Aahhhhhh! it’s stopped working!

After painstakingly putting the kit together and testing it, my contrary PIC chip has decided to stop working. Today I plugged it in to find that only some of the keypad keys are working, the LED won’t flash and the buzzer is making a constant low level bzzzzzzzzzzzzzzzzzzzzzzzzzz (very annoying when you’re trying to find out what’s wrong with it). None of my connections are loose, I’ve got no solder bridges and no crossed connections, time to ask for help from a higher power…. Dean, are you free?….

OK, time to run some diagnostics, after discussing my problem with Dean he has reccomended that we use an oscilloscope (such a cool name) to try and see what is going on inside my circuit. The oscilloscope will track any signals being sent from the legs of my PIC to the outputs on my board, the blip on the screen should jump up for a live signal or remain flat for no signal. Or in the case of my key pad it should jump up and down rapidly to indicate the PIC is scanning each of the output rows in turn.

Once I had taped over the buzzer (there’s only so long you can listen to it going bzzzzzzzzzzzzzzzzz without having serious thoughts about destroying it in an inventive fashion) we got down to investigating. Using the circuit diagram (see below) we tested each output in turn, it didn’t take long to see that outputs 7 and 12 had swapped places, the constant high/low signal that was supposed to be scanning one of the keypad rows was what was causing the buzzer to be so irritating. We’re still not sure what caused this to occur as the only thing that had happened to the board is that I had carried it home and back to the workshop, the only explanation we can come up with is that it suffered a static shock from the box I was keeping it in and it scrambled the programme. Right, keeping it in a static proof bag now…

One rapid reprogramme later and I’m back to having a working board, only 3/4 of a day lost…