Designing an astable Multi-vibrator using IC555

Theory:  Here an astable or free running Multivibrator is shown in the figure: 01. Initially when the output is high, capacitor C starts charging towards V_CC through R_A and R_B. However, as soon as voltage across the capacitor equals 2/3 V_CC, comparator 1 triggers the flip-flop, and the output switches low. Now capacitor C starts discharging through R_B and transistor Q1. When the 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.

Let Ra=4.4kΩ and Rb=8.2kΩ;C=0.1µF and C1=0.01µF

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

                                                  tc=0.69Ra+Rbc=0.87 ms  

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 remains low and is given by

                                                       td=0.69Rbc=0.566 ms  

Thus the total period of the waveform is

                                                         T=tc+td=0.69RA+2Rbc=1.436 ms  

And the frequency of oscillation as
                                                           fo=1T=1.45RA+2Rbc=696.3 Hz
                   
Therefore                   % duty cycle=(tc/T×100)=60.6 

Required instrument:

a)     Timer IC555

b)    Resistor ( 4.4kΩ×1; 8.2kΩ×1)

c)     Capacitor (0.1µF×1; 0.01µF×1)

d)    V_CC (+5v)

e)     Oscilloscope

Circuit Diagram:

Fig-01: Circuit Diagram of free running mulvibrator

Procedure:

a)     First we took the instruments.

b)    Then we connected the instruments according to the circuit diagram shown in Fig-01.

c)     Then we connected the output terminal of 555 to the oscillator.

d)    After noticing that output, we connected the oscillator to the capacitor C.

e)     Then we measured the capacitor’s charging and discharging time from the oscillator.

f)      Then we calculated the duty cycle using oscillator’s output.

g)    At last we equivalent this value with the theoretical value.

h)    We took data and put them on data table.

Data table: We got the following data from oscilloscope.

Capacitor charging time t_{c  =} 6 ms

Capacitor discharging time t_{d =}  3.8 ms

Simulated output in workbench:

Fig-02: Output across Capacitor C and output terminal of 555

Calculation:

Measured tc=6 ms and td=3.8 ms    

Therefore % duty cycle=tcT×100=61.2 

Result: The calculated value and theoretical value are approximately equal. So the experiment is verified.

Discussion:  

a)    The above circuit is a free running or astable multi-vibrator because no state is stable at output and trigger pulse have to be applied.

b)    We connected the instruments with proper precautionary.

c)    We measured charging and discharging time very carefully from the oscilloscope.

d)    We calculated % duty cycle carefully.