In the kit Traffic Light Control Traffic lights alternate the right of way of road users by displaying lights of a standard color (red, yellow/amber, and green), using a universal color code (and a precise sequence to enable comprehension by those who are color blind).
In the kit Traffic Light Control the 555 astable circuit provides rapid square wave clock pulses for the 4017 counter which has ten outputs (Q0 to Q9). Each output becomes high in turn; clock pulses are received from the 555 timer IC. The clock pulse from the 555 timer is depends on the RC network connected to it. Appropriate outputs are combined with diodes to supply the yellow and green LEDs. The red LED is connected to the pin no 12 which is CO (Carry Out) as in when the counter counts 0 through 9 you get a carry function. Each positive clock transitions at pin 14 (Clock In) advances the counter one count. During clock pulses 0 through 4 pin 12 remains low and then clock pulses 5 through 9 it transitions high. i.e. ÷10 output which is high for the first 5 counts (Q0-Q4 high), this saves using 5 diodes for red and simplifies the circuit. At a time one output is higher (Positive) and other output pins of IC remains at low state. The capacitor and resistor on pin 15 of IC4017 are used to reset the counter to zero (Red light ON) at initial power.