Cannot power the Arduino Uno board from the power jack. It doesnt work as i want

Hi,

My name is Dani and I started using Arduino some months ago.

Lastly I tried to build a line following and at the end it worked out. All the time i had been using the USB connection to power Arduino but i thought it would be better to use the power jack so the line follower could work without the pc. I used a 9v battery and i soldered the wires from the battery to the adaptor.

When the line follower is stopped a LED is on. It works perfectly with the USB connection but when i power the Arduino board from the 9v battery, the LED sets on for 2 seconds then it starts blinking eventually faster.

I mean that the line follower is not the problem cause it works perfectly with the USB connection.

Any reason for this?

I used a 9v battery and i soldered the wires from the battery to the adaptor.

Can you be more specific?
Pictures might help, too.

Could be a drained 9V battery, with the blinking indicating the arduino is resetting when the motor load pulls the voltage down too low.
Ditch the 9V battery, use 6 AAs or 6 C cells instead.