Here is a good quality and low cost project of a very loud 555 horn circuit. The circuit can be used as horn for bikes and other vehicles. It can also be used for many other purposes like as a siren circuit, buzzer circuit etc. The circuit is using only few components due to which you can easily fit
it in a small box with speaker.
The heart of the circuit is a 555 timer IC that is wired as an astable multivibrator here. By adjusting the 100K variable resistor you can select the desired tone of the horn. A TIP125 darlington transistor is used to amplify the output of the 555 IC. The operating voltage of the circuit is 9 to 12V DC. For speaker use an 8 Ohms 1 to 5 watt loud speaker. Loud sound will be received on 12V DC.