I really wanted there to be a complete solution for the Woot-Off lights project that could be almost plug and play for the end user, so today I drew up a schematic and a board for a USB Power Shield for the Arduino. It pops onto the Arduino like any other shield, and by controlling pins 9-12, you can control power to four devices.
There are two USB ports for anything that takes USB power (5V) – of course, these ports are for power only (no data). You can use these to control the Woot-Off lights without cutting and soldering the cord, and something else (USB Fan? Light? Humping Dog?). There are also four pads toward the middle so you can solder in anything else that takes 5V power.
Since the Arduino can only supply maybe 300mA while on USB power and 600mA from its small 5V regulator (using DC in), I’ve included the stuff necessary to add a 7805 regulator and related components to allow up to 1A (if you use a heatsink). You can always leave these pads empty if you don’t plan on using that much power. The Woot-Off Lights will need at least the 5V regulator on the Arduino. There are also LEDs to show if the regulator and board itself are powered – these can be omitted as well.
The board uses four 2N3906 to control power of up to 200mA at 5V to four devices. By writing the corresponding pin LOW on the Arduino, you turn on the transistor and allow current to flow from 5V to the device (and then, hopefully, out to GND). Very Simple.