NE555 DCDC 12V->12V+5V Regulated

Here is a schema of a DCDC Buck based on NE555 with regulated output around 12V and 5V.

Pro: You can adjust the number of turns of the transformer to adjust the voltage you need. (in the acceptable range!)
This is regulated by a zener and NE555 only.
Most of the component can be easily replaced by others.
The input voltage can drop under 9V. You can wire the inductance by hand (DIY)

Cons: Some information on the schematic are in french.
There is no safety except a fuse at input.
The converter is buzzing a little bit.
If you want to really tune the converter, you might need a simulation software or a oscilloscope.

Info: the references are RS-components