I have a crude power sensing system I built with an Arduino Pro, a GPRS shield, and a relay that will send me a text message when my power goes out. The Arduino is powered from a UPS, which will keep it going after the power fails. To sense a failure, I have a relay held closed by a USB wall wart plugged directly into the wall (no UPS). When the power fails, the relay opens, and the Arduino senses the change in state and sends a text message. When the power comes back on, the wall wart pulls the relay closed again, and I get another message saying the power has been restored. If the power is out longer than the UPS can handle (there's a computer attached to it too), I'll get a message when the Arduino starts back up once the power is restored.
What I want to do is eliminate the relay. I know I should be able to use a transistor instead. I don't know much at all about transistors though. What kind of transistor should I use? (I know NPN, but more specifically which one?). How would I wire it? Do I need anything else in my circuit, like a resistor or a diode?
I ended up finding the permanent solution I'm going to go with. I pulled the 4N35 optoisolator out of my Arduino startup kit. My USB wall wart is plugged in to the input side (with a resistor). The collector is plugged into the Arduino ground, and the emitter is plugged into a digital pin. It's programmed to look for a button push, which the optoisolator simulates.