rbbb & power & problems

Hi, I have a ID12 rfid reader and a servo connected to a rbbb arduino board. When a card is present, the servo will rotate, wait for few seconds and go back to the initial position.

The voltage regulator on the rbbb is TO-220.

If I connect a 5V, 400mA power supply to the rbbb, everything works fine, the way I wrote the program.

If I connect a 12V 1A power supply, the servo loops between open and close position even without a card present for the rfid.

Why does the board acts different with different power supply?

Thanks, -ioan

well since the RBBB doesnt come standard with a to220 packaged regulator what model of regulator do you have on there

You are right, is not a TO-220, it came with LM2940CT, 1A Low Dropout Regulator. I wrote the original post from the office trying to remember the details. It works with 9V 210mA and 9V 400mA. The power supply that doesn't work is a 12V 1A. I am just curious what do I need to know when powering a Arduino, this is my first project using it.

Edit: Actually, while I was writing this post, I left the 9V 400mA power supply connected to the Arduino and few times, very random the servo rotate, just like when a card is present.

Classic case of lack of decoupling:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Thanks Mike, that was an interesting read but I still need help with the design modification to implement the decoupling. In the picture is the rbbb voltage regulator, what modifications should I make?

Thanks, -ioan