help regarding using Arduino to drive laser device

[rochasrochas]

Hi, I need to use Arduino uno as an interface to drive laser operation from a programable power source. The power source has 0V (as +) and -28V (as -). The idea is using LM7805 to convert the voltage down to 5V to trigger arduino.
The wiring design is like what shown in the diagram photo. When I tested this system with 12V battery, it seems works fine. But when I connected this system to the real external 0V/-28V power source, it messed up the circuit of external power source and blew the power source I think. The uno is still fine afterward though. This is only happened when uno is connected to the laser (the device that uno will drive) when 0V/-28V power source is used, but not when the uno is tested alone with external power source. Any thought for what might be the cause??

[Landtronic]

Test if 7805 is OK.
Try this schematic