I wanted to return that to a central location. I ended up building the initial prototype on an Arduino Uno, then transplanting it to a Particle Photon for the ease of WiFi integration.
After prototyping with the Photon for a while I started running into some stability issues, so am considering switching back to an arduino clone and a ESP8266 module, since the code is pretty portable at this point.
Next steps will be getting the code base into source control, probably into my Gitlab instance that also tracks this blog.
TODO
- Continuous integration and unit tests Example
- Local display
- Local controls
- Failsafe configuration for high voltage components.