Traffic Lights (555 Timer) – B2P9

We’re now going to build a traffic light circuit, not for its application but to introduce an interesting way which can be used to power ICs depending on the circuit.

Traffic light circuit

Here’s what the schematic looks like and its best we start looking at the timer ICs individually. IC1 is configured in the astable mode with R1 and C1 forming the charging network with a time period of about 11 seconds. LED1 is the red LED which is also connected in the reverse biased mode which means that it will switch ON when the output of IC1 is LOW.

IC2 is also connected in the astable mode with R3 and C2 giving it a time period of about 5 seconds. IC2 is actually powered by the output of IC1. This means that IC2 will only power ON once the red LED is switched OFF. The green LED is connected to the output of IC2 and it will switch ON when the output of IC2 is HIGH. The yellow LED is connected to the discharge pin, pin 7 and remember that the output and discharge pin have complementary states. This means that when the green LED is on, the yellow LED will be OFF. The yellow LED will only switch ON only once the green LED switches OFF.

It would take about 10 seconds for the green and yellow LEDs to cycle through the two states and by this time, the output of IC1 is ready to go change state and it will go LOW, which switches OFF IC2 but switches ON the red LED. If you piece it all together, then you will see that only 1 LED is active at any particular time and it cycles through as per the circuit design which gives us a traffic light effect.

Traffic light circuit

This is what the assembled circuit looks like and you can change the timing components to suit your requirements. Let’s move on to the next project.