Actually, it is more about replacement. Quick and dirty cheap. Instead of $12 original keypad, new one costs $2.
And that cheap 4x4 matrix keyboard is not what you are thinking, there is no Arduino inside. However I've thought about it, there's no use of it that I could think of.
I had the original matrix keypad on LG MS-2042G broken somehow.
Step 1: At the Beginning
That's how it looked like at first. The keypad is broken, tiny foil paths are torn. The rest of the microwave was absolutely fine.
Total number of contacts connected to the board is 8. I marked down the combinations, in the end it helped me figuring out layout on keypad.
Step 2: Replacement
At first I was going to make a custom keypad, but then realized that the cheap chinese one costs less and has 8 pins too!
So I drilled holes in the front plastic panel for wires to fit.
On the driver board there is connector intended for those thin "film contacts", i don't know how to call it. So I soldered wires with Dupont connectors right to the back of that connector.
Those wires are then connected to the keypad.
Step 3: Finish
Assemble everything together, test if it works, figuring out what the buttons do.
Stick the black film on the 7 segment display and the keypad beneath it.
As now it has more buttons, three unused cooking modes are now available. For two of them to work there are not enough hardware inside, though. The third one - auto reheat seems to work, however no one uses it anyway.
It took me about 2 hours to make.
To do: Make nice sticker with modes so that someone understands it.