Please explain tis circuit

[sankalani2003]

Please explain this circuit.How transisters operate
i got model answer for this but cannot understand it.
it says two diodes are used to limit the input voltage to 1.2.

 

[karesz]

... simulation says two diodes are used to limit the input voltage to 1.2.

Yes, thats right, but not all ..
It generate, triggered short time after all half sinus begin, a (100Hz)sawtooth signal with the UJT.
K.

 

[Prototyp_V1.0]

The R4, R5, R6 and TR3 act as a constant current source to the capacitor, so that the voltage increasing over C1 is constant (not like a RC circuit where change in voltage over cap decreases as voltage increase).

TR1, and TR2 is just making the capacitor discharging when input voltage gets higher than a certain level (decided by voltage divider R1 and R2).

The UJT transistor just make a voltage spike when the cap is charged to a determined voltage.

Read here and here for more info about UJT's.