03 December 2021

Reverse engineering a PCB - visually

In order to increase my "stock" of MG-17G 7-segment gas-filled displays, I bought a clone of the K80W calculator. It looks like many of them were produced and rebranded. The one I got had two (glass) broken tubes and something wrong in the electronics as only the rightmost digit would lit. Last but not least the faux leather case smelled nasty, so this calculator was set for dismantling.

Both PCB sides at once.

While I am against tearing apart calculators like these, this one was beyond repair and I already have a K80W needing a couple of new tubes.

As a sign of respect to the designers of this circuit I have decided to reverse-engineer it. In order to limit the handling of the stinky board, I opted for a visual method.

I shot a picture of the front and the back of the board paying extreme care to keep the (smartphone) camera at the same distance. Then using a photo editor program (The GIMP) I flipped the back picture, melted them together making one slightly transparent and the result is shown above. It's like a color X-ray scan.

Now I can work my way through the PCB traces either on the screen or on a printout.

Obviously this method works best with single sided circuits.