IC 555
IC 555 Timer :
The Features of the IC 555 Timer
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Highly Stable device for generating accurate time delay or oscillations
Can be used with supply voltages in the range of + 5 V to + 18 V.
Can drive load upto 200 mA
Compatible with both TTL and CMOS logic ckts.
versatile and easy to use in many applications like oscillator, pulse generator, square
wave generator etc.
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LC and UC -> ⅓ Vcc and ⅔ Vcc
Pin 5 - > control pin -> modulation voltage is applied -> in applications where it
is required to change the width of the output signal irrespective of the RC timing
network. In all other cases 0.01µF is connected between pin 5 and gnd as
recommended by manufacturers.
In standby state Q and Q' are 0 and 1 respectively. = > o/p at pin 3 is low.
-ve going trigger pulse is applied to pin 2 having dc level > Vcc /3
At the negative going edge of the trigger as the trigger passes through Vcc/3 , o/p
of LC goes high. -> sets FF. i.e. Q = 1 and Q’ = 0.
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pin 6 -> threshold -> The positive input to comparator No 2. i.e. UC . This pin is used to
reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to
switch from “HIGH” to “LOW” state. ( Q = 0 and Q’ = 1 )
Reset pin provides a mechanism to reset FF in a manner which overrides the effect of any
instruction coming to FF from lower comparator and is generally connected to a logic “1”
level when not used to prevent any unwanted resetting of the output.
IC 55 as astable multivibrator:
In electronic circuits, astable multivibrators are also known as Free-running Multivibrator as they
do not require any additional inputs or external assistance to oscillate. Astable oscillators
produce a continuous square wave from its output which can then be used to flash lights or
produce a sound in a loudspeaker.
RA, RB -> timing resistors
C -> timing capacitor
Let us consider the case when o/p =1=> Q’ = 0 => discharge transistor is OFF.
Charging path for C is Vcc RA, RB, C.
Charging time const. (RA + RB ) C
During this time o/p at pin 3 is high.
When cap. Voltage is just > ⅔ Vcc ,
UC triggers the control FF s.t. Q’ = 1
Astable Multivibrator using 555 – time calculations
The time during which the capacitor C charges from 1/3 VCC to 2/3 VCC is equal to the time the
output is high and is given as tc or THIGH = 0.693 (RA + RB) C
Voltage across the capacitor at any instant during charging period is given as, vc=VCC(1-e-t/RC)
The time taken by the capacitor to charge from 0 to +2/3 VCC be = t1
So, =>(2/3) Vcc=VCC(1-e-t1/RC)
The time taken by the capacitor to charge from 0 to +1/3 VCC is say t2, So,
(⅓) Vcc=VCC(1-e-t2/RC)
=> t1 = RC loge 3 = 1.0986 RC
t2 = …….= 0.405 RC
So the time taken by the capacitor to charge from +1/3 VCC to +2/3 VCC
= thigh = tc = t1 - t2 = 0.69 RC
In the given ckt. R = RA + RB .
So, thigh = tc = t1 - t2
= 0.69 (RA + RB)C
o/p is low while the capacitor discharges from ⅔ Vcc to ⅓ Vcc. The voltage across the capacitor
is given by,
+1/3 VCC = +2/3 VCC e– td/ RBC
=> td = 0.69RC . here R = RBSo, Tlow = tD= 0.69 RB C
T = tH + tL = ……
f = …….
Summarising , Astable 555 Oscillator Charge and Discharge Times
where R is in Ω and C in Farads.
555 Oscillator Cycle Time
555 Oscillator Frequency Equation
555 Oscillator Duty Cycle
An Astable 555 Oscillator is constructed using the following components, R1 = 1kΩ, R2 = 2kΩ
and capacitor C = 10uF. Calculate the output frequency from the 555 oscillator and the duty
cycle of the output waveform. …..……
f= 28.6Hz, duty cycle = 60 %
IC 555 as Monostable Multivibrator
A monostable multivibrator has only one stable state i.e. a state of output where the device is
able to latch or hold to forever, without external prodding.
A latch or flip-flop, being a bistable device, can hold in either the “set” or “reset” state for an
indefinite period of time. Once its set or reset, it will continue to latch in that state unless
prompted to change by an external input.
A monostable device, on the other hand, is only able to hold in one particular state indefinitely.
Its other state can only be held momentarily when triggered by an external input.
All monostable multivibrators are timed devices. That is, their unstable output state will hold
only for a certain minimum amount of time before returning to its stable state.This timing
function is typically accomplished through the use of resistors and capacitors.
Monostable Multivibrators or “one-shot” pulse generators are generally used to convert short
sharp pulses into much wider ones for timing applications. Monostable multivibrators generate a
single output pulse, either “HIGH” or “LOW”, when a suitable external trigger signal or start
pulse T is applied.
VC = VCC (1 – e-t/RC)
When the capacitor voltage is 2/3 VCC, then
2/3 VCC = VCC (1 – e-t/RC)
2/3 = 1 – e-t/RC
e-t/RC = 1/3
– t/RC = ln (1/3)
– t/RC = -1.098
t = 1.098 RC
∴ t ≈ 1.1 RC
The pulse width of the output rectangular pulse is W = 1.1 RC.
PWM using 555 Monostable multivibrator