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555 Timer
©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.
555.2
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.
555.3
555 Timer Configurations
◊ We will look at the 555 timer in 2 modes:
◊ 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.
555.4
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
555.5
555.6
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
555.7
General Configuration
◊ Basic connections:
◊ Ground
◊ Vcc
◊ Note: some 555
timers may function
at voltages other
than 5 volts.
◊ Reset (active low)
◊ Output
Ground
Trigger
Output
Reset
1
2
3
4
8
7
6
5
Vcc
Discharge
Threshold
Control
555.8
General Configuration
◊ Specialized connections:
◊ Trigger
◊ monitors low voltage
◊ Threshold
◊ monitors high voltage
◊ Discharge
◊ path to ground, to
discharge the capacitor
◊ Control
◊ specialized input
◊ filtering
◊ special applications
Ground
Trigger
Output
Reset
1
2
3
4
8
7
6
5
Vcc
Discharge
Threshold
Control
555.9
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
555.10
Astable
Calculated
Values
Note: Duty Cycle
must be > 50%
Filter Cap
0.01μF
555.11
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%
555.12
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%
555.13
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 TH and TL:
TH  78 %  0.005 s  0.0039 s  3.9ms
TL  22 %  0.005 s  0.0011 s  1.1ms
555.14
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  10F
R B  158 .7
555.15
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)10F
562 .8  R A  158 .7
R A  404 .1
555.16
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%
555.17
Sample Design
Frequency, Duty Cycle Set
◊ Build an oscillator using a 555 timer with a
frequency of 72kHz at 75% D.C. Use a 100F
capacitor.
555.18
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
555.19
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
555.20
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%
555.21
Design Solution
Frequency, Duty Cycle Set
7- Create the circuit diagram using EWB:
555 Timer, 74.2kHz @ 75.7% D.C.
555.22
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
555.23
OTHER ASTABLE CONFIGURATIONS
555.24
Astable Configuration#2
Rb must be < .5 Ra
555.25
Astable Configuration #3
555.26
Specification sheet
◊ From the specification sheet for the LM555,
discuss 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?
555.27
555 as a Monostable
◊ The 555 timer can also be configured as a
monostable.
◊ The 555 Monostable has interesting
characteristics that may be used in specific
applications.
555mono.28
Timing Diagram
The Trigger is active low, not
edge triggered.
Input
trigger
Output
pulse
Tw
Output pulse
created when
input trigger
voltage is less
than 1/3 Vcc
Tw
If the trigger is held low beyond
calculated pulse width, the output
pulse follows the input trigger
555mono.29
Monostable
Calculated
Values
Filter Cap
0.01μF
555mono.30
Calculations: Monostable
TW  1.1  R  C
Tw
Notes:
•The value 1.1 is a k-factor associated with the 555 timer.
•The trigger is active-low (not edge-triggered), and must be
brought high before the end of the pulse width.
555mono.31
Sample Monostable Calculation
◊ Design a Monostable that produces a 5S pulse.
Use a 100F capacitor.
TW  1.1  R  C
5s  1.1  R  100 F
R  45.45k
555mono.32
Animated Slide
The following slides contain animations to
demonstrate the operations of:
555 as a monostable
555mono.33
Monostable
++>
<-
01
Vc
01
101
010
++<>--
0
Animated
The
transistor
AWait-state.
low
is applied
to
the trigger
The
comparator
provides
a logic
Q’
goes
low;
Q
high.
The
Capacitor
voltage
>
2/3
Vcc.
High
Latch
reset.
Q’
logic
high.
discharges
the
Capacitor
input.
high;
the
latch
is set.
transistor
is
“off”.
Cap
charges.
comparator
output
high.
Capacitor discharges,
waits.
555mono.34
Animated Slides
The following slides contain animations to
demonstrate the operations of:
555 as an astable: charge cycle
555 as an astable: discharge cycle
555.35
555.36
+ <> 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
555.37
- >< +
01
01
10
+ <> -
10
Vc
Capacitor
is discharging.
Q output
Upper
comparator
+
voltage
less
Latch
in
a
hold
state.
Lower
comparator
+
voltage
is
Latch
is
set.
Q’
is
low.
is
low.
Q
output
is
high.
than
reference
Transistor
is “off”.
Capacitor
greater
than
– voltage.
voltage.
begins to charge.
Animated discharge cycle
555.38
END
©Paul R. Godin
prgodin°@ gmail.com
555.39