Low and Slow Offset Wood Burning BBQ Temperature Controller (with Variable Speed Fan)

After visiting Memphis and loving the blues and slow cooked BBQ I decided to build an offset BBQ when returning back home. I cut up an old LPG car fuel tank and had a local company profile cut sides for a firebox, as well as hinges etc. I welded it all together and painted it with hi temp black paint.

Several times using it, I found that too much time is spent mucking about with keeping steady temperature. This wasn't helped by poor design of my air intake. I think it was not placed low enough below the fire grate level.

After looking on the web for ready made automated solutions I found that (at least for a stick burner) there was not too many options.....even less so available locally in Australia. With not wanting to buy a pellet smoker i thought id have a crack building my own fan forced PID temperature controller. After a bit of trial and error this is the final version. With my large firebox (slightly bigger than advised by Feldons Calc) I find I can add at least a couple of hours worth of dry wood/fuel and let the controller / variable speed fan speed do the work.

Using the rotary encoder, there are adjustable temperature, low temp alarm, high temperature mode and PID setpoints. These will store in the EEPROM once adjusted so that they become the default on powerup of the unit. Anyone having a crack at this will have to build some form of ducting to get the fans output to the firebox....I used 40x40mm square tube and a custom bracket. with the fan running (even pulsing in hi temp mode) not much heat transfer occurs along the duct so anything that keeps fan far enough away from the firebox's radiant heat should suffice.

If anyone likes the build and is a jet at 3d design/printing a fit for purpose enclosure would be a great addition/improvement!!

low alarm - buzzer will sound to alert wood fire needs more fuel
low alarm silence button - will silence alarm to give fire time to recover temperature once fuel added (10 minutes)
high temp mode - will pulse fan at lowest speed on/off to help drop temperature
PID control - offers adjustable PID constants to help with tuning
Arduino sketch is set up as Celsius initially, however if button on Pin8 is pressed on powerup then default will toggle to Fahrenheit and save in EEPROM.

Build as per schematic using a suitable PVC enclosure that will allow easy view of LCD display.

Supplies

Arduino Uno
make sure all the libraries required in the ino file are installed to your Arduino IDE
12vdc 4 (or 3) wire PC Fan (at least 25cfm).
20x4 LCD Display
2 Channel Arduino Relay
12vdc Piezo Buzzer
MAXX6675 Thermocouple and probe module
12vdc switchmode power supply (with additional 12vdc > 5vdc converter)
Generic laptop power supply with 12vdc output selected is ideal
Rotary Encoder with Push Button switch
1 Normally Open / momentary switch
solder/wire etc
Optional 10K Variable resistor
Suitable enclosure with clear cover (to view LCD display)
Using SHS steel section, build a short duct to direct air from fan to bottom of the firebox