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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

    Need Help? Please leave a comment, I'll get back soon with a reply!


    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,

    2. Dear Guru,
      What will be the duty cycle of the collector outputs with regard to your previous comment.

      Is it possible to get a 50% Duty Cycle from the above schematic?

      1. Dear Sherwin, the duty cycle will be 50%

    3. Dear,
      Is it possible to use diodes between resistors' junctions to make sure the circuit outputs a fixed 50% Duty Cycle??

      If we want 150mA current at the collectors then which transistor would be suitable for this circuit?

      Can high gain type transistors be used with this circuit for efficient power output?

      In a possible disaster to avoid both collector outputs going high within the functioning of this circuit, which components could help prevent this situation? How should they be connected in this circuit?

      I wish to know whether it could be possible to get the best out of this circuit since i would be using it for an inverter application.

      1. Hi Sherwin, for 150mA you can use a 2N2222, however that's not the correct way, in order to operate a load you must add an additional buffer transistor and rate this transistor appropriately for the load, as shown in this example article:

        both transistor can never be on at a given instant, that's practically impossible, unless one of the part is of bad quality and fails.

    4. Dear,
      I have observed that the right click function of the mouse doesn't operate on the CALCULATOR SECTION page.
      Why is it so?
      Actually I was unable to go to the page address you provided me with.

      1. You can highlight the link, and press Cntrl+C for copying

    5. Dear,
      What would be the phase degree of both collector outputs?

      Will it be a push-pull configuration?

      I did mention about the use of diodes between resistor junctions to make the circuit output a fixed 50% Duty Cycle? What do you think about it?

      1. Dear, diode will not help and is not relevant to the generation 50% cycle in the above design.

        the two arms of the circuit needs to be perfectly balanced using precisely identical component values, in order to get an accurate 50% Duty Cycle

      2. yes it is a push pull across both the collectors.

    6. Dear,
      If we set the frequency of the above circuit to 50Hz will the two outputs be a 180 degree out of phase??

      I planned to use tantalum capacitors along with the normal resistors in the circuit since i knew they would provide perfect accuracy.

      I even used bypass capacitors at the input of the circuit since the caps would hold the outputs more stable and with less noise.

      I also tried SL100 transistor and it worked best.

      1. yes it will completely out of phase 180 degrees.

        all transistors will work in this circuit

    7. A few years ago, i opened a cross light and found the above components connected (the transistor astable multivibrator) and i didn't understand anything about it and how it is constantly flipping on and off the LEDs.

      Actually there were two sections of LEDs one on the cross and other surrounded at the bottom where both sections would flip on and off alternately.

      The Astable multivibrator was powered from a capacitive power supply and i began to wonder what type of power supply is this? i had no idea about it.

      In fact both circuits were new to me at that time and now after some years i happen to understand these circuits and how the work.

      1. I appreciate your investigative nature in electronics, that's how true enthusiasts learn and improve their knowledge

    8. Now i'm really happy seeing this page detailed with instruction on this little circuit regarding how to prepare it.

      I'm also impressed with its calculator section designed by you in this page.

      I were curious about the working of the above T-AMV circuit and wished to know how it operated.

      Finally I've understood its operation.

      1. Thanks Sherwin, please keep up the good work, I wish you all the best....


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