The 4093 CMOS chip contains four 2 input NAND gates.
The gate at the top of the diagram is not used
The gate at the bottom is conected to a 27K resistor and 4.7mfd capacitor to form a square wave oscillator. This provides the clock for the circuit. (Normally you may use a 555 timer as a clock but that would mean another chip and more space used up)
Note: You can speed up or slow down the scanning of the LEDs by changing the value of the 27K resistor.
The output of the clock (pin 3 of the 4093) is fed to the Count input of the 4516.
The 4516 counter will count up or down depending on status of pin 10.
The 4516 counts in binary on its Q1, Q2 and Q4. (It does have a Q8 output on pin2 but we do not need it as we are only counting to 8.)
The Q1, Q2 and Q4 outputs are connected to the 4051's inputs. The 4051 connects pin 3 to one of the 0-7 pins depending on the binary data on Asel, Bsel and Csel.
As pin 3 is connected to the +ve supply one LED in turn will be illuminated via the 470 ohm dropping resistor.
The remaining two gates of the 4093 and associated diodes and resistors form a flip flop circuit. The output of which is fed to pin 10 of the 4516 to control whether it counts up or down.
Power is applied to the circuit.
The 4093 clock output (pin 3) feeds the 4516.
The output of the 4093 flip flop (pin 4) is high. Thus the 4516 will count down.
The 4051 fed from the 4516 connects +ve voltage via the 4051's pin 3 to each of the LEDs in turn.
(eg 4 then 3 then 2, then 1,then 0,)
When voltage is applied to the 0 LED it also feeds back to the 4093 flip flop and 'resets' it making pin 4 go low.
Pin 4 going low makes the 4516 count up.
LEDs will turn on in turn (eg 0 is already on. then 1 then 2 then 3 then 4 then 5 then 6 then 7)
When voltage is applied to the 7 LED it also feeds back to the 4093 flip flop and 'sets' it making pin 4 go high.
Pin 4 going high makes the 4516 count down.
The cycle repeats......