Figure 8-2 shows the smoker system as a block diagram. Air enters the system at the left and exits at the top right. Heat is generated in the firebox and exits everywhere; the amount of heat generated is determined by the amount of charcoal in the firebox and the volume of air moving past the charcoal.

Figure 8-2

Smoker system block diagram

Under pure convection, the volume of air moving through the system is itself determined by the heat differential between the air in the system and the outside air, and by the size of the inlet and flue openings. One option to control the internal temperature is, in fact, to physically move one or the other of the dampers in the openings, but because that approach requires knowing the absolute position of the damper as well as being able to move it, we chose another approach—we decided to set the dampers so the system had slightly too little air flow, then use a fan to push air through the too-restrictive opening. That approach gives the control system the ability to reduce and increase heat, and requires only that we control the speed of a fan.

The overall control system has the block diagram shown in Figure 8-3. We used the Javelin Stamp processor to run the software, giving it a thermistor as a sensor and a variable speed fan as an actuator. There’s a balance between the damper setting and the size of the fire you build in the firebox. Getting that setting right is a matter of trial and error, but the control software can help by telling you it can’t stabilize the system and indicating whether the damper needs to be opened further or closed. Continued…

Figure 8-3

Control system block diagram