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Electronic Instrumentation
Experiment 7 555 Timer
Part A:
Controlling Oscillation Frequency with Capacitors and Resistors
Part B: Diodes and Light
555 Timer



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The 555 Timer is one of the most popular
and versatile integrated circuits ever produced!
It is 30 years old and still being used!
It is a combination of digital and analog circuits.
It is known as the “time machine” as it performs a wide
variety of timing tasks.
Applications for the 555 Timer include:
• Bounce-free switches and Cascaded timers
• Frequency dividers
• Voltage-controlled oscillators
• Pulse generators and LED flashers
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7
DIS
8
V CC
R
4
555 Timer
6
2
5
THR
TR
CV
3
GND
Q
1
NE555


Each pin has a function, the meaning of
which will become clearer later.
Note some familiar components inside
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Inside the 555 Timer
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Inside the 555 Timer

You will learn more about these components later in
the course, for now just understand the following:
• The voltage divider has three equal 5K resistors. It
divides the input voltage (Vcc) into three equal parts.
• The two comparators are op-amps which compare the
voltages at their inputs and saturate depending upon
which is greater.
• The flip-flop is a bi-stable device. It generates two
values, a “high” value equal to Vcc and a “low” value
equal to 0V.
• The transistor is being used as a switch, it connects pin 7
(discharge) to ground when it is closed.
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Periodic Pulse Train from a 555 Timer

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.
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First we must examine how capacitors charge
10V
TCLOSE = 0
1
U1
R1
2
8V
V
V
1
V
1k
6V
U2
V1
TOPEN = 0
Voltage
C1
4V
2
10V
Capacitor
1uF
2V
0V
0
0s
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

CAPACITOR
R1

CAPACITOR
1k
As the capacitor charges up, its voltage increases
and the current charging it decreases, resulting in
the charging rate shown
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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
I  Ioe
 t

Capacitor Current

Capacitor Voltage V  Vo 1  e

Where the time constant

 t


  RC  R1 C1  1ms
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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
1
6V in 1ms.
(1  e ) .632
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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
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555 Timer

At the beginning of the cycle, C1 is charged through
resistors R1 and R2. The charging time constant is
  ( R1  R2)C1

The voltage reaches (2/3)Vcc in a time
  0.693( R1  R2)C1
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555 Timer

When the voltage on the capacitor reaches
(2/3)Vcc, a switch (the transistor) is closed at
pin 7 and the capacitor is discharged to
(1/3)Vcc, at which time the switch is opened and
the cycle starts over
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555 Timer

The capacitor voltage cycles back and forth
between (2/3)Vcc and (1/3)Vcc at times
and  1  0.693( R1  R2)C1
 2  0.693( R2)C1
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555 Timer

The frequency is then given by
1
144
.
f 

0.693( R1  2  R2)C1 ( R1  2  R2)C1
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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-555timer_slow.htm
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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
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Types of 555-Timer Circuits
5V
DIS
DIS
8
4
R
8
7
VCC
7
R
V CC
R
4
Ra
5V
1K

C
NE555
Astable Multivibrator
puts out a continuous
sequence of pulses
CV
GND
5
THR
TR
3
LED
NE555
1
LED
0.01uF
CV
Q
6
2
1
C
0.01 uF
5
THR
TR
3
1
6
2
GND
Q
2
Rb

Monostable Multivibrator
(or one-shot) puts out one
pulse each time the
switch is connected
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
Monostable Multivibrator (One Shot)
8
Vcc
Reset
R Threshold Comparator
Ra
2
Vcc
3
6
-
R
Q
S
Q
Output
3
-V
R
-
2
1
Vcc
3
7
+V
+
Trigger
C
4
+V
+
-V
Trigger Comparator
Control Flip-Flop
R
1
Monstable Multivibrator
One-Shot
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Behavior of the Monostable Multivibrator
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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.
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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.
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
Astable Pulse-Train Generator (Multivibrator)
Vcc
8
R Threshold Comparator
R1
R2
4
-
6
+V
+
R
Q
S
Q
Output
3
-V
R
-
2
+V
+
-V
Trigger Comparator
7
C
Control Flip-Flop
R
1
Astable Pulse-Train Generator
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Behavior of the Astable Multivibrator
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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
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Uses of the Astable Multivibrator
•
•
•
•
Flashing LED’s
Pulse Width Modulation
Pulse Position Modulation
Periodic Timers (see mushroom timer in the
experiment).
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Flashing LED’s

40 LED bicycle light with 20 LEDs flashing
alternately at 4.7Hz
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PWM: Pulse Width Modulation

Signal is compared to a sawtooth wave
producing a pulse width proportional to
amplitude
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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.
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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
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Pulse Position Modulation
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This is an optical transmitter.
Astable is used to produce carrier pulses at a frequency we
cannot hear (well above 20kHz)
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Behavior of the Pulse Position Modulator
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This application generates a continuous stream of
rectangular off-on pulses that switch between two
voltage levels, BUT they vary in width.
The frequency of the pulses and their duty cycle are
dependent upon the RC network values AND the value
of the input signal.
When a signal is
encoded like this, it
can be transmitted and
then decoded with
a receiver.
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Optical Receiver Circuit
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
The receiver takes the optical pulses, reconstructs the
signal, amplifies it, and plays it on a speaker.
You will build this circuit in project 2.
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Part B: Diodes and Light
• The Light-Emitting Diode
(LED) is a semiconductor
pn junction diode that
emits visible light or nearinfrared radiation when
forward biased.
• Visible LEDs emit
relatively narrow bands of
green, yellow, orange, or
red light. Infrared LEDs
emit in one of several
bands just beyond red
light.
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Facts about LED’s
• LEDs switch off and on rapidly, are very rugged
and efficient, have a very long lifetime, and are
easy to use.
• They are current-dependent sources, and their
light output intensity is directly proportional to
the forward current through the LED.
• Always operate an LED within its ratings to
prevent irreversible damage.
• Use a series resistor (Rs) to limit the current
through the LED to a safe value. Usually a 330
Ω resistor is used in series with an LED when
used with a 5V supply.
• VLED is the LED voltage drop. It ranges from
about 1.3 volts to about 2.5 volts.
Vin  VLED
Rs 
• ILED is the specified forward current.
I LED
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Photodiodes and Phototransistors
• Photodiodes are designed to detect photons and
can be used in circuits to sense light.
• Phototransistors are photodiodes with some
Photodiode Light-detector
internal amplification.
Circuit
Note:
Reverse current flows through the
+
photodiode when it is sensing light.
If photons excite carriers in a reverse- V
biased pn junction, a very small
current proportional to the light
intensity flows.
The sensitivity depends on the
wavelength of light.
I
R
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