I started down the road to this method of controlling kilns and furnaces because I thought that the controllers offered for sale were more expensive than I wanted to pay. Dont get me wrong, Im not suggesting that they are "over-priced." They are specialized devices made for a very small market. Furthermore, they are built for proper certification for insurance and safety purposes. They have a lot of engineering cost behind them.
There will be a lot of engineering cost in my units, too. If I spent the amount of time on engineering consulting jobs that I did on designing and building glass equipment, I could buy a commercial furnace with the proceeds, but then, Im an amateur glass worker/studio builder - I do it for the love of the sport.
I am, by the way, an engineer. I taught Engineering Mechanics and Mechanical Engineering at West Virginia University for thirty years. I also hold an amateur radio license.
My plan focuses on "closed-loop" systems. That is, controls which look at what is going on in the kiln, and adjust things in response. These are not simple timers or simple power level controls.
Rather than construct or buy "instrumentation" I/O boards and then soldering up a variety of measurement and power circuits, I'm working with "off-the-shelf" modules. Many of the components can be found at Radio Shack, the others in a home builders supply store or else on the INTERNET. Thermocouple input is by means of a digital multimeter with serial PC interface (priced at $60 or less). Outputs rely on X10 "home automation" units. Because common X10 controllers are limited to about 20 amps, simple higher amperage relays, and probably SCR "amplifiers," will be added. I don't expect to achieve the precision of commercial controllers, but I think there are compensations.