# Multivibrators and the 555 Timer

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Multivibrators and the 555 Timer
Chapter 14 Multivibrators and the 555 Timer 1

Objectives You should be able to:
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. Connect IC monostable multivibrators to output a waveform with a specific pulse width. 2

Objectives (Continued)
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. Discuss the operation and application of crystal oscillator circuits. 3

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

Capacitor Charge and Discharge
RC circuit charge and discharge curves 5

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

Capacitor Charge and Discharge
Transposing the curve equation for t 7

Astable Multivibrators
Single Schmitt Inverter and an RC circuit 8

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

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

Monostable Multivibrators
Built from NAND gates and RC circuit 11

IC Monostable Multivibrators
74121 Nonretriggerable Connect RC components for proper pulse width Two active-LOW trigger inputs One active-HIGH trigger input 12

IC Monostable Multivibrators
74121 block diagram and function table 13

Retriggerable Monostable Multivibrators
74123 new timing cycle each time new trigger applied 14

Retriggerable Monostable Multivibrators
74123 logic symbol and function table 15

Retriggerable Monostable Multivibrators
74123 Dual multivibrator Active low reset (RD) which forces Q low No internal timing resistor Pulse width is determined using: If Cext is greater than or equal to 1000 pF, the graph in Figure can be used to select components. 16

Capacitor Selection Curves (Figure 14-18)
17

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

555 IC Timer Block Diagram 19

Discussion Point 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) 20

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

555 Timer Astable Multivibrator
22

555 Timer Monostable Multivibrator
23

555 Timer Monostable Multivibrator Waveforms
24

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

Crystal-Controlled Oscillators
25

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. Capacitor voltage charging and discharging rates are the most common way to produce predictable time duration for oscillator and timing operations. 27

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. 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

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. Retriggerable monostable multivibrators allow multiple input triggers to be acknowledged even if the output pulse from the previous trigger had not expired. 29

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. Crystal oscillators are much more accurate and stable than RC timing circuits. They are used most often for microprocessor and digital communication timing. 30