IC 555 TIMER
M.S.P.V.L. Polytechnic College,
Pavoorchatram
What is the 555 timer?
• The 555 timer is one of the most remarkable integrated
circuits ever developed. It comes in a single or dual
package and even low power cmos versions exist -
ICM7555.
• Common part numbers are LM555, NE555, LM556,
NE556. The 555 timer consists of two voltage
comparators, a bi-stable flip flop, a discharge transistor,
and a resistor divider network.
Block Diagram of Timer 555 IC
Inside the 555 Timer
– The voltage divider (blue) has three equal 5K resistors.
It divides the input voltage (Vcc) into three equal
parts.
– The two comparators (red) are op-amps that compare
the voltages at their inputs and saturate depending
upon which is greater.
• The Threshold Comparator saturates when the voltage at the
Threshold pin (pin 6) is greater than (2/3)Vcc.
• The Trigger Comparator saturates when the voltage at the
Trigger pin (pin 2) is less than (1/3)Vcc
– The flip-flop (green) is a bi-stable device.
It generates two
values, a “high” value equal to Vcc and a “low” value equal to
0V.
• When the Threshold comparator saturates, the flip flop is Reset (R) and
it outputs a low signal at pin 3.
• When the Trigger comparator saturates, the flip flop is Set (S) and it
outputs a high signal at pin 3.
– The transistor (purple) is being used as a switch, it connects pin
7 (discharge) to ground when it is closed.
• When Q is low, Qbar is high. This closes the transistor switch and
attaches pin 7 to ground.
• When Q is high, Qbar is low. This open the switch and pin 7 is no
longer grounded
What are the 555 timer applications?
• Applications include
– precision timing,
– pulse generation,
– sequential timing,
– time delay generation and pulse width modulation
(PWM).
Pin configurations of the 555 timer
•
Pin Functions - 8 pin package
•
Ground (Pin 1)
•
Not surprising this pin is connected directly to ground.
•
Trigger (Pin 2)
•
This pin is the input to the lower comparator and is used to set the latch, which in
turn causes the output to go high.
•
Output (Pin 3)
•
Output high is about 1.7V less than supply. Output high is capable of Isource up to
200mA while output low is capable of Isink up to 200mA.
•
Reset (Pin 4)
•
This is used to reset the latch and return the output to a low state. The reset is an
overriding function. When not used connect to V+.
• Control (Pin 5)
• Allows access to the 2/3V+ voltage divider point when the 555 timer is
used in voltage control mode. When not used connect to ground through a
0.01 uF capacitor.
• Threshold (Pin 6)
• This is an input to the upper comparator.
• Discharge (Pin 7)
• This is the open collector to Q14 in figure 4 below.
• V+ (Pin 8)
• This connects to Vcc and the Philips databook states the ICM7555 cmos
version operates 3V - 16V DC while the NE555 version is 3V - 16V DC.
Note comments about effective supply filtering and bypassing this pin
below under "General considerations with using a 555 timer"
Types of 555-Timer Circuits
5V
7
DI S
DI S
V CC
7
V CC
R
R
8
4
4
R
Ra
8
5V
1K
C
C
CV
NE 5 55
1
GND
CV
L ED
L ED
1
0 .0 1 u F
5
T HR
TR
5
T HR
TR
3
NE 5 55
1
6
2
3
2
Q
0 .0 1 u F
Rb
GND
Q
6
2

• Astable Multivibrator
puts out a continuous
sequence of pulses
Monostable Multivibrator (or oneshot) puts out one pulse each time
the switch is connected
• Monostable Multivibrator (One Shot)
8
V cc
R
Ra
2
3
Vcc
4
Reset
Threshold Com parator
-
+V
+
6
R
Q
S
Q
3
-V
R
Trigger
-
+V
2
1
3
7
C
+
Vcc
-V
Trigger Com parator
Control Flip-Flop
R
1
M onstable M ultivibrator
O ne-Shot
O utput
Behavior of the Monostable Multivibrator
• The monostable multivibrator is constructed by adding an
external capacitor and resistor to a 555 timer.
• The circuit generates a single pulse of desired duration when
it receives a trigger signal, hence it is also called a one-shot.
• The time constant of the
resistor-capacitor
combination determines
the length of the pulse.
Uses of the Monostable Multivibrator
– Used to generate a clean pulse of the correct height
and duration for a digital system
– Used to turn circuits or external components on or off
for a specific length of time.
– Used to generate delays.
– Can be cascaded to create a variety of sequential
timing pulses. These pulses can allow you to time and
sequence a number of related operations.
Astable Pulse-Train Generator (Multivibrator)
V cc
8
R
R1
4
Threshold Com parator
-
+V
+
R2
6
R
Q
S
Q
3
-V
R
-
+V
2
+
-V
Trigger Com parator
7
C
Control Flip-Flop
R
1
O utput
Astable Pulse-Train G enerator
Behavior of the Astable Multivibrator
• The astable multivibrator is simply an oscillator. The astable
multivibrator generates a continuous stream of rectangular off-on
pulses that switch between two voltage levels.
• The frequency of the pulses and their duty cycle are dependent
upon the RC network values.
• The capacitor C charges through the series resistors R1 and R2
with a time constant
(R1 + R2)C.
• The capacitor discharges
through R2 with a time
constant of R2C
Uses of the Astable Multivibrator
– Flashing LED’s
– Pulse Width Modulation
– Pulse Position Modulation
– Periodic Timers
Flashing LED’s
• 40 LED bicycle light with 20 LEDs flashing
alternately at 4.7Hz
Understanding the Astable Mode Circuit
• 555-Timers, like op-amps can be configured in different ways to
create different circuits. We will now look into how this one
creates a train of equal pulses, as shown at the output.
First we must examine how capacitors charge
10V
TC LO SE = 0
1
U1
R1
2
8V
V
V
1
V
1k
6V
U2
Capacitor
V1
TO PE N = 0
C1
4V
2
1 0V
Voltage
1 uF
2V
0V
0s
0
1ms
V(U2:1)
V(R1:2)
2ms
3ms
4ms
5ms
6ms
7ms
8ms
9ms
10ms
V(V1:+)
Time
• Capacitor C1 is charged up by current flowing
through R1
V1 V
10  V
I 
C A P A C IT O R
R1

C A P A C IT O R
1k
• As the capacitor charges up, its voltage increases
and the current charging it decreases, resulting in
the charging rate shown
Capacitor Charging Equations
10mA
10V
8mA
8V
6mA
Capacitor
and
Resistor
6V
Current
Capacitor
4mA
4V
2mA
2V
0A
Voltage
0V
0s
1ms
I(R1)
2ms
3ms
4ms
5ms
6ms
7ms
8ms
9ms
10ms
I(C1)
0s
1ms
V(U2:1)
2ms
V(R1:2)
3ms
4ms
5ms
6ms
7ms
8ms
9ms
10ms
V(V1:+)
Time
Time
 t
• Capacitor Current
I  Ioe
• Capacitor Voltage
V  V o  1  e

• Where the time constant

 t



  R C  R1  C 1  1 m s
Understanding the equations
10V
8V
6V
Capacitor
Voltage
4V
2V
0V
0s
1ms
V(U2:1)
V(R1:2)
2ms
3ms
4ms
5ms
6ms
7ms
8ms
9ms
10ms
V(V1:+)
Time
• Note that the voltage rises to a little above 6V
1
in 1ms.
(1  e )  .632
Capacitor Charging and Discharging
• There is a good description of capacitor charging
and its use in 555 timer circuits at
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
555 Timer
• At the beginning of the
cycle, C1 is charged through
resistors R1 and R2. The
charging time constant is
 ch arg e  ( R 1  R 2 ) C 1
• The voltage reaches
(2/3)Vcc in a time
t ch arg e  T 1  0 . 693 ( R1  R 2 ) C 1
555 Timer
• When the voltage on the
capacitor
reaches
(2/3)Vcc, a switch (the
transistor)
is
closed
(grounded) at pin 7.
• The capacitor is discharged
to (1/3)Vcc through R2 to
ground, at which time the
switch is opened and the
cycle starts over.
 disch arg e  ( R 2 ) C 1
t disch arg e  T 2  0 . 693 ( R 2 ) C 1
555 Timer
• The frequency is then given by
f 
1
0.693( R1  2  R 2 ) C 1

1.44
( R1  2  R 2 ) C 1
555 Animation
Output is high for
0.693(Ra+Rb)C
Output voltage high
turns off upper LED
and turns on lower
LED
Capacitor is charging through Ra and Rb

http://www.williamson-labs.com/pu-aa-555-timer_slow.htm
555 Animation
Output is low for
0.693(Rb)C
Output is low
so the upper
LED is on and
the lower LED
is off
Capacitor is discharging
through Rb
PWM: Pulse Width Modulation
• Signal is compared to a sawtooth wave
producing a pulse width proportional to
amplitude
What Can Be Done With PWM?
Low
Duty Cycle
Medium
Duty Cycle
High
Duty Cycle
• Question: What happens if voltages like the
ones above are connected to a light bulb?
Answer: The longer the duty cycle, the longer
the light bulb is on and the brighter the light.
What Can Be Done With PWM?
• Average power can be controlled
• Average flows can also be controlled by fully opening and
closing a valve with some duty cycle
The End
…..Thank you…..