This circuit produces the famous Big Ben sound. It produces the `ding dong` sound when switched ON.
Basically the circuit alternates between two frequencies which are adjustable. This produces the `ding-dong` sound.
The first IC(left) oscillates at about 1Hz. The second IC's tone is modulated by the changing voltage at the output of the first IC.
The first IC determines how fast the changeover from one frequency to the other takes place and second IC determines the tone of the final output.
By varying the VR1, the changeover rate can be adjusted. By varying VR2 the tone can be adjusted.