How can the Arduino check if it is connected to a PC and the PC is on?
As suggested by Paul &amp; Gerben, you could check the 5V USB line to see if it has a voltage or not.

I am assuming from your question that the Arduino is not powered from the USB line. I am also assuming that the USB port is not powered when the PC is turned off.

All you have to do is connect VUSB to a digital input through a resistor like so:




Alternatively, you could have the computer send a pre-determined message telling the Arduino that its alive.

If the Arduino receives this message then it lights up the LED on pin 13. If the Arduino hasn't received the "I'm alive" message for a while (ie. a timeout) it then turns off the LED.

A simple PC program could be written to automate this task.