Post on 22-Jan-2018
transcript
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
– 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
Inside the 555 Timer
– 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
• Astable Multivibrator puts out a continuous sequence of pulses
5V
Ra
C
0.01uF
LED
NE555
2
5
3
7
6
4 81
TR
CV
Q
DIS
THR
R
VCC
GND
Rb
5V
12
1K
0.01uF
C
R
LED
NE555
2
5
3
7
6
4 81
TR
CV
Q
DIS
THR
R
VCC
GND
� Monostable Multivibrator (or one-shot) puts out one pulse each time the switch is connected
• Monostable Multivibrator (One Shot)
+V
-V
-
+
+V
-V
-
+R
S
Q
Q
3
4
1
7
2
6
8
R
R
R
Control Flip-FlopTrigger Comparator
Threshold Comparator
Output
ResetVcc
Trigger
Monstable MultivibratorOne-Shot
C
Racc
2V
3
cc
1V
3
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.
– 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.
Uses of the Monostable Multivibrator
Astable Pulse-Train Generator (Multivibrator)
+V
-V
-
+
+V
-V
-
+R
S
Q
Q
3
4
1
7
2
6
8
R
R
R
Control Flip-FlopTrigger Comparator
Threshold Comparator
Output
Vcc
Astable Pulse-Train Generator
C
R1
R2
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
– Flashing LED’s
– Pulse Width Modulation
– Pulse Position Modulation
– Periodic Timers
Uses of the Astable Multivibrator
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
• Capacitor C1 is charged up by current flowing through R1
• As the capacitor charges up, its voltage increases and the current charging it decreases, resulting in the charging rate shown
VV V
R1
1k
U2
TOPEN = 01
2C1
1uF
U1
TCLOSE = 0
1 2
0
V110V
IV V
R
V
kC A P A C I T O R C A P A C I T O R= − = −1
1
1 0
1
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 msV( U2 : 1 ) V( R1 : 2 ) V( V1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
Ca p a c i t o r Vo l t a g e
Capacitor Charging Equations
• Capacitor Current
• Capacitor Voltage
• Where the time constant
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 msI ( R1 ) I ( C1 )
0 A
2 mA
4 mA
6 mA
8 mA
1 0 mA
Ca p a c i t o r a n d Re s i s t o r Cu r r e n t
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 msV( U2 : 1 ) V( R1 : 2 ) V( V1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
Ca p a c i t o r Vo l t a g e
I I eo
t= − τ
V V eo
t= −
−1 τ
τ = = ⋅ =R C R C m s1 1 1
Understanding the equations
• Note that the voltage rises to a little above 6V in 1ms.
T i me
0 s 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms 7 ms 8 ms 9 ms 1 0 msV( U2 : 1 ) V( R1 : 2 ) V( V1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
Ca p a c i t o r Vo l t a g e
( ) .1 6 3 21− =−e
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
• The voltage reaches (2/3)Vcc in a time
1)21(693.01arg CRRTt ech +==
1)21(arg CRRech +=τ
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. 1)2(arg CRedisch =τ
1)2(693.02arg CRTt edisch ==
555 Timer
• The frequency is then given by
fR R C R R C
=+ ⋅
=+ ⋅
1
0 6 9 3 1 2 2 1
1 4 4
1 2 2 1. ( )
.
( )
Output voltage high turns off upper LED and turns on lower LED
Capacitor is charging through Ra and Rb
Output is high for 0.693(Ra+Rb)C
555 Animation
� http://www.williamson-labs.com/pu-aa-555-timer_slow.htm
Output is low so the upper LED is on and the lower LED is off
Capacitor is discharging through Rb
Output is low for 0.693(Rb)C
555 Animation
PWM: Pulse Width Modulation
• Signal is compared to a sawtooth wave producing a pulse width proportional to amplitude
What Can Be Done With PWM?
• 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.
Low
Duty Cycle
Medium
Duty Cycle
High
Duty Cycle
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…..