I'm using servo motor in my Arduino based project, I've tried to provide servo with external power supply, So I've used 9v battery with IC7805 to step down to 5v, but the motor is not working, so what should I do? FYI: I used the same battery with IC7805 to power IR proximity sensor which works fine.
A 9v "transistor" battery is unsuited to powering servos - at best it might work very briefly.
Use 4 alkaline (or depending on the servo ratings, perhaps 5 rechargeable) AA or AAA cells to power the servo.
Either power your Arduino from something else, or get a 3.3V version that you can operate from a low dropout regulator on the 4 or 5 cell pack.
A 7805 has a high dropout voltage and is out of place in a battery powered system.