An arduino in its simplest form is an ATMEGA chip with bootloader, a crystal, 2 capacitors and reset switch. An LED on pin 13 and of course some possibilities to make easy connections are ‘nice to have’.
A simple circuit is found to the right. It is a small adaptation from Iñigo Zuluaga, who is a master in designing nifty circuits with a paper ‘silk screen’. Other people have done similar things like in the paperduino project van Guillherme Martins or the oomlaut arduino and also here.
The circuit is quite easy. Programs can be uploaded with either one of the many USB or serial connector boards or cables