Prerequisite Information Circuit 4 Schmitt Trigger Objective Upon completion of this procedure, you will be able to determine the functional characteristics of a typical Schmitt trigger. You will verify your results by taking voltage and waveform readings. Discussion 9V Figure 4-1 The Schmitt trigger is a wave squaring circuit used to convert a sinusoidal or irregular input wave into a square or rectangular output wave. The circuit is basically a two-stage electronic gate. The gate is comprised of two dc coupled transistors that employ regenerative feedback through a common emitter resistor (). Regenerative feedback is necessary to produce a positive switching action. 59
Prerequisite Information 9V Initially is biased off. Initially is on. Figure 4-2 Transistors and are biased so that initially is off and is on. Bias levels and stability for are provided by voltage divider, variable resistor, and by. Transistor forward bias and stability are determined by load resistor, base resistor and by common emitter resistor. 60
Prerequisite Information 9V Figure 4-3 Variable resistor sets the bias level for and thus controls the circuit trigger or threshold level. When the positive going input signal exceeds the trigger level, rapidly switches into full conduction forcing to switch off. This results in the output signal at the collector to change from a low value to a high positive value (V CC ). is driven low again when the input signal drops below the value required to hold on. This on/off action converts a sinusoidal input wave into a square or rectangular output wave. 61
Prerequisite Information turn on trigger level 7V. turn off trigger level 1V. Figure 4-4 As shown, the value of input signal voltage required to switch on and that required to cause it to switch off may not be equal. The difference between the two values of trigger voltage depends on the circuit design and the intended purpose of the circuit. The difference between these two voltage levels is referred to as hysteresis voltage, or backlash. In this circuit, the setting of variable resistor has a significant effect on the hysteresis value. 62
Prerequisite Information Wiring Scheme Figure 4-5 The above photo shows average part placement and wiring scheme. Feel free to design your own and use as much space on the breadboard as needed for your layout. Component parts in your kit may be different in color or size from in the photo but should be the correct value or part number from the bill of materials. Equipment and Materials In order to complete this job sheet, you will need the following equipment: o FACET base unit o Multimeter/ Generator o Oscilloscope, dual trace o BREADBOARD MODULE o -Electrolytic, 10μF o, -NPN Transistor, o,, -Resistor, 4.7KΩ, ¼ W o, -Resistor, 1KΩ, ¼ W o -Resistor, 100Ω, ¼ W o -Potentiometer, 1KΩ, 0.7 W 63
Prerequisite Information Safety Safety rules are common sense ideas that help prevent injury. Use the following list as a reminder of standard safety rules before you begin any procedure. Rules to avoid injury should be remembered. You cause safety, it doesn t just happen. Machinery and equipment can be dangerous. Always be interested in working safety. FACET trainers have current and voltage levels that, under normal circumstances, are harmless to a normal, healthy person. The sensation of current flow through the body is called electric shock. A surprise shock can cause involuntary muscle spasms, which can result in secondary injuries. Know electricity and respect it. For additional information on the proper use of FACET equipment, refer to Appendix A Safety found at the back of this guide. 64
9V 10uF 20% 100 Figure 4-6 o 1. Connect the Schmitt trigger circuit as shown. o 2. Adjust the variable positive dc power supply so that V CC equals 9.0 Vdc. o 3. Adjust to set the voltage at the base of to 0.3V. In the following procedure steps you will measure and record the DC operating voltages of a Schmitt Trigger. 65
9V 10uF 20% 100 Figure 4-7 o 4. Measure and record the dc voltage at the collector and emitter of. V = V B1 = V E1 = Vdc Vdc Vdc o 5. Measure and record the dc voltage at the collector, base, and emitter of. V C2 = V B2 = V E2 = Vdc Vdc Vdc 66
9V 10uF 20% 100 Figure 4-8 o 6. As a result of your measurements, transistor a. is reverse biased and transistor is forward biased b. is forward biased and is transistor reversed biased. In the following procedure steps you will measure the dc hysteresis voltage associated with a typical Schmitt trigger. 67
9V CH 1 Figure 4-9 o 7. Connect channel 1 of the oscilloscope to the collector of. Oscilloscope Settings CH1 Vertical Deflection 5V/cm Horizontal 0.5ms/cm Coupling DC Trigger CH1, Internal A horizontal trace should be displayed on the screen. The upward and downward deflection of the trace will be used to indicate the switching action of the Schmitt trigger. 68
9V 10uF 20% 100 Figure 4-10 o 8. Inititally, adjust so that there is zero resistance between the base of and ground. The trace of the oscilloscope should be at zero. o 9. Slowly increase the resistance of until the circuit triggers on as indicated by the upward deflection of the oscilloscope trace. Stop increasing the instant the trigger occurs. o 10. Using the multimeter, measure and record the trigger on voltage at the base of. V B1_on = Vdc Note: You may have to repeat the measurement several times to obtain an accurate reading the instant trigger occurs. 69
9V 10uF 20% 100 Figure 4-11 o 11. Slowly decrease the resistance of until the oscilloscope trace deflects downward, indicating that the circuit has triggered off. Stop decreasing the instant the trigger occurs. o 12. Using the multimeter, measure and record the trigger off voltage at the base of. V B1_off = Vdc Note: You may have to repeat the measurement several times to obtain an accurate reading the instant trigger occurs. 70
o 13. The hysteresis voltage is the difference between the two values of trigger voltage, trigger on and trigger off. Calculate the hysteresis voltage (V hystersis ). V hystersis = V B1_on - V B1_off V hystersis = Vdc The significance of the hysteresis voltage depends on the circuit application because it affects the turn-on and turn-off characteristics of the Schmitt trigger. o 14. Remove the channel 1 oscilloscope probe from the circuit. In the following procedure steps you will apply an ac sine wave to the input of the Schmitt trigger. 71
9V CH 11 10uF V - V1 Hz 20% 100 Figure 4-12 16. Connect the AF Generator to the input capacitor. 17. While observing the signal on Channel 1 of the oscilloscope, adjust v1 for a 20 Vpk-Vpk sine wave output at a frequency of 1 khz. 1) CH1 2) CH2 1 2 Oscilloscope Settings CH1 Vertical Deflection 10V/cm CH2 Vertical Deflection 5V/cm Horizontal 500uS/cm Coupling DC Trigger CH1, Internal Mode Dual (CH1 and CH2) Figure 4-13 72
9V CH 11 10uF CH 2 V - V1 Hz 20% 100 Figure 4-14 o 18. With the oscilloscope in the Dual Mode (CH1 and CH2), connect channel 2 of the oscilloscope to the collector output of. Trigger on channel 1. 1) CH1 2) CH2 1 2 CH 2 Oscilloscope Settings Vertical Deflection 5V/cm Coupling DC Trigger CH1, Internal Figure 4-15 73
9V CH 1 10uF V - V1 Hz 20% 100 Figure 4-16 19. While observing the oscilloscope display, increase and decrease the value of. 20. Explain in detail why the output pulse width varies by varying? Name: Date: Instructor approval: 74
Review Questions Review Questions 9V Figure 4-17 1. The initial conditions of a Schmitt trigger are: a. Both and biased on. b. on and off. c. off and on. d. Both and biased off. 2. Hysteresis in a Schmitt trigger is the difference between the a. output switching times. b. on and off trigger voltage levels. c. on and off output voltage levels. d. input frequency and output frequency. 75
Review Questions 9V 10uF V - V1 Hz 20% 100 Figure 4-18 3. Which of the following statements is false? a. The Schmitt trigger is a wave squaring circuit. b. controls the circuit threshold level. c. Bias levels and stability for are,, and. d. A positive going input signal forces to switch on. 76