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The 555 Timer circuit 555 is a versatile integrated timer circuit first introduced by Signetic Corporation. Like

Op-Amps, 555 is a general purpose IC which finds applications in

• Oscillators

• Pulse generators

• Ramp or square wave generators

• Multivibrators

• Voltage monitors & Burglar alarms

• Delay generators etc.

555 can also be in touch switches, frequency dividers etc. One extension of 555 type

circuitry is a programmable timer counter, XR2240 which can be used for further

applications such as long-interval timers delay generators etc. 555 has a maximum timing

range of 15 minutes, whereas counter timers have maximum range of days.

555 can operate from supply voltages +5 to +18 V, making it compatible with Op-Amps

and TTL circuits.

The actual 555 circuit can be considered as a functional block that contains

• 2 comparators

• 2 transistors operating in switching mode

• 1 flip-flop of S-R type, and several other components

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Output terminal (Pin 3)

Output terminal is pin number 3 and it can either source or sink current. Maximum sink

or source currents is technically 200mA but more realistically it is about 50 mA. The

high output is about 0.5 V below VCC, and the low output voltage is nearly zero (0.1 V).

( Note that the complementary output of the F-F is a active low type output which is

identical to Q .)

3

Reset terminal (Pin 4)

Reset terminal is connected to the base of the pnp transistor, the emitter of which is

connected to the two gates, and a pull-up resistor to the power supply. Q can be reset by

applying a high to low signal ( reset ). i.e. when reset is low, the transistor will be on and

the reset line is held low (0). This makes S =0 and R = 1 resetting the output Q to 0.

When reset is high the transistor will be off and the logic levels of S and R inputs of the

F-F are determined by the logic levels of the set and clear inputs. Reset line has to be kept

high when using the device.

Discharge terminal (Pin 7)

The discharge transistor Q1 controls the status of the discharge terminal. Note that the

transistor becomes on when Q of the F-F is high, i.e. when Q = 0. When the transistor

becomes on the discharge terminal acquire ground potential through the on transistor

(10Ω ). The discharge terminal, pin 7, is usually used to discharge an external timing

capacitor during the time when the output is low. When the output is high, pin 7 acts as

an open circuit and allows the capacitor to charge at a rate determined by an external

resistor or resistors and a capacitor. See figures below.

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Control voltage terminal (Pin 5)

A 0.01 µ F capacitor is usually connected between pin5 and the ground. It bypasses noise

and ripple voltage of the power supply to ground. See the 555 circuit diagram. The

control voltage terminal can also be used to change both trigger and threshold voltage

levels. This is done by connecting a resistor between the terminal 8 (power supply

terminal) and the pin 5. For example, connection of a 10 k resistor between pins 5 and 8

as shown below will change threshold voltage UTV to 0.5 VCC and the threshold voltage

LTV to 0.25 VCC.

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(A power supply can also be connected to pin 5 to change the threshold and trigger

voltages) This terminal can also be used to apply another signal for modulation purposes.

Trigger (Pin 5) and Threshold (Pin 6) terminals

The 555 has two possible operating states and one memory state. They are determined by

the trigger input, Pin 2, and the threshold input, Pin 6. The trigger input is compared by

the comparator C1 with a lower threshold voltage, VLT that is equal to VCC/3. See figure

below. The Threshold input is compared by comparator C2with a higher threshold voltage

VUT that is equal to 2VCC/3.

+ _

5

8 VCC

10 k R

R 5k

5k

+ _

6

8 VCC

R

R

+ _

C2

C1 2

Trigger

Threshold

R

VLT

VUT

VUT = 2VCC/3 VLT = VCC/3

Clear

Set

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Each of the inputs, trigger and threshold has two possible voltage levels, either above or

below those reference voltages VLT and VUT. Thus with two inputs there are four possible

combinations that will cause four possible operating states. The four possible

combinations and corresponding states of the 555 are given in the table below.

State Trigger

input (2)

Threshold

input (6)

State S

of F-F

State R

of F-F

Output Q

(3) Q Discharge

transistor

A Below

VLT

(<VCC/3)

Below

VUT

(<2VCC/3)

1 0 High Low Open

B Below

VLT

(<VCC/3)

Above

VUT

(>2VCC/3)

1 0 High Low Open

C Above

VLT

(>VCC/3)

Below

VUT

(<2VCC/3)

0 0 Remembers

old output

Last

state

D Above

VLT

(>VCC/3)

Above

VUT

(>2VCC/3)

0 1 Low High Closed

(Capacitor

discharges)

In order to understand its operation, let us apply an input voltage Ei to both trigger and

threshold inputs. Consider the triangular signal Ei shown in figure below being applied to

the combined input. See figure below.

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When Ei is less than both VUT and VLT, the above table indicates that the output (VO3) is

High. (State A of the table)

When Ei is less than VUT but greater than VLT, the output remembers the previous output

(which is High) and therefore VO3 is continued to be High. (State C of the table)

When Ei is greater than both VUT and VLT, the output becomes low.

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When Ei is less than VUT but greater than VLT, the output remembers the previous output

(which is Low now) and therefore VO3 is continued to be low. (State C of the table).

Finally when Ei is less than both VUT and VLT, the above table indicates that the output

(VO3) is High. (State A of the table).

By plotting output VO3 against Ei we see a hysteresis characteristic. Hysteresis means the

circuit has a memory. This also means that if the inputs are in one of the memory states,

one can not tell what state the output is now in unless one knows the previous state.

Applications

Power on time delays In certain electronic circuits we may want to apply power to one section of the circuit and

wait for a short interval before starting some other part of the circuit. For example we

may want to reset all the counters before starting a personal computer at the beginning of

a business day.

A circuit that solves such a problem is shown below.

Initially the charge on C is zero, and therefore when the switch is closed, the entire input

voltage (15 V) will appear across R. This means that both the trigger and the threshold

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voltages are higher than VUT and VLT and V0 is zero. When the capacitor charges the

voltage across R drops and the output switches to high level The charging of the

capacitor is given by

1t

CRC CCV V e

− = −

1t

CRR CC CCV V V e

− ∴ = − −

tCR

CCV e−

=

When 13R ccV V= ,

13

TCR

CC CCV V e−

=

ln 3T orCR

∴ =

1.1T CR=

When R and C are interchanged the output of the 555 goes high initially and then reduces

to zero after a time T = 1.1 CR. See the circuit below.

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Free running or Astable operation A555 connected as a free-running multivibrator is shown in figure below.

Assume that at t = 0 the capacitor C is uncharged. The trigger and threshold voltages are

therefore below VLT (&VUT) which makes output V0 high. When the output is high ( i.e.

the Q output of the flip-flop is high), the Q is low , and it puts the discharge-transistor

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OFF. The capacitor then starts charging towards VCC (= 5 V) through RA and RB with a

time constant of C (RA+RB), and this charging will continue until the voltage VC across

the capacitor reaches VUT (=2VCC/3). (Draw your own waveforms without considering

the situation given in figure b above). When VC attempts to go beyond VUT value, the

output of 555 will go low ( and Q will go high) making the discharge transistor ON

providing a discharge path (R=10 Ω )for the capacitor. The capacitor then discharges

through RB with a different time constant (CRB), and the voltage across C starts to drop

towards 0 V. However, when VC reaches VLT(both Trigger and threshold voltages tend to

become lover than VCC/3), the output again goes high and the discharge transistor

becomes OFF. This will make the capacitor to recharge again with a time constant

C(RA+RB), and the cycle repeats.

Period T of the cycle is given by

High LowT T T= + (See fig b above)

According to the above figure, V0 is high during the period when VC rises from VLT to

VUT, and during this period,

( ) ( )A BtC R R

C CC CC LTV V V V e− += − −

Where t=0 is the instant at which the interval TH begins.

Substituting

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1 ,3

C CC UT

C CC LT

V V at V

and V V at V

=

=

( ) ln 2H A BT C R R= +

( )0.69 A BC R R= +

V0 will be low during the time interval TL, and the exponential fall of VC is described by

BtCR

C UTV V e−

=

Where we have taken t=0 as the beginning of the time interval TL.

Substituting 1 2, ,3 3C CC L UT CCV V at t T and V V= = =

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ln 2 0.69T B BL CR CR= =

( )0.69 2H L A BT T T C R R∴ = + = +

Frequency ( = 1T

) can be varied by varying RB.

H

H L

TDuty cycleT T

=+

2

A B

A B

R RR R

+=+

It is always greater than 50% and it approaches to50% when RA is very much less than

RB.

Tone-Burst Oscillator It is a circuit which produces oscillation bursts

When the switch in the above circuit is set to the “continuous” position (which is equal

to the open position of the switch), the timer 555B functions as a free-running

multivibrator . the frequency of the 555B timer can be varied from about1.3 kHz to 14

kHz by the 10-k potentiometer. If the potentiometer is replaced by a thermistor or

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photoconductive cell, the oscillation frequency will be proportional to temperature or

light intensity, respectively.

The 555A timer oscillates at a lower frequency. The 1 MΩ potentiometer sets the lowest

frequency at about 1.5 Hz. Lower frequencies are possible by replacing the 1 µ F

capacitor with a large value. When the connecting switch is thrown to the “burst”

position, output pin 3 of the A timer alternately places a ground or high voltage on the

reset pin 4 of the 555B timer. When pin 4 of the B timer is grounded, it cannot oscillate,

and when ungrounded the timer oscillates. This causes the B timer to oscillate in bursts.

The output of the tone-burst generator is V0 and is taken from the pin 3 of timer B. V0

can drive either an audio amplifier or stepdown transformer directly to a speaker.

Voltage –controlled frequency shifter This is a low cost, low- frequency voltage-controlled frequency shifter. See figure below.

Since the 555 timer is powered by VCC= 5 V, VUT= 23

5 V and VLT= 13

5 V,.

Capacitor voltage VC will charge to VUT at which time the 555 will ground pin 7 to

rapidly discharge C to VLT.

The capacitor C is charged by a constant current I. I is set by the voltage across RE and

the value of RE. The voltage across RE is set by the 15 V supply and the non inverting

input of the voltage follower (741B). The voltage follower has a transistor driven output

to provide an adequate magnitude for I. The voltage at the non inverting input is

determined by the inverting amplifier output OAV (Note pin 2 follows the pin 3 of the op

amp B).

0 10AV V E= − , where E is the input voltage.

( )15 10ERV E= − −

5 E= +

The charge lost by C for each cycle equals ( )CC V∆ where

2 1 15 5 53 3 3CV V V V ∆ = − =

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Charge stored by C equals charging current I times period T (the charging time). For

equilibrium

Charge stored = Charge lost

IT C V= ∆

5 53E

E VT CR

+ =

Since period =1/fout we can write the above equation as

outf = center frequency Cf + shift frequency f∆

Where 3C

E

fR C

= (occurs when E = 0)

And 5Cf Ef∆ =

i.e. E f∝ ∆

One-shot or monostable operation

(Monostable multivibrator) The monostable multvibrator circuit is shown below. It is a circuit which produces a

single rectangular pulse of known duration.

When a negative going pulse is applied to pin 2 (trigger input), the output goes high and

terminal 7 removes a short circuit from capacitor C. The capacitor then starts charging

from 0 V towards VCC at a rate determined by RA and C. When the capacitor voltage

reaches VUT ( 2VCC/3) the output becomes low again. (Note: This is true only if the width

of Ei is small compared to the width T of the output pulse. Check what happens if the

pulse width is very large. The circuit below has an input circuit to produce a narrow

pulse) the input and output voltage waveforms are shown in above figure. The output is

high for a time given by

1.1high At R C=

Fig (b) is a plot of the above equation and quickly shows the wide range of output pulses

that are obtainable and the required values of RA and C

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The figure shown below is also a multivibrator wired for monostable operation. Here the

negative going pulse Ei applied to the trigger input is differentiated by the Ci Ri

combination to produce narrow positive and negative voltage spikes. The positive spike

is removed by the diode. This input circuit therefore provide a narrow negative voltage

pulse to the trigger input

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Advantages of the above input pulse network

(1) Production of a narrow trigger pulse

(2) Makes idling trigger voltage = VCC

(3) Keep idling output voltage = 0 V