This circuit consists of a strip of six LED working through the inverted output signals of a CD4017 counter, so that you will see a dark light running back and forth.

The first and the last LED are turned by the single outputs 0 and 5, so I just connected the base of a PNP transistor through a 12K resistor.

Each middle LED is switched by two outputs and gets dark when one of them takes the high level, so that the base of the transistor is bypassed by the low resistor.

The trimmer is supposed to set  the running speed of the circuit.