Simple & cheap Reflow soldering oven

Reflow oven controller based on Atmega88, 16x2 LCD, rotary encoder and thermocouple with MAX6675. One-sided pcb diy-friendly.

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Atmega88 controls backlight and contrast of the LCD by software and PWM outputs. It works pretty well - the values will be stored in EEPROM. In my case good value for contrast is 50 (on 8-bit PWM, that is 50/256 duty cycle), and for backlight is everything above 0 ;) The display is readable even in lowest brightnes, so we can save some current and heat on linear regulator.

The power supply is something between 6-12V and its regulated by 78M05, LF50C or other with compatible pinout.

The output to control oven is matched to optotriac (in my case MOC3041).

To debounce encoder i'm using RC filter with exact values that i found in microwave oven, so it must be good ;)

Board design:

The pcb design is not ideal, but is good enough to work in finished oven.


-no ISP programming connector, or even test points :(. I workaroud this by bodge Kanda header with kynar.

-Also small imperfections like no mounting holes, T- from the thermocouple should be connected to gnd - this is simple to repair by a drop of solder or a wire.

-CS pin from MAX6675 should be connected to vcc via 100K resistor to allow programing atmega - they are on the same pins.

Software will be published on Github - link on the left <---

I will also upload to another github repository files from Eagle.

  • 1 × Cheap small electric oven from trash
  • 1 × LCD 16x2 HD44780
  • 1 × Rotary encoder with push button
  • 1 × Thermocouple type K
  • 1 × MAX6675 Evaluation, Demonstration Kits, Boards and Modules / Evaluation Kits, Boards and Modules

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  • first triac test

    Aleksander Kawęczyński10/26/2015 at 09:04 0 comments

    This weekend i connected the board with microprocessor to the board with triac and the oven. I temporary power the board from 7.4V li-ion battery. I inserted the thermocouple inside the oven, on the center of the plate. The task for controller was to keep temperature about 30 deg. Celsius. Actually i use simple on/off algorithm. The temperature in the my workshop was about 18 deg. C. The controller turned on the heater, waited for temperature to reach up to 31 deg. C, and turned off the oven.

    The electronic part worked well, the triac didn't heat up itself when he drive 4A of current (900W oven at 230V).

    But the temperature inside reach up 70 deg. C, even though the heater was turned off. That is no good, but i think that effect may be decreased if the oven will be work at higher temperatures - i must do some test with 150, 200 and 250 deg. Now i don't want to mount thermocouple near heater, because i want real temperature of the soldered board.

  • Time for firmware

    Aleksander Kawęczyński10/23/2015 at 09:52 0 comments

    Now i have done:

    -schematic and pcb design,

    -build first prototype of controller board (not tested with oven yet)

    -make small board with triac & optotriac to control heater (not tested yet)

    -start writing the code; hardware looks nice; encoder, display, internal sensor, max6675 with thermocouple works.

    -software menu structure

    TODO now:

    -implementation of menu in software

    -implementation of calculate temperature from internal temperature sensor (inside Atmega)

    -configuration Timer1 to drive speaker

    -configuration Timer2 to make 1s interrupts

    -calculate temperature change rate (derivatide dtemp/dt)

    -test triac

    I'm waiting for new themocouple from eBay, cause one i'm using now have huge metal end, and it can make problems to mount in the oven.

    It's possible that the software will be to big for atmega88, and i change to atmega168/328.

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