Build DC/DC (Buck) Converter using 555 timer
The circuit that you linked is a horrible voltage converter. It has no feedback in the circuit to dynamically adjust the duty cycle to regulate the output voltage. The additional circuit on that web page showing to use a power transistor with a zener diode to compensate for the short comings of the 555 circuit is also a horrible approach. 

There are a plethora of available switching power supply controller chips and modules available that will do a far more superior job of converting the voltage from 12 to 5 volts. Get one and use that instead.