I've just found a totaly useful site.
Ten most needed circuits for the DIYerI have decided to compile a list of the top ten most needed circuits that are a must know for anyone interested in DIY projects. These are the basics that can all be interchanged and used in conjunction with each other to make many of the projects that we all love so much.10. The 5 v voltage regulator is a must, and is used to convert any 9v battery to the much needed 5 volts. This is used to power IC (integrated circuit) chips, as well as can be connected to a USB port to charge your ipod. The most common part is the 7805 chip, and here is the schematic.