This is a very interesting a.c. voltage stabilizer that uses a LDR or photoresistor to stabilize an alternating current (AC).
If the voltage increases then the brightness of the light bulb increases too and the LDR’s resistance will decrease. If the potentiometer is properly adjusted then the AC voltage will stay constant.

The optimal adjustment of potentiometer is done experimental, feeding the circuit with a variable ratio transformer, which can simulate variations of voltage network.

The Author Name: P. Marian
Link: http://electroschematics.com/6021/voltage-stabilizer-with-ldr-photoresistor/