This is a direct follow up to my completion of the repairs and modification of my cheap chinese component tester. When I last left off the display was wired properly, but the code had it mirrored because apparently that’s how the controller for the original built in display was set up. The device also did not want to latch on so you had to hold the button and eventually it timed out.
It looks a bit messy, but I really don’t need a case for this thing. I’m not going to be throwing it in a backpack, I just need there to not be any stress on the wires so they don’t break off.
The first thing I did was wire around the first transistor on the board to disable the ability of the device to soft power off (kill itself and need a button press to latch back on for a while). I tried replacing each of the transistors in the circuit but that didn’t seem to change anything so I figured it was simpler to just leave it on. After that a power switch was warranted, I put that in line with the positive wire from the 9-volt battery so the whole device was really off and had no possible quiescent draw.
The build of software that I put on this expected a rotary encoder for input, so I installed one. The one I got came pre-populated with pull up resistors on the encoder lines, but I removed the pull up on the button line and I cut the traces for the other leg of the button so I could seperate it from ground. After those mods it electrically looked like the schematic above and I wired it in (to the same lines that are used by the LCD) and it worked great.
There’s two things interesting about the software portion of this. First is my discovery of AVRDUDESS which is a helpful graphical interface to avrdude allowing you to read and write AVR chips, the eeprom, the fuse bits, and it even explains the fuse bits. This gave me a lot more confidence that I wasn’t making some stupid mistake in the syntax. The second part is that I found a pre-compiled binary and eeprom file for this exact board and screen combination in the trunk of the SVN for the original authors of the component tester software. They maintain a really comprehensive codebase that you can customize with makefiles, but they also have prebuilt makefiles and binary images for all the platforms I imagine are commercially available (or common hobbyist configurations). Mine happened to be the mega328_st7565_kit.
Evan's Techie-Blog 