Astable Multivibrator using 555


  To simulate an astable multivibrator using IC 555.



EDWin Components Used


Number of components required

RES RC05 Resistor 2
CAP CAP Capacitor 2
555 LM555 Timer 1
VDC SMB_VDC Dc voltage source 1
GND SMB_SPL0 Ground 3


  The circuit diagram for the astable multivibrator using IC 555 is shown here. The astable multivibrator generates a square wave, the period

  of which is determined by the circuit external to IC 555. The astable multivibrator does not require any external trigger to change the state of the  

  output. Hence the name free running oscillator. The time during which the output is either high or low is determined by the two resistors and

  a capacitor which are externally connected to the 555 timer.

  The above figure shows the 555 timer connected as an astable multivibrator. Initially when the output is high capacitor C starts charging towards 

  Vcc through RA and RB.

  However as soon as the voltage across the capacitor equals 2/3 Vcc , comparator1 triggers the flip-flop and the output switches to low state. 

  Now capacitor C discharges through RB and the transistor Q1. When voltage across C equals 1/3 Vcc, comparator 2ís output triggers the flip-   

  flop and the output goes high. Then the cycle repeats.

  The capacitor is periodically charged and discharged between 2/3 Vcc and 1/3 Vcc respectively. The time during which the capacitor     

  charges from 1/3 Vcc to 2/3 Vcc is equal to the time the output remains high and is given by

  where RA and RB are in ohms and C is in Farads. Similarly the time during which the capacitor discharges from 2/3 Vcc to 1/3 Vcc is equal to the

  time the output is low and is given by

Thus the total time period of the output waveform is

  Therefore the frequency of oscillation

  The output frequency, f is independent of the supply voltage Vcc.


  EDWin 2000 -> Schematic Editor: The circuit diagram is drawn by loading components from the library. Wiring and proper net assignment

  has been made. Values are assigned.

  for relevant components.

  EDWin 2000 -> Mixed Mode Simulator: The circuit is preprocessed. The waveform marker is placed at the output of the circuit. GND net is set as

  the reference net. The Transient Analysis parameters are also set and the Transient Analysis is executed. The output waveform is observed in

  the Waveform Viewer.


  The output waveform may be observed in the waveform viewer.