Hi,
This is a easy question (but I don't know the answer :-/)
I have placed arduino into a plastic box. It's always connected by the usb port to the computer (to get 5V) but I would like to disconnect power when the device would be idle.
Easy answer would be 'Disconnect the usb cable' but I would prefer to place a switch on the box 
Thanks.
Well you could hook a switch up to the header that controls whether power is from the usb or power jack.
Hi wandrson,
If USB is connected and I switch to External Power Supply while power jack isn't connected, does it mean that there isn't any power in Arduino?
I have to tried it.
Thanks.
Here's an idea:
If you dont mind cutting up USB cables then cut off Arduino end of the cable. Put a SPST switch on the red or black lead (not green or white). then wire up the switch and remaining leads to a USB B plug. Cover the messy wires with electrical tape or heatshrink, and mount the switch to the top of the box.
Hope that helps,
Macmaniac ;D
replace the jumper next to the USB plug with a swtich, that will disconnect the 5v supply coming
from the USB cable
massimo
A single on off on switch.
Wire the common to the pin in the center of the header that has the jumper for USB or External power. Wire up one connection to the USB power power pin and one to the external power pin. Now you can flip the switch to use USB or external power or be off. I'm not sure but I think a three pin connector similar to the ones used on the board would allow you to solder to the connector and slide that onto the three pins on the board.
Also called:
Single pole changeover or Single pole, center off