> This is a serious question.  How do you "understand" how a circuit
> works and how do you create with your imagination a new one ?

It really really helps if you've got an end product as the goal. Also
a good general knowledge of what devices are out there and what's
probably appropriate to use to reach that goal. Experience and practice
are musts, IMHO. Have a look at magazine articles and the 'How It
Works' panel to see how they accomplished various circuit tasks

For example

http://www.siliconchip.com.au/cms/A_30551/article.html

is a good mix of analogue, digital and PIC, and is well documented

>From a personal perspective, not having had any formal electronics
training, I think the initial idea is more important than the understanding.
That can come later when you try to get the different parts of the
circuit working with selected components

We here would generally be working with components that obey
a few simple rules, so the limits of what you can do with those
components are not too difficult to stay within

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist