9/16/2015

1

EXPERIMENT 22

555 MONOSTABLE

MULTIVIBRATOR (MV) Ferdinand M. Fernando

A s s t . P r o f e s s o r I

Logic Circuit F. M. Fernando URSM-College of Engineering 2

9/16/2015

2

Logic Circuit F. M. Fernando URSM-College of Engineering 3

Logic Circuit F. M. Fernando URSM-College of Engineering 4

9/16/2015

3

Logic Circuit F. M. Fernando URSM-College of Engineering 5

Logic Circuit F. M. Fernando URSM-College of Engineering 6

9/16/2015

4

PROCEDURE First, draw the circuit using your simulation software

(i.e. Ckt Wizard):

1. Run the simulation software.

2. Place the components and wire them together as

shown in the given circuit diagram.

3. Simulate the circuit & observe the signal indicated

by the logic probe when the Graph Window is

shown. What is the output signal and how much

output voltage is measured and for how long it is

ON? . . . . . . . . . . . . . . . . . . . . . . . . .

Logic Circuit F. M. Fernando URSM-College of Engineering 7

555 Monostable MV using Ckt Wizard

Logic Circuit F. M. Fernando URSM-College of Engineering 8

9/16/2015

5

4. Press and immediately release the push button

switch at the input to generate a pulse source

from the signal generator. Observe what happens

to the output signal as reflected by the two logic

probes. How much output voltage is measured

and for how long it was ON? . . . . . . . . . . . . . . . . .

5. Replace R3 (1 MΩ) with 2.2 MΩ. Press and

release immediately the input button again. What

does the signal probe indicate in terms of output

voltage and for how long it is ON? . . . . . . . . . . . .

Logic Circuit F. M. Fernando URSM-College of Engineering 9

6. State the difference between the result observed

between Procedure 4 & 5. . . . . . . . . . . . . . . . . . . . .

7. Calculate the period (T) using the formula

T=1.1RC and based from the measured values in

the previous steps, find the % accuracy of the

simulation using the absolute difference between

true and measured value divided by true value

and this result multiplied by 100%. . . . . . . . . . . . .

8. Using a stop watch (from your cell phone) verify

and compare the duration (T) of the 555’s ON

period in the previous steps. Again, measure the

software’s % accuracy. . . . . . . . . . . . . . . . . . . . . . .

Logic Circuit F. M. Fernando URSM-College of Engineering 10

9/16/2015

6

9. Using the formula for determining the ON

period of a 555 monostable MV, compute T

for the following RC combinations:

a) 4.7 MΩ & 1 µF, T =……….. d) 1 MΩ & 0.1 µF, T =……. .. ……

b) 2.2 MΩ & 0.47 µF, T =……. e) 560 KΩ & 0.1 µF, T =…………

c) 2.2 MΩ & 0.22 µF , T =…….

10.Draw the complete schematic diagram of a

555 monostable MV that generates a pulse

width of 1/2 second when it is triggered.

Logic Circuit F. M. Fernando URSM-College of Engineering 11

WRITTEN REPORT

THE 74121 NONRETRIGGERABLE

ONE-SHOT

9/16/2015

7

NONRETRIGGERABLE ONE-SHOT ACTION

In the figure (a) is the nonretriggerable one-shot being

triggered at intervals greater than its pulse width and at

intervals less than the pulse width. Notice that in (a), the

additional pulses are ignored. Logic Circuit F. M. Fernando URSM-College of Engineering 13

LOGIC SYMBOLS FOR THE 74121 NONRETRIGGERABLE

The 74121 is an example of a nonretriggerable one-

shot IC. It has provisions for external R and C. The

inputs (A1 , A2 , & B) are gated trigger inputs. The

RINT input connects to a 2KΩ internal timing

resistor. Logic Circuit F. M. Fernando URSM-College of Engineering 14

9/16/2015

8

3 WAYS TO SET THE PULSE WIDTH OF A 74121 IC

When a 74121 receives a trigger signal on one of its trigger inputs Q goes HIGH, &

after a period of time determined by a resistor value and a capacitor value, the Q

output returns LOW (Q’ returns HIGH). The time that the outputs remain in their

triggered states is gives as tW = 0.7RC. Logic Circuit F. M. Fernando URSM-College of Engineering 15

PROBLEMS 1. A certain application requires the 74LS121 one-shot

with an output pulse width of 10 µs. Use an external C

in conjunction with RINT. Show the connections and

component values using the ANSI/IEEE std. symbol.

2. Using a simulation software, perform the following

laboratory procedures:

a) Calculate R needed to give an output pulse width of 600

µs with C equal to 0.1 µF.

b) Draw the schematic for a 74121 ckt. which uses these

values. Choose the trigger input which will cause the

74121 to be triggered on the rising edge of the trigger

signal. Show this on the Graph Window. Connect the

other trigger inputs to the proper logic levels so that

they are permanently enabled. Logic Circuit F. M. Fernando URSM-College of Engineering 16

9/16/2015

9

c) Simulate the circuit for a 1.2-kHz TTL-level square-wave signal to the trigger input.

d) Use a dual-trace scope to analyze and show the input and output signals.

e) If the output pulse width is not about 600 µs, change the value of R to correct it. Make sure you do not use a value lower than the specified minimum R value.

f) To demo that the 74121 circuit is nonretriggerable, you need to investigate its response with 3 different trigger frequencies. To start, draw a section of the input and output waveforms with the 1200-Hz trigger signal.

Logic Circuit F. M. Fernando URSM-College of Engineering 17

g) Decrease the frequencies of the trigger signal to 500 Hz and draw a section of the trigger and the Q output waveforms.

h) Compare the width of the Q output pulse for a 500 Hz input signal with the Q output pulse width for the 1200-Hz trigger signal. Did the output high pulse width change as you went to a lower trigger frequency? Justify your answer by a graph of the signals.

i) Increase the frequency of the trigger signal to 2400 Hz and draw a section of the trigger and the Q output waveforms.

Logic Circuit F. M. Fernando URSM-College of Engineering 18

9/16/2015

10

j) Compare the width of the output pulse for this trigger signal with the output high pulse width for the previous cases.

k) Use the waveforms you drew for the 2400-Hz trigger signal to help you explain how a 74121 will respond to a trigger signal that occurs when the Q output is already high.

l) Compare the frequency of the output signal produced by a 1200-Hz trigger signal with the frequency of the output signal produced by a 2400-Hz trigger signal. Several applications of devices such as the 74121 take advantage of the frequency relationship you observe here.

Logic Circuit F. M. Fernando URSM-College

of Engineering 19