When switching the 9V ON, the buzzer starts making sound even if there is light on photoresistor
but it will stop after some time because of 555IC and then the circuit functioning with no problem
HOW can I solve the buzzer problem (when the circuit starts up) ????
Now you have pin 2 of the 555 at a low voltage all the time. With pin 2 low then it makes its pin 3 high which turns on Q3 and the buzzer all the time.
Pin 2 of a 555 must be pulsed low for a moment to trigger it then it must return to being high.
The signals at the emitters of the transistors are at a very low level. The trigger pin 2 of a 555 needs a high signal level.
In my circuit the 555 is a monostable exactly like in its datasheet. The 10k resistor holds its pin 2 high so it can time out and stop the buzzer.
When it gets dark then Q1 turns off. Then R4 turns on Q2. The collector of Q2 then has a negative-going pulse through the 47nF capacitor that triggers pin 2 of the 555 so it makes a timed output for the buzzer. R4 and R9 are 10k ohms and R5 is 470 ohms.