|  | [Raspberry Pi](https://www.adafruit.com/category/105) | Virtually any Raspberry Pi will work since only a few GPIO pins are being used. Any board supported by [blinka](https://circuitpython.org/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. |
|  | [Thermocouple](https://www.auberins.com/index.php?main_page=product_info&cPath=20_3&products_id=39) | 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. |
|  | 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](https://www.auberins.com/index.php?main_page=product_info&cPath=2_30&products_id=331). 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. |
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](https://github.com/jbruce12000/kiln-controller/blob/main/config.py) file for gpio pin configuration.
**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:** The GPIO configuration in this schematic does not match the defaults, check [config](https://github.com/jbruce12000/kiln-controller/blob/main/config.py) and make sure the gpio pin configuration aligns with your actual connections.
*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.*
Download [Raspberry PI OS](https://www.raspberrypi.org/software/). Use Rasberry PI Imaging tool to install the OS on an SD card. Boot the OS, open a terminal and...
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:
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:
| 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 |
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:
Run the [autotuner](https://github.com/jbruce12000/kiln-controller/blob/main/docs/ziegler_tuning.md). 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](https://github.com/jbruce12000/kiln-controller/blob/main/docs/pid_tuning.md) if you end up having to manually tune.
There is a state view that can help with tuning. It shows the P,I, and D parameters over time plus allows for a csv dump of data collected. It also shows lots of other details that might help with troubleshooting issues. Go to /state.
If you want to schedule a kiln run to start in the future. Here are [examples](https://github.com/jbruce12000/kiln-controller/blob/main/docs/scheduling.md).
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](https://github.com/jbruce12000/kiln-controller/blob/main/docs/watcher.md).
If you're having trouble with hardware, I did too. Here is a [troubleshooting guide](https://github.com/jbruce12000/kiln-controller/blob/main/docs/troubleshooting.md) I created for testing RPi gpio pins.
