## Monday, April 4, 2011

### TIMER IC APPLICATIONS - I(ASTABLE MULTIVIBRATOR)

TIMER IC APPLICATIONS - I
(ASTABLE MULTIVIBRATOR)

Design an astable multivibrator with 65% duty cycle at 4 KHz frequency, assume C= 0.01 µF.
[Design can be changed by changing the Duty cycle and frequency]

AIM:

To design an astable multivibrator circuit for the given specifications using 555 Timer IC.

APPARATUS REQUIRED:

 S. No Name of the Apparatus Range Quantity 1. Function Generator 3 MHz 1 2. CRO 30 MHz 1 3. Dual RPS 0 – 30 V 1 4. Timer IC IC 555 1 5. Bread Board 1 6. Resistors 7. Capacitors 8. Connecting wires and probes As required

THEORY:
An astable multivibrator, often called a free-running multivibrator, is a rectangular-wave-generating circuit.  This circuit do not require an external trigger to change the state of the output.  The time during which the output is either high or low is determined by two resistors and a capacitor, which are connected externally to the 555 timer.  The time during which the capacitor charges from 1/3 Vcc to 2/3 Vcc is equal to the time the output is high and is given by,

tc = 0.69 (R1 + R2) C

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,

td = 0.69 (R2) C

Thus the total time period of the output waveform is,

T = tc + td = 0.69 (R1 + 2 R2) C

The term duty cycle is often used in conjunction with the astable multivibrator.  The duty cycle is the ratio of the time tc during which the output is high to the total time period T.  It is generally expressed in percentage. In equation form,

% duty cycle = [(R1 + R2) / (R1 + 2 R2)] x 100

PIN DIAGRAM:

CIRCUIT DIAGRAM OF ASTABLE MULTIVIBRATOR

DESIGN:

Given f= 4 KHz,
Therefore, Total time period, T = 1/f = ____________

We know, duty cycle = tc / T
Therefore, tc  =  ------------------------
and td = ____________

We also know for an astable multivibrator
td = 0.69 (R2) C
Therefore, R2 = _____________

tc = 0.69 (R1 + R2) C
Therefore, R1 = _____________

PROCEDURE:

1.  Connections are given as per the circuit diagram.
2.  + 5V supply is given to the + Vcc terminal of the timer IC.
3. At pin 3 the output waveform is observed with the help of a CRO
4. At pin 6 the capacitor voltage is obtained in the CRO and the V0 and Vc voltage waveforms are plotted in a graph sheet.

OBSERVATIONS:

 S.No Waveforms Amplitude( No. of div xVolts per div ) Time period( No. of div x Time per div ) tc td 1. Output Voltage , Vo 2. Capacitor voltage , Vc

MODEL GRAPH:

RESULT:

The design of the Astable multivibrator circuit was done and the output voltage and capacitor voltage waveforms were obtained.