LED Chaser with only 4017 – YouTube
Posted by jpluimers on 2020/10/21
Cool:
The triggering in this case is the result of a main power drop and recover. The battery is unable to keep the votage at 9V level at the very moment any led is turned on, the voltage is droped down. That is what makes pin 14 to get the clock pulse so the 4017 shifts to the next led. The moment the led is off and right before the next led is turned on the main power gets recovered. When the next led is on again, the main power drops down, and so on.
Schema but less explanation on: [WayBack] LED Chaser with only 4017 – RevealNew
–jeroen
The Wiert Corner - irregular stream of stuff 
Peter Wright said
Intriguing.
The how-it-works however – complete Dingo’s kidneys.
The multicolour LED used contains an Integrated circuit that switches between showing red and blue. Those two LEDS have different forward voltages, so the current flowing in the resistor varies, depending on which LED is active. This provides the trigger-input. Notice how the leds start “running” when the displayed colour changes. I’d suggest that this is because the 4017 experiences a clock input that vacillates about its trigger threshold.
Rgds…P