ok...I really need help now. So I tested the entire setting (uno + the device that will be driven by uno) with 12v battery again and it worked fine (the program demands delivery of a 3s pulse twice and 3s LED blink twice). Next I powered the system with real external power source (0V and -28V) I planned to use. Without connecting the real device that uno is to drive, the power input to uno seems work well, as LED did blink according to the program. Then I hooked up the uno to the device, run the program again. This time uno run only once of the what the program demands, and then power source got shut down (we have a LED panel to read the output of power source), seems the circuit in the power source has been messed up. Any thought of what could be the potential problem here?? not having capacitors??