i built a circuit with 2 power inputs (arduino USB and 9v input). Maybe i have everything right but i am not sure.
Before i start to explain i attach my circuit drawing.
When i just connect the 9v input then my led on the arduino start to shine weak but everything work fine if i connect both the arduino and the 9v input. So i am not sure if my common ground is connected correctly. So maybe some of you could check my circuit.