History[ edit ] A vacuum tube Abraham-Bloch multivibrator oscillator, France, small box, left. Its harmonics are being used to calibrate a wavemeter center. Since it produced a square wave , in contrast to the sine wave generated by most other oscillator circuits of the time, its output contained many harmonics above the fundamental frequency, which could be used for calibrating high frequency radio circuits. For this reason Abraham and Bloch called it a multivibrateur. It is a predecessor of the Eccles-Jordan trigger  which was derived from the circuit a year later.
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It can blink just one LED. Or it can create a tone to play on a speaker. First, let me show you the circuit in action: Want to know the theory behind how the circuit works? There have been many attempts to explain this circuit. Most have failed to explain it to other than those already beyond the beginner level. The voltage on the right side of C1 controls transistor Q2. After a short while, the voltage of C1 rises back up and turns on the transistor Q2.
When transistor Q2 turns on, it changes the voltage of C2 so that Q1 turns off. This keeps repeating. What if you want to understand why this happens?
Before we jump in If you want to really understand how the astable multivibrator circuit works, you have to look more detailed at how the voltages over the two capacitors behave. What do you need to know? And it is important that you have a good understanding of how voltages behave in a circuit, and how current flows. The Detailed Explanation A couple of things to help you before diving into the explanation… 1. Voltage is always measured between two points.
When we talk about the voltage at one specific point, it means the voltage measured from that point to the minus of the battery. Think about the transistor as a switch. It needs 0. When it is ON, its top pin collector connects down to its bottom pin emitter so that current can flow through it.
This also means that the top pin has the same voltage as the bottom pin when the transistor is on. When the transistor is OFF, there are no connections between the top pin and the bottom pin, so no current can flow.
Use this simulator to see for yourself I recommend verifying the things I am writing here by using a simulator. L1 is only lit when transistor Q1 is ON. We know from how transistors work that Q1 is only turned ON if it has 0.
The right side of the capacitor C2 connects to 9V through R4 and L2, so it is charging and the voltage is rising. A capacitor charges exponentially, which means the voltage rises quickly in the beginning, then slows down more and more. The voltage reaches V quickly, but from there the voltage rises slowly. The voltages around transistor Q2 The voltage on the right of C1 is somewhere below 0. The right side of C1 connects to the base of Q2, so that means this is also lower than 0.
But the right side of C1 is also connected to 9V through the resistor R2, which means it is being charged. That means the voltage is below 0. The Turning Point So, the voltage on the right side of C1 is rising. And when it reaches 0. When the right side of C1 reaches 0. But when Q2 turns on, something interesting happens with the voltages we had over the capacitor C2… Getting a negative voltage We had that C2 had 0.
Or to say it in another way, the left side was 7. But now that Q2 turns on, the voltage on the right side of C2 is suddenly pulled down to 0V through the transistor. The internal charge of the capacitor does not change though, so the left side keeps being 7. But now that the right side is 0V, that means the left side becomes 7. Transistor Q1 gets minus on its base With When transistor Q2 turns on, the transistor and LED on the left turn off.
So now, the left LED and transistor have turned off. And the right LED and transistor have turned on. The left side of C2 starts at Since it connects to the base of transistor Q1, when it reaches 0. And so it continues. The two transistors keep alternating between on and off, which makes the two LEDs alternate between on and off.
I had so much trouble understanding the astable multivibrator circuit when starting out. And it frustrated me because I was thinking it was a simple and easy-to-understand circuit. But the truth is that you need to have a good understanding of the basics of electronics before you will be able to understand it.
And you should have looked at several much simpler circuits first. Something I take all my Ohmify students through. Click to learn more about Ohmify. Did this explanation help you understand the circuit? Or are you just more confused than when starting out? Let me know your comments and questions in the comment field below!
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How Astable Multivibrator Circuits Work