An Astable Multivibrator or a free running oscillator circuit is generally used to generate square waves for a specified time period. The name Astable Multivibrator is used for it because, it does not have a stable state and it switches between two quasi stable states.
Consider the instant at which the DC supply is switched on. Even though, both the transistors are made of the same material their is a chance for disproportion among the two and That is the key to the generation of vibrations. This phenomenon is further enhanced by the transistor configuration as shown in the figure.
The collector terminal of T1 is connected to the base of T2 via a capacitor C1 and similarly the collector terminal of T2 is connected to the base of transistor T1. So, due to this configuration only one transistor can remain switched-on at a time. The time period is determined by the capacitor connected to its base.
As said above, at the time of providing supply to this circuit , any imbalance among the transistors can cause any one of the transistor to conduct more, let it be T1 now. As T1 turns on its collector potential will be VCEsat which is approximately equal to 0.3v therefore the transistor T2 will get a base potential less than the cutin voltage 0.7 and this prevents transistor T2 from turning on. Now since the capacitor C1 is connected in series with the base terminal of T2, the capacitor charges to vcc through RC1 resistor. As the capacitor charges, the base voltage of T1 increases exponentially and when it is more than 0.7 v, the T2 turns on.
But the collector voltage of T2 drops to VCEsat which in turn is connected to the base of T1 and it drives T1 to switched-off state. Now C2 charges to vcc through Rc2 resistor and when the base voltage of T1 is more than 0.7v T1 turns on and T2 goes off . This regenerative action continues until the power supply is maintained. Thus the time period depends on the value of C1 and C2
Design of a simple practical Astable Multivibrator
To make an led glow for 2 sec and remain off for 1 sec
Component selection parameters
Choose supply voltage as 9v (for easy design)
Select transistor BC107 as it is commonly available in market
Design of R1 and R2
We know R=V/I,therefore R1=(Vcc-VCEsat)/Ic
we choose R1=R2=4.7K(standard)
Design of RC1 and RC2
RC1 and RC2 are the base resistors . They should provide enough base current to keep the transistor in saturation.
Base current Ib=Ic/hfe (hfe or gain of BC 107 is about 100 from datasheet)
Consider an over driving factor of 5 such that the transistor will surely be in saturation
Choose RC1=RC2=82K standard
The above designed values of resistors will not change as long as the transistor and supply potential is not replaced.
Design of time interval of switching
The time period of switching is controlled by the design of capacitor C1 and C2
let off time t2=1sec
and on time t1=2sec
The time constant of a capacitor is given by T=R*C (where R is the resistor through which the capacitor charges)
For practical situations we consider an over driving factor 0.69 therefore T=0.69RC
From the above equation C2=t2/(0.69*RC2)=17.67uF choose 15uF standard
Now C1=t1/(0.69*RC1)=35.34uF choose 33uF standard
We will connect a resistor 220R series with LED to provide protection for LED from high voltage
This circuit can function as as per our design. But, there may be some small variation subsequently, because of the approximation provided in the values of capacitor and resistors. in fact we used only standard valued components available in market.
Read How Phase Shift Oscillator work ? find your answer from here
This is a simple written article about the working and design of astable multivibrator . After reading this article you will be able to design this circuit on your own