10 12V to 5V converter (555IC + Mosfet)

Buck converter circuit powered by a 12V battery down to 5V DC. Circuit is controlled by 555 timer IC that is operated in monostable mode.  The circuit is similar to 12V to 5V converter using BJT transistor but its much more stable, efficient, and higher current output (around 5A or even higher). The converter output voltage is given by the approximate equation:
Vout/Vin = (1-D); D is the Duty Cycle of 555 IC output.
See 555 timer calculator for more info about output frequency, duty cycle and combinations of resistors and capacitor.

schematic diagram

Part List:
R1 - 2 kilo ohms 1/4W
R2 - 5 kilo ohms 1/4W
R3 - 1.2 kilo ohms 1/4W
R4 - 27 ohms 1W (optional and can be removed)
C1, C2 - 100 nano-Farad capacitor
C3 - 2200uF electrolytic capacitor rated 16V
Q1 - IRF9530 or any similar P-channel Mosfet (10A current min)
D1 - SR506 or any diode rated 6A min (SCHOTTKY preferable)
L1 - 1mH Inductor minimum 6A current rating
555 - LM555 IC

555 pins

P-Channel Mosfet

When the converter is unloaded, its output is usually high but it drops to 5V when loaded. If you want to make your converter  more stable  (5V output loaded or unloaded), try to add the regulator circuit across its output pins.

regulator circuit

Regulator part list:
Dz - 1N4732A or 4.7V zener diode minimum 1W
R1 - 56 ohms 1W
Q - TIP41C, 2N3055 or similar NPN transistor