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555 Timer
by
D.V.Kamat , Faculty
Department of E&C Engg., MIT
555 Timer IC
The 555 Timer IC is an integrated circuit (chip) implementing a
variety of timer and multivibrator applications.
The IC was brought to market in 1971 by Signetics (later
acquired by Philips).The original name was the SE555 (metal
can)/NE555 (plastic DIP).
It has been claimed that the 555 gets its name from the three
5 kΩ resistors used in typical early implementations.
555 IC specifications
The typical specifications of NE555 are as follows:
Supply voltage (VCC)
Output current
(maximum)
Compatible with
Operating temperature
3 to 15 V
200 mA
TTL,CMOS ICs
0 to 70 °C
555 Timer IC
Functional diagram of 555 Timer IC
555 Timer IC
Inside 555 timer IC ,there are two comparators : Upper noninverting comparator and Lower inverting comparator
A resistive voltage divider consisting of three 5 KΩ resistors
provide reference voltages of 2/3 of the Vcc to the upper
comparator and 1/3 of the Vcc to the lower comparator. These
two reference voltages aid in determining the timing interval.
The outputs of upper and lower comparators are fed to R
and S of SR FF respectively.
The Qbar of SR FF is driving the discharge transistor(NPN)
whose collector terminal is connected to pin 7 (Discharge
terminal).
According to the output of FF, the transistor switches to its
cut-off region or saturation region.
Control input(pin 5)
• The reference voltage to the upper comparator can be
varied by applying a modulation voltage to the control
input terminal(pin 5).
• In most of the time this pin is unused. In applications
,where no such modulation is intended ,it is
recommended that a 0.01uF capacitor be connected
between control pin and ground to bypass the power
supply noise.
Trigger input(pin 2)
Trigger input(pin 2) is connected to the inverting input
of the lower comparator.
When the trigger input at pin 2 is pulled down below
+VCC/3, the output of lower comparator sets the
internal flip-flop that makes the output (pin 3) of timer
high.
Threshold input(pin 6)
Threshold input(pin 6) is connected to the non-inverting
input of the upper comparator. However, if the threshhold
input(pin 6) is now raised above +2VCC/3,the output will
return to ground and the discharge transistor will be turned
on again. When the threshhold input returns to ground, the
IC will remain in this state.
The easiest way to allow the
threshhold voltage (pin 6) to
gradually rise to (2/3)+VCC is to
connect it to a capacitor being
allowed to charge through a
resistor. In this way we can
adjust the R and C values for
almost any time interval we
might want.
555 Timer IC
The IC 555 is quiescent so long as the trigger input (pin 2)
remains at +VCC and the threshold input (pin 6) is at ground.
The outputs of both comparators are low and the FF remains
in the last state.
Reset Input(pin 4)
• The reset input is of active-low type and can be used to
reset the FF in a manner to override the effect of set
input of the FF coming from lower comparator.
• When reset input is at Low(less than about 0.4V) the
reset transistor conducts and forces the output of timer
to Low.
• When the reset is not used or to be disabled, it is to be
connected to VCC.
Discharge pin(pin 7)
• If the SR FF is in reset condition (S=0 , R=1) ,then Qbar is
high which turns on the discharge transistor ,thus
grounding whatever is connected to the discharge pin.
555 Timer IC
Pin Name Purpose
1 GND Ground, low level (0 V)
OUT rises, and interval starts, when this input
2 TRIG
falls below 1/3 VCC.
3 OUT This output is driven to +VCC or GND.
A timing interval may be interrupted by driving
4 RESET
this input to GND.
"Control" access to the internal voltage divider
5 CTRL
(by default, 2/3 VCC).
The interval ends when the voltage at THR is
6 THR
greater than at CTRL.
Open collector output; may discharge a
7 DIS
capacitor between intervals.
V+, Positive supply voltage is usually between 3
8
VCC
and 15 V.
555 Timer for multivibrator applications
The 555 has three operating modes:
Monostable mode: In this mode, the 555 functions as a
"one-shot". Applications include timers, touch switches,
frequency divider, capacitance measurement etc
Astable or free running mode: In this mode, 555 can
operate as an oscillator. Uses include LED and lamp
flashers, moving displays, square wave/pulse generation,
clock generation for logic circuits, tone generation,
security alarms etc.
Bistable mode : the 555 can operate as a flip-flop.
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Applications of 555 Timer
• The 555 can operate in either monostable or astable
mode, depending on the connections to and the
arrangement of the external components.
• In the monostable mode , it can either produce a
single pulse when triggered.
• In astable mode , it can produce a continuous pulse
train as long as it remains powered. No trigger input
needs to be applied.
Astable Multivibrator
Pin 8(Vcc) and Pin 1(Gnd) are connected to +5V and ground
respectively. Pin 4(Reset’) is connected to +5V(Reset’ is
disabled for astable operation).Pin 5(Control) is connected to
ground through 0.01uF capacitor for ripple suppression. The
square wave/rectangular output is observed at the output(Pin
3).
Astable Multivibrator
The trigger(Pin 2) and threshold(Pin 6) are shorted and connected to a
capacitor C .Here ,both the trigger and threshold inputs are controlled by
the capacitor voltage and thus the 555 Timer can trigger itself repeatedly.
No external trigger input is applied as in Monostable multivibrator.
In this case, two resistors RA and RB are in the capacitor charging path so
that one of them can also be in the capacitor discharge path. The common
point of RA and RB are connected to Discharge(Pin 7) terminal.
555 as Astable Multivibrator
Initially ,the capacitor is without any charge. When power is applied ,it charges
through RA and RB . When Vc > (2Vcc/3), then both comparator will (S=0, R=1) reset
the flip-flop and Vo will be low.
Qbar=1 will turn on the discharge transistor and thus the common point of RA and
RB(pin 7) will be shorted to ground. Now ,C starts discharging through RB .
When Vc < (Vcc/3), then both comparator will (S=1, R=0) set the flip-flop and Vo will
be high. Qbar = 0 will turn off the discharge transistor and thus C starts charging again
through RA and RB .
555 Timer IC
tch = 0.693(RA+ RB)C Note that D must always be > 0.5.
tdisch = 0.693 RBC
T = 0.693(RA+ 2RB)C
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555 Timer Astable operation
In this mode, the initial pulse when power is first applied is a bit longer than the
others, having a duration of 1.1(RA+ RB)C. However, from then on, the capacitor
alternately charges and discharges between the two comparator threshhold
voltages. When charging, C starts at (1/3)VCC and charges towards VCC. However,
it is interrupted exactly halfway there, at (2/3)VCC. Therefore, the charging time,
t1 = 0.693 RA + RB)C.
When the capacitor voltage reaches (2/3)VCC, the discharge transistor is enabled
(pin 7), and this point in the circuit becomes grounded. Capacitor C now
discharges through RB alone. Starting at (2/3)VCC, it discharges towards ground,
but again is interrupted halfway there, at (1/3)VCC. The discharge time,
t2= 0.693(RB)C.
The total period of the pulse train is t1 + t2, or 0.693(RA + 2RB)C. The output
frequency of this circuit is the inverse of the period, or 1.44/(RA + 2RB)C.
Note that the duty cycle of the 555 timer circuit in astable mode cannot reach
50%. On time must always be longer than off time, because RA must have a
resistance value greater than zero to prevent the discharge transistor from
directly shorting VCC to ground. Such an action would immediately destroy the
555 IC.
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Applications of 555 Timer
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Precision timing
Pulse generation
Sequential timing
Time delay generation
Pulse width modulation
Pulse position modulation
Linear ramp generator …
END
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