Digital Circuits III: CIRD 3131The 74LS14 Schmitt Trigger

David Poulin and Reid Maclean

Objectives 1. To understand the operation of the 74LS14 Schmitt Trigger2. To build and test circuits using the 74LS14 Schmitt Trigger

Information Read sections 14.2, and 14.3 from textbook Lecture notes (fourth chapter)

Be careful in connecting power supply. Connect all the grounds to one point on the broad.

Components1. Function generator2. Dual trace oscilloscope3. DMM4. 74LS14 5. Suitable resistors and capacitors

Pre Lab Tasks1. Study the Schmitt trigger hysteresis and its output response by simulating with as shown

below:

2. Write a short about the function of a Schmitt trigger. 3. Explain the meaning of Vt+ and Vt-.

Lab Tasks

PART 1: THE 74LS14 SCHMITT TRIGGER OPERATION

1. With power off, connect a 74LS14 Schmitt Trigger input to a variable 0 to +5V DC power supply. Look at the schematic diagram for details. Set the supply initially to 0V.

2. Connect the Vcc and gnd pins of the 74LS14 Schmitt Trigger to a fixed +5V DC supply for normal operation.

3. From the spec sheets, record nominal values of the positive and negative threshold voltages Vt+ and Vt-

VT+ = ____________ VT- = _____________

4. With power on, measure Vout with Vin = 0V.

VIN = 0V __________ VOUT = ____________

5. Slowly raise the input voltage and record the input voltage and the out put voltage when the output voltage goes through a sharp change (= Vt+ )

VIN = VT+ __________ VOUT = _____________

6. Next, raise the input to +5V, and then slowly reduce the input voltage and record the input voltage and the output voltage when the output voltage goes through a sharp change (= Vt- )

VIN = VT- __________ VOUT = _____________

7. Calculate the hysteresis voltage Vt ________________

8. Based on your results, accurately draw the transfer function graph for the 74LS14 Schmitt Trigger.

Let Vin = +x axis, Vout = +y axis. Note: Refer to the Kleitz text book for details.

9. Explain in detail the operation of the Schmitt trigger in Part 1.

There are two triggers using the 7414.

When the voltage increases at the input from 0 to 5 volts, the output changes from HIGH (5 volts) to LOW (0 volts) when the input voltage reaches approximately 1.7 volts.

When the voltage decreases at the input from 5 to 0 volts, the output changes from LOW to HIGH when the input voltage is *******************?

PART 2: THE 74LS14 SCHMITT TRIGGER 60 Hz CLOCK PULSE GENERATION

10. With power off, connect the 74LS14 Schmitt Trigger to the secondary side of a transformer with a 1N749 zener diode, Vz = 4.3V. The Fig below shows circuit details.

11. With power on, measure Vin and Vout of the Schmitt trigger using an oscilloscope.

12. Record the positive and negative peak voltages of Vin and Vout, as well as the on and off times at the output of the of the Schmitt trigger circuit.

13. Draw accurate graphs of Vin and Vout, one below the other. Label them completely with peak values, as well as on/off times. Show a minimum of two cycles. Calculate the period T from the measured times. Calculate the frequency of the output from the period

V112.6Vrms 60Hz 0°

R1

1kΩ

R2

1kΩ

D11N749A

U1A

7414N

XSC1

Tektronix

1 2 3 4 T

G

P

14. Explain the operation of the 60 Hz clock pulse generator circuit in detail.

When AC input reaches its HIGH threshold level, the output of the Schmitt trigger is high. When the input voltage reaches its LOW threshold, the output of the Schmitt trigger is high.

PART 3: THE 74LS14 SCHMITT TRIGGER ASTABLE MULTIVIBRATOR CIRCUIT

15. Calculate the frequency of the free running square wave pulse circuit as shown below. Use the formulae as f = 0.8 / (RC) , where R <= 2 kΩ for the 74LS14.

f = _____________ T = _____________

16. Connect the 74LS14 Schmitt trigger astable multivibrator circuit of fig. 3, and measure Vc, Vout, ton and toff from the scope.

Vc lower = ____________ Vc upper = ____________ Vout = ____________

ton = ____________ toff = ____________ T = ____________ f =___________

17. Draw accurate graph of the measured of Vc and Vout one below the other. Label them completely including ton, toff, Vc lower, Vc upper. Show minimum of two cycles.

U1A

7414NC11µF

R11kΩ

XSC1

Tektronix

1 2 3 4 T

G

P

Questions

1. From the Kleitz book, do Q. 11-9, 11-10, 11-11, and 11-12.

Questions

11-9 – 3.6V

Vout

0.2V

Vin 1.9V 0.7V

11-10

11-11