NE555 is a Timer IC, which is connected in an astable multivibrator mode. In NE555, 8th pin & 1st pin is connected to Vcc & Ground. The 4th pin is Reset pin which is active low and is connected to Vcc to avoid accidental resets. 5th pin is the Control Voltage pin which is not used, so to avoid high frequency noises it is connected to a capacitor C2 whose other end is connected to ground. Usually C2 = 0.01μF. The Trigger (pin 2) and Threshold (pin 6) inputs are connected to the capacitor C1 which determines the output of the timer. 2nd & 6th Pin are connected together allowing the circuit to re-trigger itself. During each cycle, capacitor C1 charges up through both timing resistors, R1 and R2 but discharges itself only through resistor, R2 as the other side of R2 is connected to the discharge terminal, pin 7. Finally circuit works as a free running multivibrator. It has astable outputs (Pin 3). Diode 1N4007 is used for preventing the negative voltages while discharging the Capacitor C1. In Output, If LED1 is ON then LED2 is OFF and vice versa.  

         t1=0.693(R1+R2)C1      (Capacitor charging time)

                     t2=0.693R2C1        (Capacitor discharging time)

                        T=t1+t2         (Total time period)

NE555 is a Timer IC, which is connected in an astable multivibrator mode. In NE555, 8th pin & 1st pin is connected to Vcc & Ground. The 4th pin is Reset pin which is active low and is connected to Vcc to avoid accidental resets. 5th pin is the Control Voltage pin which is not used, so to avoid high frequency noises it is connected to a capacitor C2 whose other end is connected to ground. Usually C2 = 0.01μF. The Trigger (pin 2) and Threshold (pin 6) inputs are connected to the capacitor C1 which determines the output of the timer. 2nd & 6th Pin are connected together allowing the circuit to re-trigger itself. During each cycle, capacitor C1 charges up through both timing resistors, R1 and R2 but discharges itself only through resistor, R2 as the other side of R2 is connected to the discharge terminal, pin 7. Finally circuit works as a free running multivibrator. It has astable outputs (Pin 3). Diode 1N4007 is used for preventing the negative voltages while discharging the Capacitor C1. In Output, If LED1 is ON then LED2 is OFF and vice versa.  

         t1=0.693(R1+R2)C1      (Capacitor charging time)

                     t2=0.693R2C1        (Capacitor discharging time)

                        T=t1+t2         (Total time period)