Kiln Controller

Turns a Raspberry Pi into an inexpensive, web-enabled kiln controller.

Features

• supports many boards into addition to raspberry pi
• supports Adafruit MAX31856 and MAX31855 thermocouple boards
• support for K, J, N, R, S, T, E, or B type thermocouples
• easy to create new kiln schedules and edit / modify existing schedules
• no limit to runtime - fire for days if you want
• view status from multiple devices at once - computer, tablet etc
• real-time firing cost estimate
• NIST-linearized conversion for accurate K type thermocouple readings
• supports PID parameters you tune to your kiln
• monitors temperature in kiln after schedule has ended
• api for starting and stopping at any point in a schedule
• accurate simulation
• support for shifting schedule when kiln cannot heat quickly enough
• prevents integral wind-up when temperatures not near the set point
• automatic restarts if there is a power outage or other event
• support for a watcher to page you via slack if you kiln is out of whack
• easy scheduling of future kiln runs

Run Kiln Schedule

Edit Kiln Schedule

Hardware

Parts

Image	Hardware	Description
	Raspberry Pi	Virtually any Raspberry Pi will work since only a few GPIO pins are being used. Any board supported by blinka and has SPI should work. You'll also want to make sure the board has wifi. If you use something other than a Raspberry PI and get it to work, let me know.
	Adafruit MAX31855 or Adafruit MAX31856	Thermocouple breakout board
	Thermocouple	Invest in a heavy duty, ceramic thermocouple designed for kilns. Make sure the type will work with your thermocouple board. Adafruit-MAX31855 works only with K-type. Adafruit-MAX31856 is flexible and works with many types, but folks usually pick S-type.
	Breadboard	breadboard, ribbon cable, connector for pi's gpio pins & connecting wires
	Solid State Relay	Zero crossing, make sure it can handle the max current of your kiln. Even if the kiln is 220V you can buy a single 3 Phase SSR. It's like having 3 SSRs in one. Relays this big always require a heat sink.
	Electric Kiln	There are many old electric kilns on the market that don't have digital controls. You can pick one up on the used market cheaply. This controller will work with 110V or 220V (pick a proper SSR). My kiln is a Skutt KS-1018.

Schematic

The pi has three gpio pins connected to the MAX31855 chip. D0 is configured as an input and CS and CLK are outputs. The signal that controls the solid state relay starts as a gpio output which drives a transistor acting as a switch in front of it. This transistor provides 5V and plenty of current to control the ssr. Since only four gpio pins are in use, any pi can be used for this project. See the config file for gpio pin configuration.

My controller plugs into the wall, and the kiln plugs into the controller.

WARNING This project involves high voltages and high currents. Please make sure that anything you build conforms to local electrical codes and aligns with industry best practices.

Note: I tried to power my ssr directly using a gpio pin, but it did not work. My ssr required 25ma to switch and rpi's gpio could only provide 16ma. YMMV.

Software

Raspberry PI OS

Download Raspberry PI OS. Use Rasberry PI Imaging tool to install the OS on an SD card. Boot the OS, open a terminal and...

$ sudo apt-get update
$ sudo apt-get dist-upgrade
$ git clone https://github.com/jbruce12000/kiln-controller
$ cd kiln-controller
$ python3 -m venv venv
$ source venv/bin/activate
$ pip install -r requirements.txt

Note: The above steps work on ubuntu if you prefer

Raspberry PI deployment

If you're done playing around with simulations and want to deploy the code on a Raspberry PI to control a kiln, you'll need to do this in addition to the stuff listed above:

$ sudo raspi-config
interfacing options -> SPI -> Select Yes to enable
select reboot

Configuration

All parameters are defined in config.py. You need to read through config.py carefully to understand each setting. Here are some of the most important settings:

Variable	Default	Description
sensor_time_wait	2 seconds	It's the duty cycle for the entire system. It's set to two seconds by default which means that a decision is made every 2s about whether to turn on relay[s] and for how long. If you use mechanical relays, you may want to increase this. At 2s, my SSR switches 11,000 times in 13 hours.
temp_scale	f	f for farenheit, c for celcius
pid parameters		Used to tune your kiln. See PID Tuning.
simulate	True	Simulate a kiln. Used to test the software by new users so they can check out the features.

Testing

After you've completed connecting all the hardware together, there are scripts to test the thermocouple and to test the output to the solid state relay. Read the scripts below and then start your testing. First, activate the virtual environment like so...

 $ source venv/bin/activate

then test the thermocouple with:

 $ ./test-thermocouple.py

then test the output with:

 $ ./test-output.py

and you can use this script to examine each pin's state including input/output/voltage on your board:

 $ ./gpioreadall.py

PID Tuning

Run the autotuner. It will heat your kiln to 400F, pass that, and then once it cools back down to 400F, it will calculate PID values which you must copy into config.py. No tuning is perfect across a wide temperature range. Here is a PID Tuning Guide if you end up having to manually tune.

Usage

Server Startup

$ source venv/bin/activate; ./kiln-controller.py

Autostart Server onBoot

If you want the server to autostart on boot, run the following command:

$ /home/pi/kiln-controller/start-on-boot

Client Access

Click http://127.0.0.1:8081 for local development or the IP of your PI and the port defined in config.py (default 8081).

Simulation

In config.py, set simulate=True. Start the server and select a profile and click Start. Simulations run at near real time.

Scheduling a Kiln run

If you want to schedule a kiln run to start in the future. Here are examples.

Watcher

If you're busy and do not want to sit around watching the web interface for problems, there is a watcher.py script which you can run on any machine in your local network or even on the raspberry pi which will watch the kiln-controller process to make sure it is running a schedule, and staying within a pre-defined temperature range. When things go bad, it sends messages to a slack channel you define. I have alerts set on my android phone for that specific slack channel. Here are detailed instructions.

License

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.

Support & Contact

Please use the issue tracker for project related issues. If you're having trouble with hardware, I did too. Here is a troubleshooting guide I created for testing RPi gpio pins.

Origin

This project was originally forked from https://github.com/apollo-ng/picoReflow but has diverged a large amount.