The electronics are relatively simple. You have three parts.

1. Thermostat: Go to the Adafruit learning center and look up what they have to say on thermistors. It is a wonderful and detailed explanation and I used quite a bit of their software. There is a good reason to buy things from this wonderful company--they seem to invest a lot of time and resources helping design these fun things. The thermistor in the 3D print head is 100,000 K and the paired resistor in the voltage divider was a 10,000 K to provide the correct range for the temperature wanted. All the rest of the constants are in the software. Obviously try out the heater with the thermistor through the serial port and make sure it is reading the correct temperatures. Go through the examples with the programs listed just to make sure both are working correctly. The software queries the thermistor about once every 5 seconds and either turns the relay on or off. The obsession most people have with temperature control in these devices is a little misplaced. Even with the thermistor accurately judging the temperature the real temp measured with the infrared thermometer can be 50 degrees off on the surface of the organic material depending on packing. You have to stir the stuff around on the plate to get it all up to temp. The power supply for the heater unit comes directly from the 12 volt 2 amp wall wart through the relay. It also powers a cheap car 12v to USB power supply wart that is used to drive the neopixels and the servo as well as a separate supply to drive the computer and relay boards.

2. Neopixels: The neopixels are programmed initially to glow green off/on until the button is pushed to close the chamber. Their color is then linked to the thermistor output to get a reading on the temperature--blue cold to red hot. They are then switched to a neopixel fire script when the temperature hits target. When the button is pushed to turn off heat it switches back to green.

3. Servo: This is powered with the neopixels with a separate line from the car USB wart.

The fritzing diagram is included above. As before try out everything on a breadboard first.