Servo and separate power supply - not working

Hi

I have a servo connected to my Arduino Duemilanove. It runs fine as long as the 5V and GND of the servo are connected to the 5V and GND pins on the Arduino board.
However it does not run when I have the servo connected to a separate power supply. The signal wire is still connected to the digital PWM output pin on the Arduino.

Is there a setting I am missing?

Thanks.

You've still got the grounds connected?

The 5V and the GND are connected to the separate power supply. It is just the signal wire that is connected to the Arduino.

But you've got the ground of the supply connected to the Arduino and the servo?

No.
I tried connecting the two GNDs together and now it works.

I knew it was something simple that I was missing. :blush:

Thanks.

The clue is in the word "circuit" - the current has to return on a second wire, otherwise you don't have a circuit, you have an antenna...