Simple Buzzer Using Only Passive Components
This circuit is a simple buzzer circuit and it called novel buzzer. It only uses a relay in series with a small audio transformer and speaker. The relay will operate via the transformer primary and closed relay contact, when the switch is pressed. The normally closed contact will open as soon as the relay operates, removing power from the relay, the contacts close and the sequence repeats.
Because the pulse of current is very fast, resulting in fluctuations in the transistor and hence secondary. The speakers tone is thus proportional to relay operating frequency. To “tune” the note, this circuit uses the capacitor C with value 0.001uF. Increasing capacitance lowers the buzzers tone. [Source: www.zen22142.zen.co.uk]