Pedestrian Cross Traffic signal using Astable Multivibrator

Pedestrian Cross Traffic Signal using

Astable Multivibrator

Our aim is to make a traffic light system that lets pedestrians to cross a busy highway. We implement this with an Astable Multivibrator which produces periodic ON and OFF signal. The output signal drives the RED and GREEN LEDs placed in the suitable fashion. The cool thing about this project is that it makes us realise we don't need a microcontroller to produce periodic signals. Welcome to Analog World!

Here is a list components, I used

1. Resistors (470 Ω,330 kΩ, 68 k Ω)
2. Capacitors (1 mF, 220μF)
3. Transistors (BC547)
4. LEDs (Red , Green) Of course!
5. Bread-board
6. Connecting wires
7. Power supply (A typical 9V alkaline battery)

    I made this circuit entirely on a bread-board as it is just  a Proof-Of-Concept.

Circuit Diagram:

The Idea behind  the circuit:

  The astable multivibrator is a kind of oscillator circuit. It gives two outputs in which one is the complement of another. When one transistor is in ON state another would be in OFF state.

An important pre-requisite for this circuit is that R2 and R3 should be much higher than R1 and R4. The reason for this condition would become obvious as we proceed further. When the power supply is given for the very first time, the transistor Q2 turns ON (V BE > 0.7 V) first as the voltage drop across R2 is less than that of R3. This action immediately turns transistor Q1 OFF, as the left plate of C2 would have a negative voltage as soon as Q2 turns ON. So, only 4 suitable LEDs glow in this state.

As of now, Q1 is OFF; Q2 is ON

Now, C2 charges through R3 via Q2 and C1 charges through R1 via the same Q2. As R1<<R3, C1 charges up fast to 7.7 V. Then slowly C2’s left plate will touch 0.7 V which makes the p-n junction of Q1 active and makes Q1 ON. This makes the capacitor C1 to discharge via Q1 and abruptly the right plate of C1 gets a potential of -7.7 V. As a result, Q2 turns OFF as soon as Q1 turns ON.

Thus, Q1 is ON; Q2 is OFF

The process continues with C1 charging through R2 via Q1 and C2 charges through R4 and via the same Q1. Because of R4 having low resistance than R2, C2 quickly charges to 7.7 V. At the same time, C1’s right plate loses its negative voltage (of course by discharging) and charges-up until it acquires 0.7 V. When it reaches 0.7 V, the transistor Q2 turns ON. So, the left plate of C2 gets a -7.7 V potential because of sudden discharge of C2 making Q1 OFF.

And now, Q1 is OFF; Q2 is ON

This process continues and repeats as long as the supply voltage is available. Apparently, resistors R2 and R3 control the time period of this oscillation as they limit the current to the capacitors. We can adjust the time period of each signal by changing either R2, C1 or R3, C2. The outputs of the transistors are taken from their emitter end and connected to LEDs in such a way that when it is GREEN for pedestrians, it will be RED for vehicles and vice-versa. When a transistor (Q1/Q2)
is ON, it forward biases the LEDs which are attached in parallel connection to ground. Hence they glow. On the other hand, when it is OFF the LEDs are unbiased.

The time duration for the two different cycles are measured to be 25 s and 20 s respectively when I checked.


I. Green for Pedestrians and RED for Vehicles (duration : 20 s):

II. RED for Pedestrians and GREEN for Vehicles (duration : 25 s):

That's all. Thanks for checking out my blog. Stay tuned to learn more.