Transistor Astable Multivibrator Calculator

With this calculator, you only enter R1 and R2 in k ohm, and you enter C1 and C2 in microfarad, and then the tool instantly gives you Ton1, Ton2, duty cycles, total period, and frequency. So now both beginners and experts can design any astable multivibrator without doing any math manually.

How BJT Astable Multivibrator Works

We know that a BJT astable multivibrator always uses two transistors, two resistors, and two capacitors, and we connect them in a cross-coupled manner.

So now we create a small positive feedback loop that keeps both transistors switching ON and OFF forever, without any stable point. That is why we call it astable.

So at the start, we assume one transistor, let us say Q1, turns ON first because of some tiny imbalance. Now the collector of Q1 goes low and that low point pulls the capacitor of the opposite side through coupling. So now Q2 gets forced OFF.

Now the capacitor C2 starts charging through resistor R2 slowly. When the voltage across C2 reaches around 0.6 V to 0.7 V, then Q2 suddenly switches ON.

And the moment Q2 switches ON, then it pulls down its collector, and that low again forces Q1 to switch OFF.

So now C1 starts charging through R1. And this loop repeats forever.

So now because each capacitor charges slowly through its own resistor, that charging time becomes the ON time of the opposite transistor. So C1 with R1 creates Ton2 or Ton1 depending on the side, and C2 with R2 creates the other timing.

How The Timing Actually Happens

Let us go more slowly, when Q1 is ON, then C2 is doing the job of charging through R2. So this time interval becomes Ton2, because Q2 is OFF and waiting to turn ON when C2 charges enough.

So after Ton2 finishes, Q2 turns ON.

Now when Q2 turns ON, then C1 starts charging through R1 and that becomes Ton1 because Q1 is OFF and waiting to turn ON.

So if we take R1 in k ohm and C1 in microfarad, then the time constant automatically becomes in milliseconds. So our simple standard formula becomes:

Ton1 = 0.693 × R1(kΩ) × C1(µF)
Ton2 = 0.693 × R2(kΩ) × C2(µF)

So now the total period T becomes:

T = Ton1 + Ton2

And the frequency becomes:

f = 1000 / T (because T is in milliseconds)

So everything becomes easy.

Why Two Duty Cycles Exist

Now let us explain the duty cycles. We have two transistors, so we have two separate ON times. So naturally each ON time becomes a part of the total period. So we calculate:

Duty1 = (Ton1 / T) × 100
Duty2 = (Ton2 / T) × 100

So both duty cycles add to 100 percent. And this is how both alternating arms create the square wave.

How To Use The Calculator

Now let us explain step by step how you must use the calculator.

1. We enter R1 in kilo ohms.
2. We enter C1 in microfarads.
3. We enter R2 in kilo ohms.
4. We enter C2 in microfarads.
5. Then we press the “Calculate” button.
6. Then the calculator tells us the ON time of Q1, the ON time of Q2, the total period, the frequency, and the individual duty cycles.

• So if we want a faster frequency, then we reduce R or we reduce C.
• And if we want a slower frequency, then we increase R or increase C.
• And if we want asymmetrical wave, then we make R1 ≠ R2 or C1 ≠ C2.
• And if we want symmetrical waveform, then we make both resistors equal and both capacitors equal.

So everything becomes very easyHow To Pick R And C Values In Real Circuits

• Now let us explain slowly how we decide the real part values.
• So normally we keep the timing resistors between 4.7 kΩ and 470 kΩ.
• And we keep the timing capacitors between 0.01 µF and 100 µF based on frequency.
• So for high frequency, like above 1 kHz, we use small capacitors like 0.01 µF to 0.1 µF.
• And for low frequency like 1 Hz to 10 Hz, we use bigger capacitors like 10 µF to 47 µF.

So now we also must make sure the collector current is not too heavy, so we keep the collector resistors between 4.7 kΩ and 10 kΩ for normal circuits.

Important Practical Notes

Now bro, let us share some practical crude tips:

1. When the capacitors are electrolytic, then we must connect the positive to the base of the transistor that gets pulled positive during charging.
2. When the frequency is not stable, then we check leakage of electrolytic capacitors.
3. When the circuit stops oscillation, then we check that the two collector resistors are equal.
4. When we want sharp switching, then we can add small 100 nF capacitors between the supply and ground for stability.
5. When we want higher amplitude, then we increase the collector resistor.
6. When we want low power operation, then we increase both resistors to 100 kΩ or more.

So this way everything becomes stable.

How The Calculator Helps You In Real Life

Now this calculator saves your time because you do not need to calculate 0.693 × R × C again and again. So you just enter k ohm and microfarad and instantly get the ON times, duty cycles, period and frequency. So this helps when we design LED blinkers, relay drivers, tone generators, square wave generators, and timing circuits.