I set up a project with Arduino and the moment the output shield is activated it resets. I checked the program, but I did not find anything wrong. What can it be?"
The problem in this case seems to be the signal feedback that occurs when a higher current or inductive load is activated, for example, a relay or motor. If they are not properly decoupled, a signal feedback can occur, which causes the microcontroller to become unstable and may cause it to reset. See in our article IP067E (Decoupling) how to proceed to eliminate this problem.