Transcript 555 Timer
Doshi Hiral
130270109008
Electrical Engineer
©Paul Godin
Updated February 2008
Oscillators
We have looked at simple oscillator designs using an
inverter, and had a brief look at crystal oscillators.
In this presentation, we introduce the 555 timer; a
versatile device that is easier to calculate, design and
configure in a variety of ways.
555.3
A
Versatile
Device
The
555 Timer is one
of the best known IC’s.
The 555 is part of every experimenter's tool kit
Capable of creating a wide variety of circuits, including:
Oscillators with adjustable frequency and Duty Cycle
Monostable Multivibrators
Analog to digital Converters
Frequency Meters
Many other applications….
The clock on the Vulcan Board is generated by a 555
timer.
555.4
In thisTimer
course we Configurations
will use the 555 timer in 2 modes:
555
Astable
With some calculations, we can determine the values of the
capacitor and the 2 resistors (Ra and Rb) for astable operations.
Monostable
We can determine the value of the resistor and the capacitor
with a simple formula for one-shot operations.
There are many other configurations and applications
for this device.
555.5
555 Layout
Ground
Trigger
Output
Reset
Vcc
8
7
6
5
1
2
3
4
Discharge
Threshold
Control
Also available:
•556 (two-555’s in one DIP package)
•555 in a “metal can” configuration
Vcc
Ground 1
8
Trigger 2
Output 3
7
Discharge
6 Threshold
4
5 Control
Reset
555.6
555.7
Astable Multivibrator
Oscillator Configuration
Externally, the 555 requires an RC circuit to create the
time delays required for the time high and the time
low.
Standard configuration requires
common capacitor
a resistor for the charge cycle
a resistor for the discharge cycle
555.8
General Configuration
Basic connections:
Ground
Vcc
Note: some 555 timers
may function at voltages
other than 5 volts.
Ground
Trigger
Output
Reset
1
2
3
4
8
7
6
5
Vcc
Discharge
Threshold
Control
Reset (active low)
Output
555.9
General Configuration
Specialized connections:
Trigger
monitors low voltage
Threshold
monitors high voltage
Ground
Trigger
Output
Reset
1
2
3
4
8
7
6
5
Vcc
Discharge
Threshold
Control
Discharge
path to ground, to discharge
the capacitor
Control
specialized input
filtering
special applications
555.10
Astable Configuration #1
(“Standard” Configuration)
Vcc
Vcc
Vcc
Ground
Output
Reset
Ra
1
2
3
4
8
7
6
5
Control
Discharge
Rb
Threshold
Trigger
C
555.11
Astable
Calculated
Values
Note: Duty Cycle
must be > 50%
Filter Cap
0.01μF
555.12
Astable Configuration #1
(“Standard” Configuration)
t1 .693(Ra Rb )C
t 2 .693 Rb C
f
1
1
t1 t 2 .693(Ra 2Rb )C
t1
Ra Rb
DC
t1 t 2 Ra 2Rb
Minimum duty cycle > 50%
555.13
Calculations: Astable
Time High, Time Low Set
TL 0.693 R B C
TH 0.693(R A R B )C
TH
TL
Notes:
•The value 0.693 is a factor associated with the
charge/discharge cycle of the 555 timer.
•Duty Cycle must be > 50%
555.14
Sample Calculation
Time High, Time Low Set
Design an oscillator with a frequency of 200Hz with
a duty cycle of 78%.
1.
Determine Period (T):
1
1
T
0.005s
F 200Hz
2.
Determine
THand
TL:s 0.0039 s 3.9ms
TH 78 %
0.005
TL 22 % 0.005 s 0.0011 s 1.1ms
555.15
Sample Calculation
Time High, Time Low Set
3.
Since there are 2 variables in the TL equation, select
C:
C=10μF
4. Determine RB by using the TL equation:
TL 0.693R BC
1.1ms 0.693 R B 10F
R B 158 .7
555.16
Sample Calculation
Time High, Time Low Set
5.
Determine the value for RA:
TH 0.693(R A R B )C
3.9ms 0.693(R A 158 .7)10F
562 .8 R A 158 .7
R A 404 .1
555.17
Calculations: Astable
Frequency, Duty Cycle Set
(R A R B )
DC
(R A 2R B )
F
1
0.693 (R A 2R B ) C
Notes:
•The value 0.693 is a factor associated with the
charge/discharge cycle of the 555 timer.
•Duty Cycle must be > 50%
555.18
Sample Design
Frequency, Duty Cycle Set
Build an oscillator using a 555 timer with a
frequency of 72kHz at 75% D.C. Use a 100F
capacitor.
555.19
Design Solution
Frequency, Duty Cycle Set
1- Determine the ratio of the resistors Ra and Rb:
Ra Rb
0.75
Ra 2Rb
Ra Rb 0.75(Ra 2Rb ) 0.75Ra 1.5Rb
Ra 0.75Ra 1.5Rb Rb
DC
Ra 2Rb
2- Use the ratio in the frequency equation (substitution):
f
1.44
1.44
1.44
(Ra 2Rb )C (2Rb 2Rb )C 4RbC
555.20
Design Solution
Frequency, Duty Cycle Set
3- Solve for Rb:
72kHz
1.44
1.44
50k
4RbC
4 72k 100
4-Solve for Ra:
Ra 2Rb 100k
5-Use standard values (optional step):
Ra=100k
Rb=47k
555.21
Design Solution
Frequency, Duty Cycle Set
6- Calculate actual frequency and DC:
f
1.44
74 .2kHz
(100k (2 47k)) 100 F
Error 3.1%
DC
(100k 47k)
0.757
(100k (2 47k))
Error 1%
555.22
Design Solution
Frequency, Duty Cycle Set
7- Create the circuit diagram using EWB:
555 Timer, 74.2kHz @ 75.7% D.C.
555.23
Minimum Value for Ra
The discharge transistor
causes the capacitor to
discharge to ground.
Ra must have a minimum
value of 25 to prevent a
short circuit of the power
supply through the
discharge transistor.
Minimum Value
555.24
In-Class Practice Problem
Design an oscillator with a frequency of 500Hz and a
duty cycle of 80%. Use a 10μF Capacitor. Calculate
using each of the equation sets.
555.25
555.26
Astable Configuration#2
Rb must be < .5 Ra
555.27
Astable Configuration #3
555.28
555.29
Internal Operation of the 555
Timer
Understanding the internal operation of the 555 timer
is important
The device combines various circuit theories.
Various aspects provide a preview of other circuits, such
as the Analog to Digital decoders.
Understanding its internal function makes it easier to
create new designs with the device.
555.30
Comparator
configuration.
The comparator is an operational amplifier (op-amp)
The comparator compares 2 analog voltages and provides
a digital output.
+
-
If V+ > V-, the output is a digital 1
If V- > V+, the output is a digital 0
555.31
Reference and Comparators
The Comparators
A 3-resistor
voltage divider
provides
reference
voltages
provide digital logic
to an SR Latch
Comparators compare voltage
levels.
•If “+” is higher, output = 1
•If “-” is higher, output = 0
555.32
Reference and Comparators
Pin 6: “Threshold”
Pin 8: Vcc
connection
connection (high
Comparator)
Pin 5: “Control”
connection,
used with filter
cap.
Pin 1: Gnd
connection
Pin 2: “Trigger”
connection (low
Comparator)
555.33
The Q’ output controls
the transistor (“on” or
“off”).
Latch
and Output
The SR Latch
holds its output
states
The reset input can be
used to enable or
disable the timer.
The transistor
acts like a switch.
The buffer has important
electrical functions.
555.34
Latch and Output
Pin 4: Reset
connection (active
low reset)
Pin 7:
“Discharge”
connection
Pin 3: Digital
Output
555.35
Charge Animation
Discharge Animation
555.36
Specification sheet
From the specification sheet for the LM555,
determine the following:
Operational voltage range
Maximum current output for each state
Frequency Range
Output rise and fall time
What is a disadvantage of the 555 as a timing device?
555.37
Function of the Control Input (Pin5)
Pin 5 is the control voltage connection, and is used to
access the 2/3 Vcc point of the voltage divider.
Normally, a 0.01F capacitor is connected to ground to
provide output voltage stability.
Used as an input for some applications.
AC to pulse modulation
voltage controlled oscillator
555.38
Design Exercises
555.39
Design Exercises
1.
Using a 555 timer, design an oscillator with an output
of 6Hz with a duty cycle of 60%. Use a 100μF
capacitor. Build in EWB.
2. Using a 555 timer, design an oscillator with an output
of 10KHz with a duty cycle of 50%. Use a 3.3μF
Capacitor. Build in EWB.
555.40
The following slides contain animations to
demonstrate the operations of:
555 as an astable: charge cycle
555 as an astable: discharge cycle
555.41
+ <> 01
10
01
- <> +
10
Vc
Capacitor
Charges
viastate
Ra
and
Rb
Latch
inQ aoutput
set
Q’
is
low;
is
high
Capacitor
continues
to
charge
Lower
comparator
provides
logic
Upper
comparator
+ input
is
Latch
receives
a
reset
state
Q’
is
high;
Q
output
is
low
Transistor
is
“on”
and
a
connection
Capacitor
to state.
discharge
low. Latch
in hold
greater
thanbegins
2/3
Vcc
reference
to ground is made.
Animated charge cycle
555.42
- >< +
01
01
10
+ <> -
10
Vc
Transistor
isdischarging.
“off”. Capacitor
begins
Capacitor
is
Q
output
Upper
comparator
+
voltage
less
Latch
in charge.
a hold
state.
Lower
comparator
+
voltage
is
Latch
is
set.
Q’
is
low.
to
is
low.
Q
output
is
high.
than
reference
greater
than – voltage.
voltage.
Animated discharge cycle
555.43
THANK YOU
555.44