Overview

If you don't have a Piezo sounder with integral oscillator to hand or you want greater
volume, then this simple circuit can be used as a replacement for the Piezo sounder.

This circuit is based on the popular 555 timer and runs from a 12V supply,
the two transistors provide a simple interface from the TTL level logic of the PIC.

By gating the 555 via the reset input of the 555 oscillation can be controlled. 

Circuit

Links:

The 555 Astable

Engineer's Mini-Notebook - 555 Timer IC Circuits