# Multivibrators with 2 transistors: astable, monostable and bistable

A multivibrator is a circuit which has two states in output, high or “1” and low or “0”. The Multivibrators can be of three types: Astable, monostable and bistable. In this post is shown operation and design of these circuits.

### Astable multivibrators

This is a classic astable multivibrator with 2 NPN transistors. How it works? When C1 is loading, T2 is in saturation allowing passage of electrical current through T2 and C2 is discharged. After C2’s discharge, it starts to charge by a current which pass through R4 until T1’s base terminal, allowing T1’s saturation. So T1 conducts, C1 is discharged and the cycle continues. In the videos shown, green part is positive voltage, yellow points are electric current, grey part is ground, which is zero voltage. While red is negative voltage.

To design this circuit, first has to define the astable’s oscillation frequency, in this example will be 20 Hz. So, period $T$ will be:

$T=\frac{1}{f}=\frac{1}{20}=0,05s$

Period must be divided in two, because it’s necessary to calculate time conduction of two transistors. $T1$ and $T2$ are periods of T1 and T2 transistors respectively.

$T=T1+T2$

$T1=0,69\cdot R2\cdot C2$

$T2=0,69\cdot R1\cdot C1$

Values of capacitors C1 and C2 are 10 μF, was chosen arbitrarily. T1 and T2 value 0.025 s, therefore:

$T1=0,69\cdot R2\cdot C2$

$0,025=0,69\cdot R1\cdot 10\mu$

$R1=3623,1\Omega$

Was used the closest value I have available, which is 3.9 kΩ. The transistor used can be a BC, I chose BC548. LEDs are red, therefore let’s considerate voltage drop $V_{LED}$ as 2 V. Voltage supply V1 is 6 V.

$Rc=\frac{(V1-V_{LED})}{Ic}$

The maximum of collector current $Ic$ BC548 can support is 100 mA, let’s use 10 mA. Therefore, resistances of R3 and R4 are:

$Rc=400\Omega$

Let’s use commercial value of 470 Ω. This is the circuit in printed circuit board, to LEDs blink slower, increase the value of resistors or capacitors. Output signal in one of transistor’s collector. #### Astable with PNP

It is possible to make an astable with 2 PNP transistors. Using the same values of previous circuit, replacing BC548 for BC558, will have the same result.  Output signals in collector of transistors.  ### Monostable multivibrators

There is only one stable state. In this circuit, transistor’s base is linked to collector of another one with a resistor. In stable state, T2 is cut while T1 is in saturation and C1 capacitor is charged. When switch S1 is closed, T2 conducts and T1 is cut. C1 is discharged, sending current to T2’s collector and D1 LED is blinked because T1 in cut produces a voltage between collector and emitter.

The duration of unstable state depends on R1 and C1, below is the period’s formula.

$T=0,693\cdot C1\cdot R1$

These are the schematics and printed circuit of monostable respectively.  R3 and R4 limit collector current and R2 protects BC548BP_2 from heating and limits base current. Pressing the button, LED will stay on for a period of time before turns off.

### Bistable multivibrators

In this bistable multivibrator, a transistor stays cut and another saturated or vice versa. When SET input goes to low level, T1 stays in cut and D1 LED turns on because voltage is in high level in T1’s collector. While T2 conducts and D2 LED stays off. If RESET stays in low level, T2 enters in cut, D2 turns on and T1 saturates. In video below, outputs change with high level (H) pulse.

In practice, in the circuit at beginning of topic, transistors operate independent of each other, outputs can stay both in high or low level. A flip-flop doesn’t work that way. This bistable was made to behave more realistically like a flip-flop. If S1 is closed, two output can stay in equal states of low or high. With S1 open, will behave like the circuit in video above.   