IC 555 has an internal Flip-Flop and its working determines the output state of the IC. The discharge input of the IC has an NPN transistor with open collector. The On/Off of this transistor also depends on the state of the Flip-Flop. The Flip-Flop is under the control of the lower and upper comparator of the IC which can be independently controlled by external voltage. Here two LDRs are used for controlling the comparators.