Arduino not functioning with external power

I am using an Arduino Duemilanove to run a LED matrix using MAX 7219.
As I am using it for a display on a car, I would like to externally provide it 12V using either the Vin pin or the power jack.

But on supplying 12V at the Vin pin or when given power through the power jack, the LED matrix isnt functioning as expected. So then I tried supplying 5v to the 5V pin using an IC 7805, but still no go.

The LED matrix display is working properly only when the arduino is powered through the USB cable.
Any suggestions.

If you hook a 12v power supply to the normal external power input .. and the 12v power has enough power (i.e. 500ma at least) .. it should work.

That said .. you may want to just get a standard 5v iphone or other phone adapter that has the USB interface, add a USB cable .. then you have a USB Plug. I do this all the time .. and it makes it convenient to only have one cable for either the computer or the wall.

Good luck

the LED matrix isnt functioning as expected

How is it functioning?
You did connect the ground up as well as the +ve ?

The LED matrix display is working properly only when the arduino is powered through the USB cable.

Firstly are you sure it is an official arduino? I have to ask as people have hed various regulator issues with non-official arduino boards in the past.

Secondly, do you still have the USB cable plugged in when you connect it to external power?

Mowcius

I have two (2) boards. Both are Arduino Duemilanove and both have 5 volts on the USB port and 3.5 volts on the external power port. Board 1 is two months old, the other arrived today. The 3.5 volts is not enough to run the LED display or power the real time clock

The USB cable is not plugged in when I use the external power port. If I need to, I can supply 12v to the external power port or 5V to the Vin pin.

Thank you all for the replies.

I did the connections again and used a 7805 to provide a regulated 5V to the 5v pin. Its working now so there must have been a problem with the external connections when it was mounted. Or maybe there was differential grounding.

also, its an official Arduino form Italy, so no problems there.
Thanks for the suggestions.

Must be a funny 12v supply then.