555 Timers

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Electronic Instrumentation

1

Experiment 7 Digital Logic Devices and the 555 Timer

Part A: Basic Logic GatesPart B: Flip FlopsPart C: Counters

Part D: 555 Timers

5V

1

2

+

-14161

345

6

710

921

141312

11

15

P1P2P3P4

PETELDCLKCL

Q1Q2Q3Q4

CO

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20 March 2007 Electronic Instrumentation 2

Part A Basic Logic Gates

Combinational Logic DevicesBoolean AlgebraDeMorgans Laws Timing Diagrams

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Combinational Logic DevicesLogic Gates perform basic logic operations, such asAND, OR and NOT, on binary signals.We can model the behavior of these chips byenumerating the output they produce for all possibleinputs.In order to show this behavior, we use truth tables ,

which show the output for all input combinations.The outputs of combinational logic gates depend onlyon the instantaneous values of the inputs.

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Logic Gates

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Logic Gate Example: XOR

Input Input OutputA B X0 0 0

0 1 11 0 11 1 0

Question: Whatcommonhouseholdswitchconfiguration

corresponds toan XOR?

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Boolean Algebra

The variables in a boolean, or logic, expression can takeonly one of two values, 0 (false) and 1 (true).

We can also use logical mathematical expressions to analyze binary operations, as well.
http://www.play-hookey.com/digital/derived_gates.htmlhttp://www.play-hookey.com/digital/derived_gates.htmlhttp://www.play-hookey.com/digital/derived_gates.htmlhttp://www.play-hookey.com/digital/basic_gates.htmlhttp://www.play-hookey.com/digital/basic_gates.htmlhttp://www.play-hookey.com/digital/basic_gates.html

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The basis of boolean algebra lies in the operations oflogical addition , or the OR operation, and logicalmultiplication , or the AND operation.

OR Gate If either X or Y is true (1), then Z is true (1)

AND Gate If both X and Y are true (1), then Z is true (1)

Logic gates can have an arbitrary number of inputs. Note the similarities to the behavior of the mathematical

operators plus and times.

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Laws of Boolean Algebra

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DeMorgans Laws

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Timing Diagrams sequential logic

When we deal with binary signals, we are not worried aboutexact voltages.

We are only concerned with two things: Is the signal high or low?

When does the signal switch states? Relative timing between the state changes of different binary

signals is much easier to see using a diagram like this.

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Part B Flip Flops

Sequential Logic DevicesFlip FlopsBy-Pass Capacitors

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Sequential Logic Devices

In a sequential logic device, the timing or sequencingof the input signals is important. Devices in thisclass include flip-flops and counters.

Positive edge-triggered devices respond to a low-to-high (0 to 1) transition, and negative edge-triggereddevices respond to a high-to-low (1 to 0) transition.

01 positive

edgepositive

edgenegative

edges

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Flip-Flops A flip-flop is a sequential device that can store and

switch between two binary states. It is called a bistable device since it has two and only

two possible output states: 1 (high) and 0 (low). It has the capability of remaining in a particular state

(i.e., storing a bit) until the clock signal and certaincombinations of the input cause it to change state.

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Simple Flip Flop Example: The RS Flip-Flop

Q = 1

Q = 0

Note that the output depends onthree things: the two inputs andthe previous state of the output.

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Inside the R-S Flip Flop

Note that the enable signal is the clock, which regularly pulses.This flip flop changes on the rising edge of the clock. It looks atthe two inputs when the clock goes up and sets the outputsaccording to the truth table for the device.

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Inside the J-K Flip Flop

Note this flip flop, although structurally more complicated, behavesalmost identically to the R-S flip flop, where J(ump) is like S(et) andK(ill) is like R(eset). The major difference is that the J-K flip flopallows both inputs to be high. In this case, the output switches stateor toggles.
http://www.play-hookey.com/digital/jk_nand_flip-flop.html

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By-Pass Capacitors

In a sequential logic device, a noisy signal can generateerroneous results.By-pass capacitors are placed between 5V and 0V to filterout high frequency noise.A by-pass capacitor should be used in any circuit involving asequential logic device to avoid accidental triggering.

V+

GND

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Part C: Counters

Binary NumbersBinary Counters

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Binary Decimal -- Hexadecimal Conversion10110101110001011001110011110110 binary number

11 5 12 5 9 12 15 6

B 5 C 5 9 C F 6

equivalent base 10 value foreach group of 4 consecutive

binary digits (bits)

corresponding hexadecimal(base 16) digit

B5C59CF6

equivalent hexadecimal

number

Decimal 8 = 1x2 3 + 0x2 2 + 0x2 1 +0x2 0 = 01000 in BinaryCalculator Applet
http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.htmlhttp://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html

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Binary CountersBinary Counters do exactly what it sounds like they should.They count in binary.Binary numbers are comprised of only 0s and 1s.

Decimal QD QC QB QA

0 0 0 0 01 0 0 0 12 0 0 1 03 0 0 1 1

4 0 1 0 05 0 1 0 1

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Binary Counters are made with Flip Flops

Each flip flop corresponds to one bit in the counter.Hence, this is a four-bit counter.

DC BA = 1100

DCB A = 1111
http://www.play-hookey.com/digital/synchronous_counter.htmlhttp://www.play-hookey.com/digital/synchronous_counter.htmlhttp://www.play-hookey.com/digital/synchronous_counter.htmlhttp://www.play-hookey.com/digital/synchronous_counter.html

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Typical Output for Binary Counter

Note how the Q outputs form 4 bit numbersDCBA = 1111DCBA = 1100

1100=12

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Part D: 555-Timers

The 555 TimerInside the 555-TimerTypes of 555-Timer CircuitsUnderstanding the Astable Mode CircuitModulationPulse Width Modulation

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The 555 Timer is one of the most popularand 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 widevariety 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

The 555 Timer

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555 Timer

Each pin has a function Note some familiar components inside

NE555

2

5

3

7

6

4 8

1

TR

CV

Q

DIS

TH R

R

V C C

G N D

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Inside the 555 Timer

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The voltage divider (blue) has three equal 5Kresistors. It divides the input voltage (Vcc) intothree equal parts.

The two comparators (red) are op-amps thatcompare the voltages at their inputs and saturatedepending upon which is greater.

The Threshold Comparator saturates when the voltageat the Threshold pin (pin 6) is greater than (2/3)Vcc.

The Trigger Comparator saturates when the voltage atthe Trigger pin (pin 2) is less than (1/3)Vcc

Inside the 555 Timer

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The flip-flop (green) is a bi-stable device. Itgenerates two values, a high value equal to Vccand a low value equal to 0V.

When the Threshold comparator saturates, the flip flop isReset (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 isclosed.

When Q is low, Qbar is high. This closes the transistorswitch and attaches pin 7 to ground.

When Q is high, Qbar is low. This open the switch and pin 7 is no longer grounded

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Types of 555-Timer Circuits

Astable Multivibrator puts out a continuoussequence of pulses

5V

Ra

C 0

. 0 1 u

F

LED

NE555

2

5

3

7

6

4 8

1

TR

CV

Q

DIS

THR

R

V C C

G N D

Rb

5V

1

2

1K

0 . 0

1 u

F

C

R

LED

NE555

2

5

3

7

6

4 8

1

TR

CV

Q

DIS

THR

R

V C C

G N D

Monostable Multivibrator(or one-shot) puts out one

pulse each time theswitch 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 Multivibrator One-Shot

C

R acc

2V

3

cc1

V3

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Behavior of the Monostable MultivibratorThe monostable multivibrator is constructed by adding anexternal capacitor and resistor to a 555 timer.The circuit generates a single pulse of desired durationwhen it receives a trigger signal, hence it is also called aone-shot.

The time constant of theresistor-capacitorcombination determinesthe length of the pulse.

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Used to generate a clean pulse of the correctheight and duration for a digital system

Used to turn circuits or external componentson 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 ofrelated 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

R 1

R 2

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Behavior of the Astable MultivibratorThe astable multivibrator is simply an oscillator. The astablemultivibrator generates a continuous stream of rectangular off-o

pulses that switch between two voltage levels.The frequency of the pulses and their duty cycle are dependentupon the RC network values.

The capacitor C charges through the series resistors R 1 and R 2with a time constant(R 1 + R 2)C.The capacitor dischargesthrough R 2 with a timeconstant of R 2C

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Flashing LEDs Pulse Width Modulation Pulse Position Modulation

Periodic Timers

Uses of the Astable Multivibrator

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Flashing LEDs

40 LED bicycle light with 20 LEDs flashingalternately at 4.7Hz

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Understanding the Astable Mode Circuit

555-Timers, like op-amps can be configured in different ways tocreate different circuits. We will now look into how this onecreates 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 flowingthrough R1

As the capacitor charges up, its voltage increasesand the current charging it decreases, resulting inthe charging rate shown

VV V

R1

1k

U2

TOPEN = 0

1

2C1

1uF

U1

TCLOSE = 0

1 2

0

V110V

I V V

RV

k CAPACITOR CAPACITOR

11

101

T i m e

0 s 1 m s 2 m s 3 m s 4 m s 5 m s 6 m s 7 m s 8 m s 9 m s 1 0 m s

V ( U2 :1 ) V ( R1 :2 ) V (V 1 :+ )

0V

2V

4V

6V

8V

10 V

C a p a c i t o r V o l t a g e

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Capacitor Charging Equations

Capacitor Current

Capacitor Voltage

Where the time constant

T i m e


I (R 1) I (C 1)

0A

2m A

4m A

6m A

8m A

1 0 m A

C a p a c i t o r a n d R e s i s t o r C u r r e n t

T i m e


V (U 2: 1) V (R 1: 2) V (V 1: +)

0V

2V

4V

6V

8V

10 V

C a p a c i t o r Vo l t a g e

I I eot

V V eot

1

RC R C ms1 1 1

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Understanding the equations

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

T i m e


V ( U 2 : 1 ) V ( R 1 : 2 ) V ( V 1 : + )

0 V

2 V

4 V

6 V

8 V

1 0 V

C a p a c i t o r Vo l t a g e

( ) .1 6321 e

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Capacitor Charging and Discharging

There is a good description of capacitor chargingand its use in 555 timer circuits athttp://www.uoguelph.ca/~antoon/gadgets/555/555.html
http://www.uoguelph.ca/~antoon/gadgets/555/555.htmlhttp://www.uoguelph.ca/~antoon/gadgets/555/555.html

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555 Timer At the beginning of thecycle, C1 is charged throughresistors R1 and R2. Thecharging time constant is

The voltage reaches

(2/3)Vcc in a time

1)21(693.01arg C R RT t ech

1)21(arg C R Rech

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555 Timer

When the voltage on thecapacitor reaches (2/3)Vcc,a switch (the transistor) isclosed (grounded) at pin 7.

The capacitor is dischargedto (1/3)Vcc through R2 toground, at which time theswitch is opened and thecycle starts over.

1)2(arg C Redisch

1)2(693.02arg C RT t edisch

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555 Timer

The frequency is then given by

f R R C R R C

10 693 1 2 2 1

1441 2 2 1. ( )

.( )

A i i

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Output voltage highturns off upper LEDand turns on lowerLED

Capacitor is charging through R a and R b

Output is high for0.693(R a+R b)C

555 Animation

http://www.williamson-labs.com/pu-aa-555-timer_slow.htm

555 A i i
http://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htmhttp://www.williamson-labs.com/pu-aa-555-timer_slow.htm

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Output is lowso the upperLED is on andthe lower LEDis off

Capacitor is dischargingthrough R b

Output is low for0.693(R b)C

555 Animation

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PWM: Pulse Width Modulation

Signal is compared to a sawtooth wave producing a pulse width proportional toamplitude

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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, thelonger the light bulb is on and the brighter

the light.

LowDuty Cycle

Medium

Duty Cycle

High

Duty Cycle

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What Can Be Done With PWM?

Average power can be controlledAverage flows can also be controlled by fully openingand closing a valve with some duty cycle

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555 Timers