Sometimes you need a more precise control of things rather than switching on and off with some hysteresis. This is why PID control was developed. PID control is known as Proportional, Integral and Derivative control where each part brings a portion of fine tune to end control signal. So instead of having rough characteristics you get pretty fine and optimal curve, say of oven temperature. osPID have published their initial version of open hardware PID controller which is based on Arduino compatible Atmega328P microcontroller.
The design of osPID is modular that consists of three plug-able boards: Main board with microcontroller, buttons and LCD, Input card where sensor input information is captured an output card which controls actual device. Depending on controlled device input and output cards can be swapped to match measured parameter or generate control signal. Of you thinking of building SMD re-flow soldering oven this controller might be a good choice.