IC1 ( NE 555) is used as an Astable multivibrator that gives output pulses around 1 Hz with the oscillating components R1,R2 and C1. The output pulses from IC1 are given to the input (pin14) of the Decade counter IC2 (CD4017). It has 10 outputs and 6 outputs are used for driving the LEDs. 7th output pin 9 is connected to the reset pin 15 so that the counter stops and the cycle repeats. This gives the running display of LEDs.
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