I bought the toaster oven in a local super market for about 40 €. There was also a cheaper oven on sale, but I wasn’t sure if it did 250°C, so I bought the more expensive and prettier one.
Since I don’t want to loose the warranty and also haven’t any interest of the oven’s internals, I designed the reflow controller as an oven-external device which directly switches the oven’s mains on/off.
The relay I use is for 6V, but it seems to work just fine with the 5V supply from the ATtiny. On the mains side, the relay is rated for 230V/16A.
The whole controller is based on a ATtiny 45 µController. I use one pin as input for the 100k thermistor for temperature measurement (connected to JP3), one pin controls the mains relay (via a BC140 transistor), one pin controls a LED for feedback and one pin is connected to a switch for user input.
I built the whole thing on a 5x5cm stripboard:
Disclaimer: If you plan to build such a thing, be aware that 220V (or 110V) are involved in this circuit. That voltage potentially kills you. So be careful and don’t do it if you’re not sure, what you’re doing!
Here are the Eagle files for the hardware:
I managed to pack full PID controlled temperature management, 3 “programs” and a minimalistic UI into the 4k program memory of the ATtiny 45.
The main program (1) handles the reflow soldering process. It controls the oven’s temperature with the following temperature profile:
- heat up to 150°C
- hold this temperature for 40 seconds
- heat up to 190°C
- hold this temperature for 10 seconds
- heat up for 245°C
- hold this temperatiure for 10 seconds
- cool down
And here’s a short video of the Reflow Soldering Oven in action:
The whole SMD reflow soldering program runs for several minutes (see above). The video shows only the one important minute, where the soldering paste melts. On the left, you can see the thermistor…