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Electronic InstrumentationExperiment 7 555 Timer

Part A: Controlling Oscillation Frequency with Capacitors and ResistorsPart B: Diodes and Light

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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

555 Timer

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555 Timer

Each pin has a function, the meaning of which will become clearer later.

Note some familiar components inside

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

4

Inside the 555 Timer

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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.

Inside the 555 Timer

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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

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 IT O R C A P A C IT O R

1

1

1 0

1

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

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Capacitor Charging Equations

Capacitor Current

Capacitor Voltage

Where the time constant

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msI(R1) I(C1)

0A

2mA

4mA

6mA

8mA

10mA

Capacitor and Resistor Current

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

I I eo

t

V V eo

t

1

R C R C m s1 1 1

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Understanding the equations

Note that the voltage rises to a little above 6V in 1ms.

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

( ) .1 6 3 21 e

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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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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

555 Timer

( )R R C1 2 1

0 6 9 3 1 2 1. ( )R R C

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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 6 9 3 1 2 1 . ( )R R C

2 0 6 9 3 2 1 . ( )R C

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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. ( )

.

( )

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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

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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

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Types of 555-Timer Circuits

Astable Multivibrator puts out a continuous sequence of pulses

5V

Ra

C

0.01

uF

LED

NE555

2

5

3

7

64 8

1TR

CV

Q

DIS

THRR

VC

CG

ND

Rb

5V

12

1K

0.01

uFC

R

LED

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

Monostable Multivibrator (or one-shot) puts out one pulse each time the switch is connected

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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

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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.

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• 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

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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

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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

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• Flashing LED’s• Pulse Width Modulation• Pulse Position Modulation• Periodic Timers (see mushroom timer in the

experiment).

Uses of the Astable Multivibrator

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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?

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

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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

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

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

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 near-infrared 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.

• ILED is the specified forward current.

in LEDs

LED

V VR

I

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34

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• Photodiodes are designed to detect photons and can be used in circuits to sense light.

• Phototransistors are photodiodes with some internal amplification.

IV

+

R

Photodiode Light-detectorCircuit

Note:Reverse current flows through the photodiode when it is sensing light.

If photons excite carriers in a reverse-biased pn junction, a very small current proportional to the light

intensity flows.The sensitivity depends on the

wavelength of light.

Photodiodes and Phototransistors