After using my DIY Ambilight for a few days I got tired of plugging in and unplugging the power supply to the ShiftBrite shield, so I finally broke down and added a switch to turn it on and off. Now that I've been using it for a few weeks, I'm tired of having to turn it on and off manually every time I want to watch TV.
I noticed that my Samsung TV had a serial interface in the form of the EX-Link connector to see if I could electrically tell if the TV was on or not. No luck. I tested the USB ports next and found that when the display is off, the USB interfaces have about .15V and just over 5V when the display is on. Bingo. Now to get a relay.
This relay requires 3.5V to activate and less than .25V to deactivate.
My next step was to add a relay to my ShiftBrite shield and cut an extra USB extension cable and wire the +5V wire (red) and the ground (black) wire to the coil of a relay. My DIY ShiftBrite shield is now wired as the above illustration indicates with the USB +5V & Ground lines coming from the TV. Since the coil in the relay will cause a brief voltage spike that could damage your TV's USB port, it is important to make sure that the protection diode is added. Just add it 'backwards' across your relay coil so the energy stays in the coil and doesn't go back into the USB port.
Here is what my finished ShiftBrite shield looks like (the protection diode is on the bottom out of view):
Since most LCD TVs now have USB ports, I figured that this build could end up helping someone else. I certainly hope so!