This would be blog post how to make smart solder reflow oven step by step
First thing to do is create simple heater controller. My idea is to use PWM signal what will turn on/of heating element, and because PWM is running more than 1kHz, i can get very fine power control. Here is original circuit:
Idea very simple. First i have DC-1MHz isolator from analog devices. so my main controller is electrically isolated from mains, that is handy. NE555 will disconnect heater from mosfet for extra safety, when where is no pwm signal, or duty cycle is zero. If duty cycle is not zero, but higher, 555 will delay pwm signal, so relay will be closed, so mosfet could make circuit with heater. UCC37322 is driving mosfet with PWM signal, and again, for extra safety, i will disable mosfet driver when PWM signal is not present. Fore feedback i use 555 output signal, so i know that my heater controller is working. If for some reason power would be lost to PWM circuit, i will not get any feedback, and will know that something is not right.