Based on the CD4017 counter this circuit lets you see a dark LED running back and forth. Each middle LED is
connected to a PNP transistor controlled by two outputs. When one of them switches on, the low resistor bypasses
the base of the transistor, that switches off the LED.
The extreme stages are controlled by single outputs, so we just have to connect them to PNP transistors.
The inputs 15 and 13 of the CD4017 counter are connected to the negative pole of the power supply.The trimmer
connected to the NE555 timer is supposed to set the running speed.