1) What is it, describe how to build, what is needed
2) Explain it, what makes it work, what is it that is working
Links discouraged as sole contibution, they may be broken, but copying something from them is ok, where link is given for anyone wanting more.
This should be a couple easy questions, we'll see. I've an idea from experience, I just do not want to devote time to remember well or to look it up myself. The objective centers around a device that does not plug into wall for power source, or to use batteries either.