kumod B09064

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EXPERIMENT #1

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OBJECTIVE: -

Obtaining square wave from function generator.Convert this to a trigger pulse using wave-shaping circuit.Making variable pulse monostable multivibrator circuit using 555 timer

IC.

Vary the duty cycle using control voltage input in the timer to getpulse width modulated output.

WAVE SHAPING CIRCUIT:-

BY: KUMOD KUMAR GUPTA (B09064)

KULDEEP PUROHIT (GROUP MATE)

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The wave shaping circuit is required to trigger the monostable circuit. Output of

this circuit has the trigger pulse which goes low from high for a short duration.

Figure 1: Output (B) after the diode.

Figure 2: Output (A) after capacitor

VARIABLE PULSE WIDTH MONOSTABLE CIRCUIT:-

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HOW 555 TIMER OPERATES:-

When a negative pulse is applied to the trigger input of the timer, then

comparator-2 detects it and compares it with +Vcc/3 at the +ve terminal of

comparator and gives HIGH at the output, it will set the flip-flop and the output

goes high, at the same time the transistor becomes off and the capacitor C

charged up through the resistor R1 till it reaches +2Vcc/3, after that the

comparator-1 will have high at its output and that will reset the flip-flop and turns

the output back to its original state and at the same time transistor becomes ON

and the capacitor will get discharged.

Now, if we want to control the width of the output pulse then we may add a

control voltage at the control terminal of the timer. Now instead of comparing

threshold terminal with 2Vcc/3, the comparator-1 will compare with control

voltage, if threshold voltage becomes greater than control voltage, then flip-flop

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will be reset. So we see here that control voltage is a decisive factor for the

width of the output pulse and so for the duty cycle.

The plot is showing Vc (across capacitor). The output we get will be ON up to

the period of charging of the capacitor which is a function of resistance RA and

capacitance C.

Time (s)

0.00 25.00u 50.00u 75.00u 100.00u

Vc

0.00

3.00

output

-1.00

5.00

Figure 3: Voltage through capacitor (simulation and experimental results)

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Varying the duty cycle:

Now, to vary the duty cycle, we add an additional voltage source at the control

voltage terminal of the timer.

RESULTS:

The PWM outputs at different values of control voltage are shown below:-

Figure 4: CONTROL VOLTAGE = 2.6V

Figure 5: CONTROL VOLTAGE = 2.8.

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The above figure is showing the plot of duty cycle versus control voltage.

0

0.1

0.2

0.3

0.4

0.5

0.60.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

DUTY CYCLE vs CONTROL VOLTAGE

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CONCLUSION:-

We successfully did the experiment in the lab and are able to vary the dutycycle of the output by varying the control input in the timer.

We have obtained a trigger pulse by using the wave shaping circuit.We studied how to use 555 timer and all to get the pulse of the desired

width and got them for different values of control voltage.

Now we can use this PWM output as an input in DC-DC convertor as agate driver circuit in MOSFET (switch).

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kumod B09064