This simple electronic bicycle horn circuit uses only one gate of a 4093 quad 2-input NAND Schmitt trigger, U1, conected in a simple, low frequency, square-wave oscillator circuit.
The oscillator’s output, at pin 3, drives the gate of Q1. The drain of that FET drives a small horn speaker.
Potentiometer R1 can be adjusted to set the horn’s output frequency. Some horn speakers are frequency sensitive, so play with the oscillator’s frequency control for the best loudest sound.