Transistor Astable Multivibrator AMV Calculator

Astable Multivibrator using Transistor Working
  • The negative voltage passing through the capacitor C2 turns off the transistor Q1 which causes the capacitor C1 to initiate charging through resistor R and Vcc, as well as through the base emitter of transistor Q2. This causes the transistor Q2 to acquire the momentary ON state.
  • During the process, the capacitor C2 gradually discharges until it's completely empty, and then it starts charging from opposite direction through R2.
  • As soon as the voltage in capacitor C2 is sufficient to turn ON transistor Q1, Q1 switches ON and forces capacitor C1 to initiate discharging.
  • The above process keeps recycling causing a sustained and alternate switching of the transistors as long as the circuit in the powered state. 



    R – Collector Resistor

    The resistance R needs to be dimensioned so that it is able to limit the collector current Ic below the specified threshold.

    The following formula represents it:

    R = V/Ic ,

    where V is the voltage across the resistor R.

    Normally this could be expressed as, V = (Vcc – Vce) = (Vce – 0.3)  however in cases where an emitter load such as an LED is utilized, the expression may be modified as:

    V = (Vcc – Vce – Vled) , where Vled is the voltage drop across LED.

    In most cases the maximum collector current Ic could be much higher than than the required for emitter load current. During such instances Ic could be tailored in such a way that it stays below the max current specification of emitter load.

    • R = (Vcc – Vce – Vload) / Ic

    R1 & R2 – Base Resistors

    R1 & R2 must be selected to obtain the desired collector current during saturation state.
    • Min. Base Current, Ibmin = Ic / β, where β is the hFE of the transistor
    • Safe Base Current,Ib =  10x Ibmin= 3 x Ic / β
    • R1, R2 = (Vcc – Vbe) / Ib

    T1 & T2 – Time Period

    • T2 = OFF Period of transistor Q1 = ON Period of Transistor Q2 = 0.693R2C2
    • T1 = OFF Period of transistor Q2 = ON Period of Transistor Q1 = 0.693R1C1
    From these expressions we can evaluate the value of C1 and C2.

    Duty Cycle

    It may be defined as the ratio of  time Tc when the output is high to the total time period T of the cycle.

    Thus here, Duty Cycle = Toff/(Toff + Ton) when the output is acquired from the collector of the transistor T.




      V      Please enter 0 if there is No Emitter Load


    1. Thank you sir
      For your kind help ...... but sir i am new at this stage. i hope you will help me to Get my desired circuit . i want a circuit which can turn On the relay For 15 seconds and Turn OFF the relay For 15 seconds ... it will goes continue

      1. Thanks Saroj, first you will have to make the above circuit, then connect the following relay driver driver across the collector and ground

        Part values for the transistor astable is

        outside resistors = 585 Ω
        inside resistors = 37667 Ω
        C1 = 574.6 μF
        C2 = 574.6 μF
        Transistors BC547

      2. Sharoj give me your e.mail and I send you the proteus file of the circuit working as you expected with 15 seconds of time value,