We will now use a new IC to help us count events. The CD4017 is a binary counter that has 10 output pins.
Initially, the first output will be 1, but when a clock pulse is sent to the clock pin, the second output will be a 1, while the others will be 0. Once the next clock pulse is sent to the clock pin, the third output will be 1, while others will be 0. This cycle continues till the 10th clock pulse is sent, after which it will cycle back to the first output.
The clock pulse is nothing but a square wave that can be generated using a 555 timer IC in the astable mode.
This is what the schematic looks like. We have a timer IC with a frequency of 1Hz and whose output is used as the clock signal for the 4017 counter. The CD4017 has an active HIGH reset pin, which means that the pin needs to be LOW to keep it out of reset. The clock inhibit pin can be used to stop the count, but since we want it to count continuously, we connect this to ground.
If we were to connect 10 different LEDs to each of the 10 outputs, then we would be able to see the LEDs glow from 1 to 10 and then back to 1, but since we want to create a LED runner, we connect them as per the circuit. This gives us a sequential, back and forth effect.
This is what the assembled PCB looks like. Be sure to observe the polarity for the LEDs and both the ICs. You can watch the 4017 sequentially illuminate the LEDs and you can change the RC circuit to change the clock frequency.