Multivibrator

MultivibratorAn electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops.

Note:• Astable

Not stable in either state, switch from one state to other state continuously

• MonostableWhich one of state s is stable , but other state is unstable

• BistableStable in either state, triggered by two external trigger

IC 555

Control Voltage (5)

Threshold Voltage (6)

Trigger Voltage (2)

Ground (1)

Vcc (8) Discharge (7)

Reset (4)

Output (3)

-+

-+

RESET

SET

Q

Q

COMP1

COMP2

Flip-Flop T1

5 Volts

N/C

N/C

Discharge

Threshold /Trigger

Ground

Output1.666 V

3.333 VRA

RB

C

Schematic of 555

555 Timer Design Equations

5

tHIGH : Calculations for the Oscillator’s HIGH Time

CRR0.693 BA HIGHt

5v

3.333 v

Vc1.666 v

0 v

tHIGH

Output

HIGH

LOW

THE OUTPUT IS HIGH WHILE THE CAPACITOR IS CHARGING THROUGH RA + RB.

555 Timer Design Equations

6 C0.693RBLOWt

tLOW : Calculations for the Oscillator’s LOW Time

5v

3.333 v

Vc1.666 v

0 v

tLOW

Output

HIGH

LOW

THE OUTPUT IS LOW WHILE THE CAPACITOR IS DISCHARGING THROUGH RB.

555 Timer – Period / Frequency / DC

7

C R2R 693.0T

CR 693.0C RR 693.0TttT

CR 693.0tC RR 693.0t

BA

BBA

LOWHIGH

BLOW

BAHIGH

Period:

C R2R 693.01F

T1F

BA

Frequency:

%100 R2R

RRDC

%100C R2R 693.0C RR 693.0DC

%100TtDC

BA

BA

BA

BA

HIGH

Duty Cycle:

Detail Analysis of a 555 Oscillator

8

5v

3.333 v

Vc1.666 v

0 v

RESET HIGHLOW

SET HIGHLOW

HIGHLOW

T1 ONOFF

Q HIGHLOW

Q

9

5v

3.333 v

Vc1.666 v

0 v

RESET HIGHLOW

SET HIGHLOW

HIGHLOW

T1 ONOFF

Q HIGHLOW

Q

Detail Analysis of a 555 Oscillator

10

5v

3.333 v

Vc1.666 v

0 v

RESET HIGHLOW

SET HIGHLOW

HIGHLOW

T1 ONOFF

Q HIGHLOW

Q

Detail Analysis of a 555 Oscillator

11

5v

3.333 v

Vc1.666 v

0 v

RESET HIGHLOW

SET HIGHLOW

HIGHLOW

T1 ONOFF

Q HIGHLOW

Q

OUTPUT IS LOW WHILE THE CAPACITOR IS DISCHARGING THROUGH RB.

OUTPUT IS HIGH WHILE THE CAPACITOR IS CHARGING THROUGH RA + RB.

Detail Analysis of a 555 Oscillator

Improved 555 Duty Cycle

By connecting this diode, D1 between the trigger input and the discharge input, the timing capacitor will now charge up directly through resistor R1 only, as resistor R2 is effectively shorted out by the diode. The capacitor discharges as normal through resistor, R2.

Now the previous charging time of t1 = 0.693(R1 + R2)C is modified to take account of this new charging circuit and is given as: 0.693(R1 x C). The duty cycle is therefore given as D = R1/(R1 + R2). Then to generate a duty cycle of less than 50%, resistor R1 needs to be less than resistor R2.

Multivibrator Circuit using 555

Circuit Diagram

74HC14 Hex-Inverter Schmitt-Trigger

R = 10kC = 0.1 uF

Circuit Diagram

C = 560 pFRA and RB • RA > RB• RA = RB• RA < RB• With dioda 1N 4148 (improve duty cycle