Is it possible to use 7805 regulator without capacitors at its input and output pins?
Can a 7805 regulator be used without input/output capacitors


Short answer: YES ....but there are conditions

First you should read a datasheet for the regulator such as this one.

The datasheet clearly lays out:




If your DC in supply is not too far away (distance/length not specified) you don't need a capacitor on the input. If your input supply is a small AC/DC walwart it will have a filter capacitor on its output. With a meter or so of cable you could probably do without the input capacitor on the regulator.
You don't need an output capacitor BUT the transient response of the regulator will suffer. In your case you have a load that is quickly turning on/off (your buzzer, so it may well be advisable to add an output capacitor of at least 0.1uF.
It's also worth noting that there is a MINIMUM load current specified for the regulator (5mA) so if your loads turn off then you should consider a resistor of 620 Ohms on the output for a 5volt regulator.