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Chapter 14 Multivibrators and the 555 Timer 1. Objectives You should be able to: Calculate capacitor charging and discharging rates in series RC timing.

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Presentation on theme: "Chapter 14 Multivibrators and the 555 Timer 1. Objectives You should be able to: Calculate capacitor charging and discharging rates in series RC timing."— Presentation transcript:

1 Chapter 14 Multivibrators and the 555 Timer 1

2 Objectives You should be able to: Calculate capacitor charging and discharging rates in series RC timing circuits. Calculate capacitor charging and discharging rates in series RC timing circuits. Sketch the waveforms and calculator voltage and time values for astable and monostable multivibrators. Sketch the waveforms and calculator voltage and time values for astable and monostable multivibrators. Connect IC monostable multivibrators to output a waveform with a specific pulse width. Connect IC monostable multivibrators to output a waveform with a specific pulse width. 2

3 Objectives (Continued) Explain the operation of the internal components of the 555 IC timer. Explain the operation of the internal components of the 555 IC timer. Connect a 555 IC timer as an astable multivibrator and as a monostable multivibrator. Connect a 555 IC timer as an astable multivibrator and as a monostable multivibrator. Discuss the operation and application of crystal oscillator circuits. Discuss the operation and application of crystal oscillator circuits. 3

4 Multivibrators Changes between two digital levels Changes between two digital levels Continuous, free-running Continuous, free-running On demand On demand Three types Three types Bistable (S-R flip-flop) Bistable (S-R flip-flop) Astable Astable Monostable (one shot) Monostable (one shot) 4

5 Capacitor Charge and Discharge RC circuit charge and discharge curves RC circuit charge and discharge curves 5

6 Capacitor Charge and Discharge Exponential charge and discharge Exponential charge and discharge Time constant (  ) Time constant (  ) 6

7 Capacitor Charge and Discharge Transposing the curve equation for t Transposing the curve equation for t 7

8 Astable Multivibrators Single Schmitt Inverter and an RC circuit Single Schmitt Inverter and an RC circuit 8

9 Discussion Point Describe the operation of an astable multivibrator similar to that shown in Figure 14-5 Describe the operation of an astable multivibrator similar to that shown in Figure 14-5 How is the frequency of the multivibrator controlled? How is the frequency of the multivibrator controlled? 9

10 Monostable Multivibrators Also called a one shot Also called a one shot Block diagram and waveforms Block diagram and waveforms 10

11 Monostable Multivibrators Built from NAND gates and RC circuit Built from NAND gates and RC circuit 11

12 IC Monostable Multivibrators Nonretriggerable Nonretriggerable Connect RC components for proper pulse width Connect RC components for proper pulse width Two active-LOW trigger inputs Two active-LOW trigger inputs One active-HIGH trigger input One active-HIGH trigger input 12

13 IC Monostable Multivibrators block diagram and function table block diagram and function table 13

14 Retriggerable Monostable Multivibrators new timing cycle each time new trigger applied new timing cycle each time new trigger applied 14

15 Retriggerable Monostable Multivibrators logic symbol and function table logic symbol and function table 15

16 Retriggerable Monostable Multivibrators Dual multivibrator Dual multivibrator Active low reset (R D ) which forces Q low Active low reset (R D ) which forces Q low No internal timing resistor No internal timing resistor Pulse width is determined using: Pulse width is determined using: If C ext is greater than or equal to 1000 pF, the graph in Figure can be used to select components. If C ext is greater than or equal to 1000 pF, the graph in Figure can be used to select components. 16

17 17 Capacitor Selection Curves (Figure 14-18)

18 Astable Operation of the 555 IC Timer One shot or astable oscillator One shot or astable oscillator Voltage divider Voltage divider Comparators Comparators S-R flip-flop S-R flip-flop Discharge transistor Discharge transistor 18

19 IC Timer Block Diagram

20 Discussion Point Describe the operation and function of the 555 pins. Describe the operation and function of the 555 pins. Describe the operation of the 555 connected in astable mode (as in text Figure 14-20) Describe the operation of the 555 connected in astable mode (as in text Figure 14-20) 20

21 Astable Operation of the 555 IC Timer 50% Duty Cycle Astable Oscillator 50% Duty Cycle Astable Oscillator R A cannot = 0 ohms R A cannot = 0 ohms R A = R B and short R B with a diode R A = R B and short R B with a diode 21

22 Timer Astable Multivibrator

23 555 Timer Monostable Multivibrator 23

24 Timer Monostable Multivibrator Waveforms

25 Crystal Oscillators Quartz crystal Quartz crystal Size and shape determine specific frequency Size and shape determine specific frequency Accurate to more than five significant digits Accurate to more than five significant digits Integrated circuit packages or use external quartz crystal Integrated circuit packages or use external quartz crystal 74S124 74S124 Voltage controlled oscillator Voltage controlled oscillator 25

26 Crystal-Controlled Oscillators 25

27 Summary Multivibrator circuits are used to produce free- running clock oscillator waveforms or to produce a timed digital level change triggered by an external source. Multivibrator circuits are used to produce free- running clock oscillator waveforms or to produce a timed digital level change triggered by an external source. Capacitor voltage charging and discharging rates are the most common way to produce predictable time duration for oscillator and timing operations. Capacitor voltage charging and discharging rates are the most common way to produce predictable time duration for oscillator and timing operations. 27

28 Summary An astable multivibrator is a free-running oscillator whose output oscillates between two voltage levels at a rate determined by an attached RC circuit. An astable multivibrator is a free-running oscillator whose output oscillates between two voltage levels at a rate determined by an attached RC circuit. A monostable multivibrator is used to produce an output pulse that starts when the circuit receives an input trigger and lasts for a length of time dictated by the attached RC circuit. A monostable multivibrator is used to produce an output pulse that starts when the circuit receives an input trigger and lasts for a length of time dictated by the attached RC circuit. 28

29 Summary The is an IC monostable multivibrator with two active-LOW and one active-HIGH input trigger sources and an active-HIGH and an active-LOW pulse output terminal. The is an IC monostable multivibrator with two active-LOW and one active-HIGH input trigger sources and an active-HIGH and an active-LOW pulse output terminal. Retriggerable monostable multivibrators allow multiple input triggers to be acknowledged even if the output pulse from the previous trigger had not expired. Retriggerable monostable multivibrators allow multiple input triggers to be acknowledged even if the output pulse from the previous trigger had not expired. 29

30 Summary The 555 IC is a general-purpose timer that can be used to make astable and monostable multivibrators and perform any number of other timing functions. The 555 IC is a general-purpose timer that can be used to make astable and monostable multivibrators and perform any number of other timing functions. Crystal oscillators are much more accurate and stable than RC timing circuits. They are used most often for microprocessor and digital communication timing. Crystal oscillators are much more accurate and stable than RC timing circuits. They are used most often for microprocessor and digital communication timing. 30


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