The idea of this project is converting a £25 toaster oven into a reflow oven. Also the goal is doing it with the minimum possible modifications to the oven to keep the overall price as low as possible.
The first approach I have made is attaching a controlling box to the side that turns on and of the oven to control the heating power, that is able to measure the temperature with a thermocouple. Once the open loop controlling system is working, it will be possible to implement a closed loop controller, PID or similar. It is probable that controlling only the heating elements will not be enough for the fast temperature changes required for the reflow profiles, so may be in a second iteration a cooling system will be added.
The controlling box has a circuit that uses an ESP32. This circuit is connected to the WiFi. The final product will have a PC software where the reflow profiles will be drawn and sent to the oven, also the start, stop and monitoring options will be done from the computer.
Video assembling the control box:
Video assembling the cooling actuator and doing some tests.
The wire that comes from the mains socket is split in two, on one hand it powers the control box, and on the other, it powers the oven. A, B. Both wires use cable glands for be permanently attached to the box 1. Turning on and off the “mains switch” 2, removes the power completely for the oven and the control box. The terminal block 3, is where the mains wire is actually split in two, the part that goes back to the oven has the neutral and ground wires permanently connected, but the live wire can be switched on or off using a solid state relay 7. The part that powers the controlling box connects all the wires directly to the 5 volts 2 amps power supply 8 that powers the circuit 9. A thermocouple is used to measure the temperature 6 plus a cold junction compensated driver based on the MAX6675 integrated circuit module, that is able to provide temperature readings from 0 to 1024 Celsius degrees with a resolution of 0.25. The control box has two pilot led indicator. The yellow one 4 is going to indicate that the oven is connected to the WiFi, and the green one, 5 is going to indicate that the oven is connected to the control software in the computer. There is an available USB connection in the ESP32, that is attached to the box through a panel mount mini usb cable 10 that will be used to set the configuration parameters like the name and password of the WiFi network.
In order to generate a "negative set point" and cooling the oven faster when needed by the regulator, a door opening mechanism has been added 12. Two fans blowing directly to the gap generated when the door is open help moving the air 11.