Astable Multivibrator
Projects
Astable Multivibrator using 555
Download Astable Multivibrator.epb
Aim
To simulate an astable multivibrator using IC 555.
Components
Name
EDWin Components Used
Description
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
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
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 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 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
Astable Multivibrator using 555
Download Astable Multivibrator.epb
Aim
To simulate an astable multivibrator using IC 555.
Components
Name
EDWin Components Used
Description
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
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
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 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 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