common ground, need help for my circuit

Hello,

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.

Thank you.

The first to do is to connect the grounds together for all power supplies. Then they have a base for the Exchange of signals between the different modules. Make this connection in one "central"place.

you’re missing power to the Arduino from the power board if you’re going to supply power to the project from that board. But that’s not recommended

A circuit diagram would be most helpful because I really can't see what's connected where. Specific pins and so.

There is no 5v power supply to mega without using 9v through its own regulator

wolframore: There is no 5v power supply to mega without using 9v through its own regulator

OP indicates 5V in at the USB. That's a good 5V supply.

The regulator doesn't need 9V. 6.5-7V is enough.

Confusing because OP states with only 9v led is weak below that.