I recently bought a USB-powered desk light. Is it possible to control (turn on/off) it with the Arduino Uno? The desk light has 3W so needs 600mA.
Use a TIP120 Darlington transistor to switch the current. This webpage has a detailed guide. http://bildr.org/2011/03/high-power-control-with-arduino-and-tip120/ Use the example with the light bulb - no diode is needed in your case.
You have a couple options here. The main idea is that you're going to add a switch to the light's circuit and that switch will be controlled by the arduino. It's important that you keep the electronics for the light and the electronics for your arduino separate, 600mA will damage your arduino which no one wants. You can power the light and arduino from the same power supply, make sure it can handle the power draw though.
To actually switch the circuit you could use an NPN transistor as mentioned above, with the lamp's - terminal flowing into the C pin, an arduino GPIO pin connected to the B pin and the E pin grounded. When you set your arduino's pin HIGH the transistor will allow current to flow and light up the lamp.
Another option that actually physically separates the circuits is to use an opto-coupler. They're little IC's that serve this exact purpose. The have a light and a light sensitive transistor inside them, so when the light illuminates, the transistor allows current to flow. In your case, you would hook up an arduino pin to the + pin on the light side of the coupler and ground the - pin. Then connect your lamp in the same to the other side, +5v into the + pin on the transistor side and the lamp's positive terminal to the output pin. Again, when the arduino pin goes HIGH, the lamp will light up.
