Astable Multivibrator using 555

  Aim

  To simulate an astable multivibrator using IC 555.

  Components

  Theory

  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 V_cc through R_A and R_B.

  However as soon as the voltage across the capacitor equals 2/3 V_cc , comparator1 triggers the flip-flop and the output switches to low state. Now capacitor C discharges through R_B and the transistor Q1. When voltage across C equals 1/3 V_cc, comparator 2’s output triggers the flip- flop and the output goes high. Then the cycle repeats. The capacitor is periodically charged and discharged between 2/3 V_cc and 1/3 V_cc respectively. The time during which the capacitor charges from 1/3 V_cc to 2/3 V_cc is equal to the time the output remains high and is given by

t_c=0.693(R_A+R_B)C

  where R_A and R_B are in ohms and C is in Farads. Similarly the time during which the capacitor discharges from 2/3 V_cc to 1/3 V_cc is equal to the time the output is low and is given by

t_d=0.693R_BC

Thus the total time period of the output waveform is

T=t_c + t_d=0.693(R_A+2R_B)C

  Therefore the frequency of oscillation

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

   Procedure

  EDWinXP -> 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.

 EDWinXP-> 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.

 Result

  The output waveform may be observed in the waveform viewer.