Experiment EB2: IC Multivibrator Circuits

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1 EEE1026 Electronics II: Experiment Instruction Learning Outcomes Experiment EB2: IC Multivibrator Circuits LO1: Explain the principles and operation of amplifiers and switching circuits LO2: Analyze high and low frequency response of amplifiers LO4: Analyze the operation of JFET, MOSFET and BJT amplifiers and switching circuits. 1.0 Objectives To measure the frequency and duty cycle of an astable 555 timer To measure the pulse width and duty cycle of a monostable 555 timer To measure the frequency and duty cycle of a voltage-controlled oscillator 2.0 Apparatus Equipment required Components required Power Supply 1 Timer IC Oscilloscope 1 Resistor 10k (1/4W) 2 Multimeter 1 Resistor 100k (1/4W) 1 Breadboard 1 Resistor 33k (1/4W) 1 Function Generator 1 Resistor 68k (1/4W) 1 Resistor 47k (1/4W) 1 Resistor 1k (1/4W) 2 Mylar Capacitor 0.01 F 4 Potentiometer (1k ) Introduction Multivibrators are circuits that are designed to have zero, one, or two stable output states. The 555 timer is one of the most popular general purpose IC multivibrators. It can be used in a variety of applications requiring accurate time delays, oscillation, and pulse conditioning. Signetics Corporation first introduced it as SE555 timer, which is an 8-pin IC that can be connected with external components for either astable or monostable operation. Figure 1 shows the simplified block diagram of a 555 timer. The circuit s name is derived from the use of an internal voltage divider between V CC and ground using three resistors. This divider chain is used to set a pair of reference voltages for two comparators that drive the set and reset inputs of an R-S flip-flop. Page: 1

2 EEE1026 Electronics II: Experiment Instruction 8 +V CC THRESHOLD CONTROL TRIGGER S R Q Q 3 OUTPUT Q 1 7 DISCHARGE 1 GROUND 4 RESET Figure 1: Block diagram of 555 Timer Refer to Figure 1, a logic high voltage (+V 0 ) applied to the set S input and a logic low (0V) to the reset R input forces the output Q to high (V CC ) and Q low (0V). This is referred to as the set condition of the flip-flop. A high reset R and low set S causes the output to switch to a low Q and a high Q. This is referred to as the reset condition of the flip-flop. The circuit latches in either of the two states. In other words, a high S input sets Q to high; a high R input resets Q to low. Output Q remains in a given state until triggered into the opposite state. The comparators are simply Op-amps. Note that the upper comparator has a threshold input (pin 6) and a control input (pin 5). In most applications, the control input is not used, so that the control voltage equals +2V CC /3. However, applying an external voltage to this pin provides some control over the reference voltages for both comparators. When the voltage of pin 6 exceeds the control voltage, the high output from the Op-amp will set the flip-flop. The high Q output from the flip-flop will turn on transistor Q 1 and discharge the external timing capacitor connected to pin 7. The complementary signal (logic low) of the flip-flop goes to pin 3, the output. When the external reset (pin 4) is grounded, it inhibits the device. This ON-OFF feature is useful sometimes. In most applications, however, the external reset is not used and pin 4 is tied directly to the supply voltage. The inverting input of the lower comparator is called the trigger (pin 2) and its noninverting input has a fixed voltage of +V CC /3 developed by the three voltage divider. When the trigger input voltage is slightly less than +V CC /3, the Op-amp output goes high and resets the flip-flop. Lastly, pin 1 is the chip ground, while pin 8 is the power supply pin. The 555 timer will work with any supply voltage between 4.5 and 16V. Monostable Operation Figure 2a shows the 555 timer connected for monostable (one-shot) operation. It produces a single, fixed voltage, output pulse each time a trigger pulse is applied to pin 2 (Figure 2b). Page: 2

3 EEE1026 Electronics II: Experiment Instruction 7 R 8 +V CC C 6 + S Q TRIGGER 2 + R Q 3 V out 1 (a) +V CC +2V CC /3 +V CC (pin 2) (pin 6 & 7) (pin 3) (b) Figure 2: (a) Monostable operation; (b) ideal waveforms The trigger input is a narrow pulse with a quiescent value of +V CC. When the trigger input is slightly less than +V CC /3, the lower Op-amp has a high output and resets the flip-flop. This cuts off the transistor, allowing the capacitor to start charging up. As the capacitor charges, the voltage at pin 6 increases. Eventually, the voltage becomes slightly greater than the control voltage (+2V CC /3). The output of the upper Op-amp then goes high, forcing the RS flip-flop output to be set. As soon as Q goes high, it turns on the transistor and this quickly discharges the capacitor. As a result, we get a triangular pulse at pin 6 & 7. The capacitor C is charged through resistance R. For a larger RC time constant, the capacitor will take longer time to charge to +2V CC /3. In other words, the RC time constant controls the width of the output pulse. Solving the exponential equation for capacitor voltage gives the formula for its pulse width as W 1. 1RC (1) Astable Operation Figure 3a shows the 555 timer connected for astable or free-running operation. The output is a square-wave signal. When Q is low, the transistor is cut off and the capacitor is charged through ( RA RB ). Because of this, the charging time constant is ( RA RB ) C. When the voltage at pin 6 is slightly greater than +2V CC /3, the upper Op-amp has a high output and this sets the flip-flop. With Q high, it turns on the transistor and grounds pin 7. Now the capacitor discharges through R B. The discharging time constant is R B C. When the capacitor voltage drops slightly below +V CC /3, the lower Op-amp has a high output and this resets the flip-flop. Page: 3

4 EEE1026 Electronics II: Experiment Instruction R A 8 +V CC 7 R B 6 C S R Q Q 3 V out 1 (a) +2V CC /3 +V CC /3 +V CC 0 W T (b) Figure 3: (a) Astable operation; (b) ideal waveforms (pin 2 & pin6) (pin 3) Figure 3b illustrates the waveforms; the timing capacitor has an exponentially rising and falling voltage and the output of Q is a rectangular wave. Since the charging time constant is longer than the discharging time constant, the output is not symmetrical; the high state lasts longer than the low state. To specify how unsymmetrical the output is, we can define duty cycle as W D 100% (2) T Depending on the resistances R A and R B, the duty cycle is between 50 and 100 percent. The mathematical solutions of the charging and discharging equations give the following formulas. The output frequency is and the duty cycle is If A 1.44 f (3) ( R 2R C A B ) RA RB D 100% (4) R 2R R is much smaller than R, the duty cycle approaches 50 percent. B A B Page: 4

5 EEE1026 Electronics II: Experiment Instruction Voltage-Controlled Oscillator (Pulse Position Modulator) The free-running multivibrator can be modified to become a voltage-controlled oscillator (VCO). Recall that pin 5 (control) is connected to the inverting input of the upper Op-amp. Normally, the control voltage is +2V CC /3 because of the internal voltage divider. In VCO, however, the voltage from an external potentiometer overrides the internal voltage. In other words, by adjusting the potentiometer, we can change the control voltage level. If we increase V control, the capacitor will take a longer time to charge and discharge; therefore, the frequency decreases. As a result, we can change the frequency of the circuit by varying the control voltage. 4.0 Procedures A. Astable 555 Timer 1. Refer to the 555 timer circuit shown in Figure 4-1. The schematic diagram does not show the op-amps, flip-flop, and other components inside the 555 timer, but only the pins and external components. 2. Notice that pin 5 (control) is bypassed to ground through a small capacitor, typically 0.01 F. This provides some noise filtering for the control voltage. 3. Based on equations (3) and (4), calculate and record the frequencies (f cal ) and duty cycles (D cal ) for the resistances listed in Table Connect the circuit of Figure 4-1 on a breadboard with R A = 10k and R B = 10k. Measure and record the supply voltage V CC(meas) with a multimeter. 5. Using an oscilloscope (set CH1 and CH2 to DC coupling and trigger source to CH1), and connect the probes at pin 3 (CH1) and pin 6 (CH2), measure the waveforms V out (at CH1) and V pin-6 (at CH2). If the circuit is functioning properly, these waveforms will be similar to those in Figure 3(b). 6. Align the ground levels of CH1 and CH2 as indicated on Graph 4-1. Adjust Volts/div and Time/div to display the waveforms on the screen as big as possible with one to two cycles. Sketch V out and V pin-6 waveforms on Graph Measure and record the period, T and the high portion of the pulse width, W. Determine the frequency f and duty cycle D from the measurement results. 8. Repeat steps 5 through 7 for the other resistances of Table For R A = 100 k and R B = 10 k case, measure and record the maximum and minimum voltage levels of V out and V pin 6 waveforms. 10. Ask the instructor to check all of your results. You must show the last oscilloscope waveforms to the instructor. R A R B Timer 3 5 V out V CC =+5V C 1 =0.01 F 2 1 C 2 =0.01 F 4 5 SE/NE555 Figure 4-1: Astable 555 Timer Circuit (TOP VIEW) Page: 5

6 EEE1026 Electronics II: Experiment Instruction B. Monostable 555 Timer *Do not remove the circuit from the previous part. The connection of IC U1 is similar with Part A. 1. IC U2 of Figure 4-2 is a 555 timer connected for monostable operation. Calculate the pulse width for each resistance value R listed in Table 4-2. Record the results under W cal. 2. IC U1 of Figure 4-2 is the astable multivibrator circuit of Part A. It is used here to provide the trigger input to the monostable circuit (U2). 3. Connect the circuit of Figure 4-2 with resistance R = 33k. Measure and record V CC(meas). 4. Using an oscilloscope, measure the waveforms at pin 2, V pin2 (at CH1) and pin 3, V out (at CH2) of the monostable circuit (U2). Set CH1 and CH2 to DC input coupling and trigger source to CH1. Align the ground levels of CH1 and CH2 as indicated on Graph 4-4. Set Time/div to display the waveforms with one to two cycles on the screen. Set Volt/div to display the waveforms as big as possible but not overlapping. Sketch the waveforms. 5. Measure and record the pulse width, W, at the output of U2. 6. Repeat steps 4 to 5 for the other resistances R in Table For R = 68 k case, connect CH2 to pin 6, measure and record the maximum and minimum voltage levels of pin 6 waveform. 8. Ask the instructor to check all of your results. You must show the last oscilloscope waveforms to the instructor. V CC =+5V 100k 10k U1 555 Timer 3 5 R U2 555 Timer 3 5 V out 0.01 F 0.01 F F 0.01 F 2 1 Figure 4-2: Monostable 555 Timer Circuit Page: 6

7 EEE1026 Electronics II: Experiment Instruction C. Voltage-Controlled Oscillator 1. Connect the voltage-controlled oscillator (VCO) of Figure 4-3 (refer to Appendix A for the potentiometer legs). Measure and record V CC(meas). 2. Measure the output, V out (CH1) and pin 6, V pin 6 (CH2) waveforms with an oscilloscope. Set CH1 and CH2 to DC input coupling and trigger source to CH1. Align the channel ground levels as indicated on Graph 4-5. Adjust Volts/div to display the waveforms on the screen as big as possible. 3. Vary the 1-k potentiometer and notice the changes in the waveforms. Adjust Time/div if necessary. 4. Turn the potentiometer to get the minimum frequency. Adjust Time/div to display the waveforms with one to two cycles on the screen. Sketch V out and V pin 6 waveforms. 5. Measure and record T, W, V pin 6 (max) and V pin 6 (min). Measure and record the DC voltage at pin 5, V pin 5 with a multimeter. Calculate the frequency and duty cycle. 6. Turn the potentiometer to get the maximum frequency. Repeat steps 4 and Ask the instructor to check all of your results. You must show the last oscilloscope waveforms to the instructor k k Timer C 1 =0.01 F V out Var V CC =+5V 1k A 1k B 1k Figure 4-3: Voltage-Controlled Oscillator Circuit Report Submission You must obtain the signature of the Instructor after completing each section of the experiment. Submit your report to the Lab Supervisor, latest by 5.00pm the next day, after the experiment. Page: 7

Then, the reading 10 is located at x mm and the reading 0.1 is located at x mm. For a reading F, it is located at [1og 10 (F)]*x mm. E.g.: Reading 0.25 is located at [1og 10 (0.25)]*x mm = -0.

8 EEE1026 Electronics II: Experiment Instruction APPENDIX A The Resistor color code chart Capacitance ABC.abc AB x 10 C pf 0.abc F Potentiometer A Var B Log Scale The distance in a decade of the log scale in the figure below is x mm. Since log 10 1 = 0, it is used as a refernce point (0 mm) in the linear scale. Then, the reading 10 is located at x mm and the reading 0.1 is located at x mm. For a reading F, it is located at [1og 10 (F)]*x mm. E.g.: Reading 0.25 is located at [1og 10 (0.25)]*x mm = x mm Reading 2.5 is loacted at [1og 10 (2.5)]*x mm = 0.398x mm Reading 25 is located at [1og 10 (25)]*x mm = 1.398x mm (not shown in the figure) Reading 250 is located at [1og 10 (250)]*x mm = 2.398x mm (not shown) z / x Conversely, a point at z mm location is read as 10. E.g.: -0.3x mm is read as 10 (-0.3x/x) = x mm is read as 10 (0.6x/x) = x mm is read as 10 (1.5x/x) = 31.6 (not shown) 2.7x mm is read as 10 (2.7x/x) = 501 (not shown) -0.3x -x x 0.6x x x Linear scale (mm) Log scale (unit) Page: 8

1 F Vertically connected 8 7 6 5 555 Vertically connected 1 2 3 4 0V GND Internal connections

9 EEE1026 Electronics II Experiment EB2 Appendix B: Breadboard Internal Connections Horizontally connected Internal connections Horizontally connected +V CC 0.1 F Vertically connected Vertically connected V GND Internal connections Multimedia University General mistakes: The legs of the resistors and the transistor are shorted by the breadboard internal connections. FOE Page: 9

10 EEE1026 Electronics II Appendix C Experiment EB2: IC Multivibrator Circuits Lab Report (Submit a complete report a day after the experiment) Name: Student I.D.: Date: Majoring: Group: Table No.: A. Astable 555 Timer V CC(meas) = V [1 mark] Table 4-1: Astable Operation for various R A and R B R A (k ) R B (k ) For R A = 100 k, R B = 10 k case (Step 9): V out (max) = V V out (min) = V V pin 6 (max) = V V pin 6 (min) = V f cal D cal T W f D V CC(meas) V out (max) = V V pin 6 (max) / V CC(meas) = V pin 6 (min) / V CC(meas) = Graph 4-1: Astable Operation for R A = 10 k, R B = 10 k [3 marks] [7 marks] Time base : s/div, CH1 (V out ) : V/div, CH2 (V pin 6 ) : V/div CH1 & CH2 ground [5 marks]

11 EEN1026 Electronics II Experiment EB2 Graph 4-2: Astable Operation for R A = 10 k, R B = 100 k Time base : s/div, CH1 (V out ) : V/div, CH2 (V pin 6 ) : V/div CH1 & CH2 ground Graph 4-3: Astable Operation for R A = 100 k, R B = 10 k Time base : s/div, CH1 (V out ) : V/div, CH2 (V pin 6 ) : V/div [5 marks] CH1 & CH2 ground [5 marks] * Note: Ask your instructor to verify your results before you proceed to Part B. Signature: Time: Remarks Page: 2

12 EEN1026 Electronics II Experiment EB2 B. Monostable 555 Timer V CC(meas) = V Table 4-2: Monostable Operation for various R R (k ) W cal W [1 mark] [3 marks] For R = 68 k case (Step 7): V pin 6 (max) = V V pin 6 (min) = V V pin 6 (max) / V CC(meas) = [3 marks] Graph 4-4: Monostable Operation for R = 33 k Time base : s/div, CH1 (V pin 2 ) : V/div, CH2 (V out ) : V/div CH2 ground CH1 ground [5 marks] * Note: Ask your instructor to verify your results before you proceed to Part C. Signature: Time: Remarks Page: 3

13 EEN1026 Electronics II Experiment EB2 C. Voltage-Controlled Oscillator V CC(meas) = V [1 mark] Graph 4-5: Voltage-Controlled Oscillator at minimum frequency Time base : s/div, CH1 (V out ) : V/div, CH2 (V pin 6 ) : V/div T W = s = s V pin 6 (max) = V V pin 6 (min) = V V pin 5 f D = V = Hz = % CH1 & CH2 ground [5 + 7 marks] Voltage-Controlled Oscillator at maximum frequency T = s W = s V pin 6 (max) = V V pin 6 (min) = V V pin 5 f D = V = Hz = % [7 marks] * Note: Ask your instructor to verify your results. Signature: Time: Remarks Page: 4

14 EEN1026 Electronics II Experiment EB2 Discussion A. Astable 555 Timer 1. Compare the calculated values ( f cal, D cal ) and the measured values (f, D). Justify their difference 2. Identify how the voltages V out and V pin 6 are related in the three graphs. 3. Compare between voltages V CC(meas) and V out (max) and explain their difference. 4. Evaluate how W, f and D changes when R A and/or R B are varied. Propose the expected minimum and maximum duty cycle values. [8 marks] B. Monostable 555 Timer 1. Identify how the voltages V out and V pin 2 of the monostable circuit (U2) are related. 2. Describe how W changes when R is varied. [4 marks] Page: 5

15 EEN1026 Electronics II Experiment EB2 C. Voltage-Controlled Oscillator 1. With the help of Figure 1 and Figure 3, compare and evaluate the relationships between voltages V pin 6 (max) and V pin 5, as well as V pin 6 (min) and V pin 5. Include numerical calculations in your answer. 2. Describe how W, f and D changes when the value of V pin 5 is varied. 3. What is the voltage at pin 6 before the output of a monostable 555 timer changes from ON state to OFF state? Why? [7 marks] Conclusion [3 marks] Page: 6

16 EEE1026 Electronics II Student's Name: ID No: Subject Code and Title: EEE Electronics 2 Experiment Title: EB2 IC Multivibrator Circuits Experiment Date: Criteria Ability in constructing the IC Multivibrator Circuits: Monostable, Astable and Voltage-Controlled Oscillator 2 Ability in performing data collection using lab equipment such as DC power supply, digital multimeter, oscilloscope and function generator 3 Ability to determine and draw the time domain waveform for input and output voltages of the amplifier circuits 4 The ability to present results and summarise final outcomes which answers the objectives of the lab 5 Ability to answer the questions in by Oral Assessment Unable to demonstrate proper steps to construct the IC Multivibrator Circuits and not asking for help Unable to record data, and no effort is shown Time: Constructing the circuits and performing data collection using lab equipment Able to demonstrate Able to demonstrate good basic steps required to knowledge on the construct the IC construction of the IC Multivibrator Circuits Multivibrator Circuits with some help with minimum help Not able to determine, and draw the time domain waveform for input and output voltages (no effort was also shown) Unable to present results clearly and no attempt was made to summarise final outcomes Not able to answer the question, no attempt was made to answer Able to provide adequate data, and show some efforts in getting the data Able to at least determine and draw the time domain waveform for input and output voltages and show some efforts in the calculation Conclusions Able to present results and summarise adequate final outcomes and reasonably relating them to the objectives Able to answer questions with some basics answers and demonstrate some attempts to refer to the text books, notes, lab sheet Able to record most of the data correctly Able to determine and draw the time domain waveform for input and output voltagesand solve the calculations partiall Able to present results and summarise mostly to the final outcomes and answered most of the objectives of the lab Able to answer most part of the questions, with some explanations and elaborations and demonstrate some attempts to refer to text books, notes or lab sheet Able to demonstrate full knowledge on the construction of the IC Multivibrator Circuits without help Recorded all data neatly and correctly Completed all the drawing and calculations correctly Able to present results very clearly and excellent summary of final outcomes which answer the objectives of the lab Answered all correctly with proper explanations and elaborations, without a need to refer to any references. Rating Awarded by Assessor

Appendix D2 Experiment EB2: IC Multivibrator Circuits. Lab Report (Submit your report on the same day immediately after the experiment)

Appendix D2 Experiment EB2: IC Multivibrator Circuits. Lab Report (Submit your report on the same day immediately after the experiment) EEE1026 Electronics II Appendix D2 : IC Multivibrator Circuits Lab Report (Submit your report on the same day immediately after the experiment) Name: Student I.D.: Date: Majoring: Group: Table No.: 4.