# Astable Multivibrator Circuit – B1P6

We’re going to use everything we’ve learnt to build a circuit that alternately toggles two LEDs.

This is what the circuit diagram looks like and at first glance, you might say that it is a very symmetric design. We have two LEDs which are controlled by 2 NPN transistors. We know that the base has to be positive for the transistors to switch ON and we can see that the base terminals are connected to a different resistor and capacitor pair.

When the capacitors charge to around 0.6-0.7V, it will cause the corresponding transistor to switch ON. The voltage at C1 controls Q1, while the voltage at C2 controls Q2. One thing to keep in mind is that no two components are exactly alike and this is particularly true for semiconductors like transistors. What this means is that one transistor will conduct before the other.

Let’s take a look at the simulation circuit and let’s assume Q2 starts conducting while Q1 is still OFF. Since Q2 is ON, it will cause LED 2 to glow.

Current can also flow through the combination of R5/C1 which causes C1 to charge with the polarity as shown.

When this voltage reaches 0.6-0.7V, it will cause Q1 to start conducting and this will turn ON LED 1. Since no current previously passed through C2, it does not hold any charge at this moment, but since Q1 has now switched ON, C2 starts charging from 0V and this 0V is also present across Q2 which switches it OFF.

Capacitor C1 was previously charged to polarity as shown earlier, but now, since Q1 is ON, it starts discharging and starts charging with the opposite polarity as shown above.

All this while, capacitor C2 has been charging slowly from 0V and once it reaches the threshold voltage for Q2, it will cause Q2 to switch ON. This will connect the positive terminal of C1 to the emitter of Q1, while the negative terminal is connected to its base and this would force Q1 to switch OFF. These cycles keep repeating causing the LEDs to alternate.

It’s easier to get an appreciation of what’s going on by running the simulation. Keep a note of how the capacitor voltages vary between the positive and negative cycles – which control the LEDs.

Let’s build the circuit using the breadboard layout. This type of a circuit is called an Astable multivibrator as it keeps alternating between two states. It can be used as a waveform generator.

Electrolytic capacitors do have a fixed polarity which should not be reversed. However, most of them are designed to withstand reverse voltages of about 1.5V without being damaged. It is acceptable to use them in this circuit as we’re only trying to understand the basics but if you were to design a professional multivibrator circuit then you would use a non-polarized capacitor like a ceramic capacitor.

We’re going to start building different sensor, alarm and detection circuit starting in the next post.