see 555 Timer IC xx

EE 503, SP

555 Timer IC

EE 503, SP

Widely used IC

It can run in:

Monostable

mode

(one stable state) -

it can

produce accurate time delays from micro-secs

to hours

Astable

mode

( no stable states)

it can produce

Rectangular waves with variable duty cycle

Bistable

Mode

(two stable states)

And lots of other circuits

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Pin Configuration

EE 503, SP

Functional Block Diagram

Upper comparator

Lower comparatorDischarge Transistor

Reset Transistor

_Q

Q

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555 IC comprises of :

Two op-amp comparators

A flip-flop

A discharge transistor

A reset transistor

Three identical resistors (e.g

5 k)

Output stage

The resistors set the reference voltage

levels at the non-inverting I/P of the lower comparator & inverting I/P of the upper comparator at +Vcc/3

& +2Vcc/3

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when output Q bar is high, the discharge transistor is off (as Q low)

When output low, discharge transistor On

Pin 6 Voltage (threshold) > 2Vcc/3 : O/P of upper comparator high, S=1, Q=1, discharge transistor On

Pin 2 voltage (trigger) < Vcc/3 : O/P of the lower comparator high, R =1, Q=0, discharge transistor off

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Main Features1.

Timing from micro-secs

to hours

2.

Monostable

and astable

operation3.

Adjustable duty cycle

4.

Ability to operate from a wide range of supple voltages

5.

O/P compatible with CMOS, DTL & TTL (when used with a 5V supply)

6.

High current. O/P can sink or source 200 mA7.

Trigger and reset inputs are logic compatible

8.

O/P can be operated normal on and normal off9.

High temperature stability

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Descriptions of the Pins

Pin 1:

GroundPin 2: Trigger: Pin 2 voltage (trigger) < Vcc/3 : O/P of the lower

comparator high

Pin 3: O/P :

Two ways of connecting a loadi) between pin 3 & ground (pin 1)Or ii) Between pin 3 & +Vcc

(pin 8)

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When O/P is low:

(i)

If the load is connected between pin 3 & 1(gnd):

The current through the load is zero

normally off load

(ii) If the load is connected between pin 3 & 8(+Vcc):

The current through the load flows into the O/P terminal & is called sink current. The load connected between pin 3 & +Vcc

is called

normally on load

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When O/P is high:

(i)

If the load is connected between pin 3 & 1(gnd):

The output terminal supplies

the current to the normally off load. This current is called source current

(ii) If the load is connected between pin 3 & 8(+Vcc):

The current through the load is zero

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Pin 4: Reset: The timer can be reset or disabled (o/p voltage zero) by applying a ve

pulse to this pin

When not is use, this terminal should be connected to +Vcc

to avoid any possibility of false triggering

Pin 5: Control voltage: If we do not want to operate the comparators at +Vcc/3 or +2Vcc/3. An external voltage applied to this pin (or by connecting a pot between this pin & gnd) changes threshold as well as trigger voltage

When not is use, this pin should be bypassed to gnd

with a 0.01F capacitor to prevent any noise problem

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Pin 6: Threshold:

When pin 6 voltage > 2Vcc/3, the o/p

of the upper comparator high, S=1, Q=1,

discharge transistor On, O/P of the timer low

Pin 7: Discharge: This pin is internally connected to the collector of the discharge transistor.

Pin 8: +Vcc: The supply voltage of +5V to +18V

is applied to this pin w.r.t

ground (pin 1)

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Operation as Astable

Multivibrator

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Multivibrator

R1

R2

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Multivibrator

Initially C is fully discharged which forces the O/P to high state.

Pin 2 < Vcc/3, R=1, Q=0, discharge transistor Off, the capacitor starts charging from +Vcc

through R1

& R2

.

When Vc

> 2Vcc/3, Q=1, discharge transistor on, C is discharged through this transistor & R2

When Vc

< Vcc/3, Q=0, discharge transistor off, C will again starts charging to 2Vcc/3

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High & Low state time periods are generated by the charge (+Vcc/3 to +2Vcc/3) & discharge (+2Vcc/3 to +Vcc/3)

High state time period, Thigh

or Ton

=0.69(R1

+R2

)CLow state time period, Tlow

or Toff

=0.69R2

C

Time period, T = Thigh

+ Tlow

= 0.69(R1

+ 2R2

)Cf= 1 / 0.69(R1

+ 2R2

)CDisadvantage:1. Thigh is always > Tlow2. Thigh

& Tlow

both dependent on R2

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Operation as Monostable

Multivibrator (Monoshot)

=0.01F

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Pin 2 (trigger) is initially kept at +Vcc. So O/P low. Now pin 2 is lowered

Pin 2< vcc/3 , Q=1, C starts charging through R.

When Vc

> 2Vcc/3, Q=1, C discharges through discharge transistor.

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Everytime

the O/P is appropriately triggered, the O/P goes to high state & stays there for a time period taken by the capacitor to charge from 0 to +2Vcc/3.

The time period or the monoshot

pulse width, T,T =1.1 RA

CThe pulse width of the trigger I/P should be less

than the high time of the monoshot

O/P

555 Timer ICSlide Number 2Pin ConfigurationPin ConfigurationFunctional Block DiagramSlide Number 6Slide Number 7Main FeaturesDescriptions of the PinsSlide Number 10Slide Number 11Slide Number 12Slide Number 13Operation as Astable MultivibratorOperation as Astable MultivibratorOperation as Astable MultivibratorSlide Number 17Operation as Monostable Multivibrator (Monoshot)Operation as Monostable Multivibrator (Monoshot)Operation as Monostable Multivibrator (Monoshot)Operation as Monostable Multivibrator (Monoshot)

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see 555 Timer IC xx