Students should be able to : Objective Students should be able to : 1 Describe the characteristics of the 3 basic types of multivibrator. Astable (Free-running) Monostable (One-shot) Bistable (Flip-flop) 2 Explain the function of IC 555 in a timer circuit. 3 Identify the following pin functions of the IC 555. 4 Describe the circuit operation and applications of a monostable multivibrator. 5 Calculate the mark and space timing of the astable multivibrator. 6 Calculate the oscillator frequency using the formula: 1.44 (R1 + 2R2)C fosc = 7 Calculate the time delay of a monostable using the formula t = 1.1 RC

Multivibrator A multivibrator is a circuit whose outputs are bistate. The output may be either a Low or High. Multivibrators can be divided into three groups: - Astable multivibrator (Free-running) - Monostable multivibrator (One shot) - Bistable multivibrator (Flip-Flop)

Types of Multivibrator Astable Monostable Bistable Click to select Main Menu

555 pin connection 1 8 2 7 3 6 4 5 Trigger (Pin 2) Threshold (Pin 6) Control voltage (Pin 5) Discharge (Pin 7) Output (Pin 3) Reset (Pin 4) R 2 7 Q1 R S FF 3 6 R R 4 5 Q2 Ref

TIMER 555 1 2 Internal circuitry of a 555 in the monostable mode Vcc OUTPUT 1 + _ 1 2 3 S Q R 2 3 INPUT 1 3 1 + _ 2 Internal circuitry of a 555 in the monostable mode When power is applied initially Comparator 2 output is driven LOW by positive HIGH at the trigger input. Comparator 1 output is driven HIGH by the voltage charged across the external capacitor. S = 1 , R = 0, Q = 1 and output is LOW.

1 2 Vcc S Q R When the transistor is ON OUTPUT 1 + _ 1 2 S Q R 2 3 INPUT 1 3 1 + _ 2 When the transistor is ON This set the flip-flop, Q=1 and the transistor is ON. The capacitor then discharges to the ground through the transistor and causes the output of comparator 1 to go LOW. The flip-flop will now remain set because both inputs are LOW (unchanged state). The output will maintain at this stable state until a LOW trigger inputs is received.

1 2 Vcc S Q R When a Low trigger input is applied at pin 2 OUTPUT 1 + _ 1 2 3 S Q R 2 3 INPUT 1 3 1 + _ 2 When a Low trigger input is applied at pin 2 Inverting input of comparator 2 from pin 2 will go LOW. This is lower than the non inverting input which is 1/3Vcc. The output of comparator 2 is driven High. S = 0, R = 1, Q = 0, and it turns OFF the transistor. The circuit is now in its unstable state, output is logic ‘1’.

1 2 Vcc S Q R When the transistor is OFF OUTPUT 1 + _ 1 2 3 S Q R 2 3 INPUT 1 3 1 + _ 2 When the transistor is OFF Input has returned to HIGH, Logic “1” . The external capacitor will charge through resistor R. When the voltage across the capacitor exceeds 2/3Vcc, the output of comparator 1 is driven High. Therefore S = 1, R = 0 and this sets the flip-flop. Q = 1. The circuit returns to its stable state, output is logic ‘0’.

The Astable Multivibrator C 6 2 1 5 3 8 7 555 Output Vcc t1 t2 t1 = 0.693 ( R1 + R2 )C t2 = 0.693 R2C Formula for calculating the oscillator frequency : 1.44 (R1 + 2R2)C fosc =

= 0.693 (4 K + 100K) 0.001µf Output t2 t1 = 0.693 (100K) 0.001µf Example : To calculate the Mark , Space and the frequency of oscillations of the output waveform. Given that C = 0.001µf, R1 = 4 K and R2 = 100K. Solution : Mark, t1 = 0.693 (R1 + R2)C = 0.693 (4 K + 100K) 0.001µf = 0.693 (4 000 + 100 000) 0.001 x 10-6 = 0.000 0721s = 72.1µs Vcc Output t1 t2 Space, t2 = 0.693 R2C = 0.693 (100K) 0.001µf = 0.693 (100 000) 0.001 x 10-6 = 0.000 0693s = 69.3µs fosc Frequency, fosc = = = 7059Hz )C 2R (R 1.44 2 1 + ) 0.001 x 10-6 200 000 (4000

Monostable Multivibrator Example : To calculate the duration of the quasi-stable state of the monostable multivibrator. Given that C = 0.001µf and R = 10K. Solution : Monostable Multivibrator Circuit Input Output t T = 1.1 RC = 1.1(10 K x 0.001µf ) = 1.1(10 x 103 ) ( 0.001 x 10-6f ) = 0.000 011s = 11µs

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The Astable Multivibrator It is a free-running oscillator. It output a rectangular wave and does not requires any trigger input. • Application as oscillators, square-wave generators etc. Output Input Astable Multivibrator Circuit None 4

The Astable Multivibrator It is a free-running oscillator. It output a rectangular wave and does not requires any trigger input. • Application as oscillators, square-wave generators etc. Output Input Astable Multivibrator Circuit None 4

The Astable Multivibrator It is a free-running oscillator. It output a rectangular wave and does not requires any trigger input. • Application as oscillators, square-wave generators etc. Output Input Astable Multivibrator Circuit None Click to return to Main menu 4

Monostable Multivibrator It is also known as one-shot multivibrator. Is stable in only one of two voltage levels. When a trigger is received it switches to the other state for a period of time (t ) and then returns to the stable state. Application as a timer. Input Monostable Multivibrator Circuit Output t Click to return to Main menu 5

Bistable Multivibrator It is generally known as flip-flop. It has two inputs. It has two stables output states and will stay in either one indefinitely. When a trigger input is received it will switch to the other state and remain in that state until the next trigger input is received. Application as a digital storage register. Input Bistable Multivibrator Circuit Output T1 T1 6

Bistable Multivibrator It is generally known as flip-flop. It has two inputs. It has two stables output states and will stay in either one indefinitely. When a trigger input is received it will switch to the other state and remain in that state until the next trigger input is received. Application as a digital storage register. Input Bistable Multivibrator Circuit Output T2 T2 Click to return to Main menu 6