Multivibrator VII

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CHAPTER VII

MULTIVIBRATOR

DCC INSTITUTE


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MULTIVIBRATOR

A multivibrator is an electronic circuit used to implement a

variety of simple two-state systems such as oscillators, timersand flip-flops.

There are three types of multivibrator circuit:

Astable, in which the circuit is not stable in either stateit

continuously oscillates from one state to the other. Due to this, it

does not require a input (Clock pulse or other). Monostable, in which one of the states is stable, but the other is

notthe circuit will flip into the unstable state for a determined

period, but will eventually return to the stable state. Such a circuit is

useful for creating a timing period of fixed duration in response to

some external event. This circuit is also known as a one shot. A

common application is in eliminating switch bounce.

Bistable, in which the circuit will remain in either state indefinitely.

The circuit can be flipped from one state to the other by an external

event or trigger. Such a circuit is important as the fundamental

building block of a registerormemory device. This circuit is also

known as a latch or a flip-flop.
http://en.wikipedia.org/wiki/Oscillatorshttp://en.wikipedia.org/wiki/Timerhttp://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Processor_registerhttp://en.wikipedia.org/wiki/Computer_storagehttp://en.wikipedia.org/wiki/Latch_(electronics)http://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Latch_(electronics)http://en.wikipedia.org/wiki/Computer_storagehttp://en.wikipedia.org/wiki/Processor_registerhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Flip-flop_(electronics)http://en.wikipedia.org/wiki/Timerhttp://en.wikipedia.org/wiki/Oscillators


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555 TIMER IC

The 555 Timer IC is an integrated circuit (chip)

implementing a variety oftimerand multivibratorapplications.

It has been claimed that the 555 gets its name from

the three 5 k resistors used in typical early

implementations.
http://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/Timerhttp://en.wikipedia.org/wiki/Multivibratorhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Multivibratorhttp://en.wikipedia.org/wiki/Timerhttp://en.wikipedia.org/wiki/Integrated_circuit


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555 IC INTERNAL DIAGRAM


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Pin1: Ground. All voltages are measured with

respect to this terminal. Pin2:Trigger. The output of the timer depends on

the amplitude of the external trigger pulse applied

to this pin. The output is low if the voltage at this pin

is greater than 2/3 VCC. When a negative goingpulse of amplitude greater than 1/3 VCC is applied tothis pin, comparator 2 output goes low, which in

turn switches the output of the timer high. The

output remains high as long as the trigger terminal

is held at a low voltage.

555 TIMER IC


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Pin3:Output. There are two ways by which a load canbe connected to the output terminal: either between

pin 3 and ground or between pin3 and supply voltage+VCC.

Pin4: Reset. The 555 timer can be reset (disabled) byapplying a negative pulse to this pin. When the reset

function is not in use, the reset terminal should beconnected to +VCC to avoid any possibility of falsetriggering.

Pin5: Control Voltage. An external voltage applied tothis terminal changes the threshold as well as trigger

voltage. Thus by imposing a voltage on this pin or byconnecting a potbetween this pin and ground, thepulse width of the output waveform can be varied.When not used, the control pin should be bypassed toground with a 0.01F Capacitor to prevent any noise

problems.

555 TIMER IC


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Pin6: Threshold. This is the non-inverting input ofcomparator 1, which monitors the voltage across the

external capacitor. When the voltage at this pin is

greater than or equal to the threshold voltage 2/3 VCC,the output of comparator 1 goes high, which in turn

switches the output of the timer low.

Pin7: Discharge. This pin is connected internally tothe collector of transistor Q1. When the output is high

Q1 is OFF and acts as an open circuit to external

capacitor C connected across it. On the other hand,

when the output is low, Q1 is saturated and acts as ashort circuit, shorting out the external capacitor C to

ground.

Pin8: +VCC. The supply voltage of +5V to + 18V is

applied to this pin with respect to ground.

555 TIMER IC


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555 IC AS

MONOSTABLE MULTIVIBRATOR


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WORKING - MONOSTABLE This circuit diagram shows how a 555 timer IC is configured to

function as an monostable multivibrator.

Initially when the circuit is in the stable state i.e., when the output islow, transistor Q1 is ON and the capacitor C is shorted out to

ground.

Upon the application of a negative trigger pulse to pin 2, transistor

Q1 is turned OFF, which releases the short circuit across the

external capacitor C and drives the output high.

The capacitor C now starts charging up towards VCC through R.

When the voltage across the capacitor equals 2/3 VCC, comparator

1s output switches from low to high, which in turn drives the output

to its low state via the output of the flip-flop. At the same time the output of the flip-flop turns transistor Q1 ON

and hence the capacitor C rapidly discharges through the

transistor.

The output of the monostable remains low until a trigger pulse is

again applied. Then the cycle repeats.
http://ecelab.com/555-timer.htmhttp://ecelab.com/555-timer.htm


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The time during which the output remains high is

given by

where R is in Ohms and C is in Farads.

Once triggered, the circuits output will remain in thehigh state until the set time, telapses.

The output will not change its state even if an input

trigger is applied again during this time interval t.

The circuit can be reset during the timing cycle byapplying negative pulse to the reset terminal.

The output will remain in the low state until a trigger

is again applied.

WORKING - MONOSTABLE


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555 IC AS

ASTABLE MULTIVIBRATOR


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WORKING - ASTABLE

This circuit diagram shows how a 555 timer IC is configured

to function as an astable multivibrator.An astable multivibrator is a timing circuit whose 'low' and

'high' states are both unstable.

As such, the output of an astable multivibrator togglesbetween 'low' and 'high' continuously, in effect generating a

train of pulses. This circuit is therefore also known as a'pulse generator' circuit.

In this circuit, capacitor C1 charges through R1 and R2,eventually building up enough voltage to trigger an internalcomparator to toggle the output flip-flop.

Once toggled, the flip-flop discharges C1 through R2 into pin7, which is the discharge pin.

When C1's voltage becomes low enough, another internalcomparator is triggered to toggle the output flip-flop.

This once again allows C1 to charge up through R1 and R2and the cycle starts all over again.
http://ecelab.com/555-timer.htmhttp://ecelab.com/555-timer.htm


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WORKING - ASTABLE

C1's charge-up time t1 is given by:t1 = 0.693(R1+R2)C1.

C1's discharge time t2 is given by:

t2 = 0.693(R2)C1.

Thus, the total period of one cycle ist1+t2 = 0.693 C1(R1+2R2).

The frequency f of the output wave is the reciprocal of

this period, and is therefore given by:

f= 1.44/(C1(R1+2R2)),

wherein f is in Hz if R1 and R2 are in mega ohms and

C1 is in microfarads.

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Multivibrator VII