I'm looking for the simplest possible way for a script (or any sort of program, can be C, Python, whatever you like within reason) to check whether mains power is on.
The computer which is going to check this is running off batteries so will still be running (I'm not quite that stupid!).
The easiest way I can think of at the moment is to have some sort of hardware with a presence on the LAN powered by mains so that one can ping it to check that mains power is there. However the 'simplest' device I have at the moment is an old router which seems rather overkill for such a simple requirement.
Can anyone think of any better way of doing this for which I am likely to have the hardware available or which will cost very little? I'm quite happy to build simple bits of electronics. The computer running the check is an old eeePc so has only USB and ehternet connections as far as I remember.