555 Timer
Subject-PEE(Mechanical)
555 Timer
• The 555 timer IC is an integral part of electronics circuits and
used in wide verity of applications.
• These provide time delays, as an oscillator and as a clock
generator element among other applications.
• Introduced in 1971 by the American company Signetics, the
555 is still in widespread use due to its low price, ease of use
and stability.
• It is made by many companies in the original bipolar and low-
power CMOS types. According to an estimate, a billion
units were manufactured back in the year 2003 alone.
• Depending on the manufacturer, the standard 555 timer
package includes 25 transistors, 2 diodes and 15 resistors on a
silicon chip installed in an 8-pin mini dual-in-line package (DIP-
8).
Contd...
• Pin 1:GND----Ground reference voltage, low level (0 V)
• Pin 2:TRIG----Output of the timer totally depends external trigger voltage applied to this
pin.
• Pin 3:OUT----This output is driven to approximately 1.7 V below +Vcc, or to GND
• Pin 4:RESET----A timing interval may be reset by driving this input to GND
• Pin 5:CTRL----Provides “control” access to the internal voltage divider (by default, 2/3 Vcc).
• Pin 6:THR----Threshold is applied.(2/3 Vcc if CTRL is open).
• Pin 7:DIS---- Discharge a capacitor between intervals.
• Pin 8:Vcc----Positive supply voltage, which is usually between 3 and 15 V depending on the
variation.
Internal Circuit
• The internal circuit of 555 Timer is shown in the below figure.
Astable Multivibrator
• Astable multivibrator is a free running multivibrator that has no stable state but
switches continuously between two states. This action will produce a train of square
pulse at a fixed frequency.
Comparator
• A comparator is a circuit that compares two voltages or
currents and output a digital signal indicating which is larger.
Vo = 1 ,if V+ > V-
0 ,if V+ < V-
Monostable Multivibrator
• Monostable or oneshot multivibrator has only one stable state and is triggered externally
with it returning back to it’s first stable state.
Schmitt Trigger ckt.
• Schmitt trigger is an electronic circuit with positive feedback
which holds the output level till the input signal to comparator
is higher than the threshold.
• It converts a sinusoidal or any analog signal to digital signal.
• It exhibits hysteresis by which the output transition from high
to low and low to high will occur at different thresholds.
• Schmitt trigger is one of such regenerative circuits also called
as astable multivibrator because of two quasi stable (unstable)
states which are 1.positive extreme and 2.negative extreme.
Contd...
• The Op-Amp Schmitt trigger is as shown below
Consider a feedback system with forward gain of A and feedback factor β.
If we adjust the loop gain to be one then the gain with feedback becomes infinite.
This results in ever ending transition of output between extremes of output.
Circuit analysis
• From the figure shown above the let us assume a sinusoidal voltage is applied and at
first output is in positive saturation state. Then the feedback voltage Vf = β*Vcc.
• Now when input Vin more than β*Vcc then the voltage at inverting terminal is greater
than the voltage at non inverting terminal, so the output will equal to –Vcc.
• This value of input voltage at which output makes transition from positive saturation
voltage to negative saturation voltage is called upper threshold point (UTP).
It is obvious from
the circuit that
positive feedback
is employed in the
circuit. The
feedback factor. β
=Vf / Vo =
R2/(R2+R1).
Contd...
• At this point Vf = -β*Vcc now if input is allowed to fall below -
β*Vcc then the voltage at non inverting terminal is greater
than the voltage at inverting terminal, so the output makes
transition from–Vcc to + Vcc.
• This value of input voltage at which output makes transition
from negative saturation voltage to positive saturation voltage
is called lower threshold point (LTP).
• This process continues till sine wave input exists at input and
DC power supply for opamp is on.
• The transfer characteristics of schmitt trigger is shown in the
figure along with the symbol.
Contd...
• The phenomenon by which the circuit triggers at a higher voltage for
increasing voltages than for decreasing voltages is called hysteresis.
Because of
hysteresis there
exists two
triggering
voltage i.e.
upper threshold
and lower
threshold for
Schmitt trigger.
It is quantified
as follows
