This easy electronic buzzer circuit built based on timer works for gaining the frequency. The IC timer NE 555 used as astable multivibrator operating at about 1kHz and produces a sound when switched on. The sound frequency can be adjusted by varying the 10K resistor. You may change the 10K resistor with variable resistor.
The supply voltage range is about 5-15V DC, middle value is recommended for safe work of the timer IC. Small speaker can be used for this circuit. This circuit is low cost, easy to build and can be used for multi purposes such as general alarm circuit, quiz buzzer, or circuit operation tone code to indicate that a switch has been connected.