Controller
My goal was to have the circuit as tidy as possible in a little box. This is what I came up with! The 7x5cm circuit board contains everything I need. A shift register that regulates the status LEDs and the buzzer; the heating circuit, which is driven by a IRLZ34N MOSFET to regulate the heater via PWM and the DRV8825 stepper driver. I took care to have all the wires, which leave the box, well protected. I created another connector in the front to attach the RaspberryPi and the sensors. Also the stepper motor is connected via the front panel.
In case some parts stop working at one point, I decided to use sockets for the LEDs and the stepper driver. So they can easily be switched to a new one.
The DRV8825 stepper driver has to be adjusted to respective stepper motor in use. In my case, the Vref was set to 0.74V. Have a look here for more details.
In the beginning I used a cheap LED power supply to run the controller and I always had issues: MOSFET was overheating, stepper driver was working strangely etc.. Once I switched to a PC-PSU, this was solved! Guess it is just more steady in its output. I found an old Dell PSU, which is great for conversion. It has a test-button; once pressed the PSU turns on, without a mainboard attached. So no rewiring is needed as for most of the other models. Please be very careful when working with PSUs! The capacitors can contain deadly high voltage!
Luckily I had access to a laser cutter, which is just great to make beautiful boxes out of wood. Wherever needed, I cut little vents to allow better cooling of the devices. The MOSFET can get a bit warmer, so it is attached to a heat sink and a 12V fan in the back. The warm air can then flow out at the top of the box. The other top vent is just above the stepper driver, which also can heat up a bit. But since it is not running all the time. No extra cooling is needed and a little heat sink is just fine. The side vent is where the alarm buzzer is located.
In the beginning I had some problems with the MOSFET. It overheated quickly until I realized that with the relatively high load (12V, 12.5A, 150 Watt) the gate voltage also needs to be higher than the 3V from the Raspberry to fully trigger the switching. Therefore I added an amplifier circuit in front to trigger the MOSFET with 5V instead (see also the datasheet for gate voltage). Then the MOSFET fully switched even under load, drastically reducing the heat production (much safer!!). The MOSFET can burn up, if not connected correctly. It still heats up a little bit. To keep it chilled, I added a heat sink and a little 12V fan in the back. Just to be on the safe side.