TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES

College of Engineering

Department of Electronics Engineering

ADJUSTABLE EXAMINATION TIMER

(HOUR-MINUTES-SECONDS DISPLAY) WITH BUZZER

Proponents:Calderon, Noel A.

Calderon, Rhegine Raymond P.Callejo, Darwin C.

Calites, John Michael G.Cerilo, Raymond M.

Rivera, Eldrin Hanz D.Sanchez, Ruben III B.

Solito, Marizun L.Sta Ana, Reymond F.

Tejero, Jefrey G.

Engr. Nilo M. Arago

Instructor

Calderon, Noel A.

Calderon, Rhegine Raymond P.

Calites, John Michael G.

Callejo, Darwin C.

Cerilo, Reymond M.

Rivera, Eldrin Hanz D.

Sanchez, Ruben III B.

Solito, Marizun L.

Sta. Ana, Reymond F.

Tejero, Jefrey G.

ADJUSTABLE EXAMINATION TIMER

(HOUR-MINUTES-SECONDS DISPLAY) WITH BUZZER

DESCRIPTION:

A countdown timer for examination with hour, minutes and seconds display in which

the examiner can adjust the period of time (hour and minutes). The time will be display with 5

seven segment display, 1 for hour, 2 for minutes, and 2 for seconds. The buzzer will alarm if the

time left is 5 minutes. This is to remind the examinees, how much time is left. When the time

period is finished, another buzzer would be heard to remind the examiner and the examinees

that the exam is over.

I. OBJECTIVES

1. To be able to construct an adjustable digital countdown timer that has various

applications such as a time management device for students during

examinations.

2. To implement the appropriate logic circuit for proper timer operation.

3. To verify the theory presented regarding logic gates, logic circuits and switching,

and Integrated circuits for digital applications.

4. To be familiarized with pin configurations and proper application of specific

integrated circuits related to this project.

5. To practice troubleshooting logic circuits.

II. MATERIALS AND EQUIPMENT

1. Seven Segment Display

2. 74192 IC (5 pieces)

3. 7404 inverter IC (1 piece)

4. 7402 NOR IC (3 pieces)

5. 7400 NAND IC (1 piece)

6. 7408 AND IC(1 piece)

7. 7432 OR IC (2 pieces)

8. 7447 IC (5 pieces)

9. 7474 IC (2 pieces)

10. NE555 IC (3 pieces)

11. LM7805 IC

12. LM7809 IC

13. IC socket

14. Hi-rel (High-reliability) connectors

15. 2200microFarad 10V capacitor (2 pieces)

16. 0.33microFarad 10V capacitor (2 pieces)

17. Transformer 500mA Multitapped

18. Fuse 0.5A

19. Buzzer

20. Presensitized Circuit Board

21. PCB Cutter

22. Circuit Artwork

23. Exposure Box

24. Ferric Chloride Solution

25. Tray for Ferric Chloride Solution

26. Mini-drill

27. Soldering Iron

28. Soldering Lead

29. Desoldering pump

30. Wire cutter

31. Digital Logic Trainer (for troubleshooting)

32. Connecting wires

33. Push buttons

34. Plywood for the casing

35. Hand saw

36. Electric drill

37. Wood file

38. Nails and hammer

39. AC cord with rubber stopper

III. INTRODUCTION

Timers are instruments used to measure time interval, which is defined

as the elapsed time between two events. One common example of a time interval is

our age, which is simply the elapsed time since our birth. Unlike a conventional

clock that displays time-of-day as hours, minutes, and seconds from an absolute

epoch or starting point (such as the beginning of the day or year), a stopwatch or

timer simply measures and displays the time interval from an arbitrary starting point

that began at the instant the stopwatch was started. The standard unit of time

interval is the second (s). Seconds can be accumulated to form longer time intervals,

such as minutes, hours, and days; or they can be sliced into fractions of a second

such as milliseconds (10-3s, abbreviated as ms) or microseconds (10-6s, abbreviated

as µs). The second is one of the seven base units in the International System of Units

(SI).

Timers, count down from a preset time period instead of counting up

from zero. They can be small, battery-operated devices that are used to signal when

a certain time period has elapsed, or they can be larger devices that plug into a wall

outlet and control other items.

IV. THEORY

Basic Digital Logic Gates

A logic gate is an elementary building block of a digital circuit . Most logic

gates have two inputs and one output. At any given moment, every terminal is in

one of the two binary conditions low (0) or high (1), represented by different voltage

levels. The logic state of a terminal can, and generally does, change often, as the

circuit processes data. In most logic gates, the low state is approximately zero volts

(0 V), while the high state is approximately five volts positive (+5 V). There are seven

basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.

AND gate

The AND gate is so named because, if 0 is called "false" and 1 is called "true," the

gate acts in the same way as the logical "and" operator. The following illustration

and table show the circuit symbol and logic combinations for an AND gate. (In the

symbol, the input terminals are at left and the output terminal is at right.) The

output is "true" when both inputs are "true." Otherwise, the output is "false."

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 0

1 1 1

OR gate

The OR gate gets its name from the fact that it behaves after the fashion of the

logical inclusive "or." The output is "true" if either or both of the inputs are "true." If

both inputs are "false," then the output is "false."

Input 1 Input 2 Output

0 0 0

0 1 1

1 0 1

1 1 1

XOR gate

The XOR ( exclusive-OR ) gate acts in the same way as the logical "either/or." The

output is "true" if either, but not both, of the inputs are "true." The output is "false"

if both inputs are "false" or if both inputs are "true." Another way of looking at this

circuit is to observe that the output is 1 if the inputs are different, but 0 if the inputs

are the same.

Input 1 Input 2 Output

0 0 0

0 1 1

1 0 1

1 1 0

 Inverter or NOT gate

A logical inverter , sometimes called a NOT gate to differentiate it from other types

of electronic inverter devices, has only one input. It reverses the logic state.

Input Output

1 0

0 1

NAND gate

The NAND gate operates as an AND gate followed by a NOT gate. It acts in the

manner of the logical operation "and" followed by negation. The output is "false" if

both inputs are "true." Otherwise, the output is "true."

Input 1 Input 2 Output

0 0 1

0 1 1

1 0 1

1 1 0

NOR gate

The NOR gate is a combination OR gate followed by an inverter. Its output is "true" if

both inputs are "false." Otherwise, the output is "false."

Input 1 Input 2 Output

0 0 1

0 1 0

1 0 0

1 1 0

XNOR gate

The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter.

Its output is "true" if the inputs are the same, and"false" if the inputs are different.

Input 1 Input 2 Output

0 0 1

0 1 0

1 0 0

1 1 1

Here is an illustration of alternative gates that can be used as substitute.

The bubble indicates an inverter.

Seven-Segment Display

A seven-segment display (SSD), or seven-segment indicator, is a form of

electronic display device for displaying decimal numerals that is an alternative to the more

complex dot-matrix displays. Seven-segment displays are widely used in digital clocks,

electronic meters, and other electronic devices for displaying numerical information.

A seven segment display, as its name indicates, is

composed of seven elements. Individually on or off, they

can be combined to produce simplified representations of

the arabic numerals. Often the seven segments are

arranged in an oblique (slanted) arrangement, which

aids readability. In most applications, the seven

segments are of nearly uniform shape and size (usually

elongated hexagons, though trapezoids and rectangles can also be used), though in the

case of adding machines, the vertical segments are longer and more oddly shaped at the

ends in an effort to further enhance readability.

Each of the numbers 0, 6, 7 and 9 may be represented by two or more different

glyphs on seven-segment displays.

The seven segments are arranged as a rectangle of two vertical segments on

each side with one horizontal segment on the top, middle, and bottom. Additionally, the

seventh segment bisects the rectangle horizontally. There are also fourteen-segment

displays and sixteen-segment displays (for full alphanumerics); however, these have

mostly been replaced by dot-matrix displays.

The segments of a 7-segment display are referred to by the letters A to G, as

shown to the right, where the optional DP decimal point(an "eighth segment") is used for

the display of non-integer numbers.

Numbers to Seven Segment Code

A single byte can encode

the full state of a 7-segment-display.

The most popular bit encodings

are gfedcba and abcdefg - both

usually assume 0 is off and 1 is on.

This table gives the

hexadecimal encodings for

displaying the digits 0 to F:

Digit gfedcba abcdefg a b c d e f g

0 0x3F 0x7E on on on on on on off

1 0x06 0x30 off on on off off off off

2 0x5B 0x6D on on off on on off on

3 0x4F 0x79 on on on on off off on

4 0x66 0x33 off on on off off on on

5 0x6D 0x5B on off on on off on on

6 0x7D 0x5F on off on on on on on

7 0x07 0x70 on on on off off off off

8 0x7F 0x7F on on on on on on on

9 0x6F 0x7B on on on on off on on

A 0x77 0x77 on on on off on on on

b 0x7C 0x1F off off on on on on on

C 0x39 0x4E on off off on on on off

d 0x5E 0x3D off on on on on off on

E 0x79 0x4F on off off on on on on

F 0x71 0x47 on off off off on on on

74192 Up/Down Decade (0-9) Counter

74193 Up/Down 4-Bit (0-15) Counter

These are synchronous counters so their outputs change precisely together

on each clock pulse. This is helpful if you need to connect their outputs to logic gates

because it avoids the glitches which occur with ripple counters.

These counters have separate clock inputs for counting up and down. The

count increases as the up clock input becomes high (on the rising-edge). The count

decreases as the down clock input becomes high (on the rising-edge). In both cases

the other clock input should be high. For normal operation (counting)

the preset input should be high and the reset input low. When the reset input is high it

resets the count to zero (0000, QA-QD low)

The counter may be preset by placing the desired binary number on

the inputs A-D and briefly making the preset input low. Note that a clock pulse is not

required to preset, unlike the 74160-3 counters. The inputs A-D may be left

unconnected if not required.

Connecting counters with separate up

and down clock inputs in a chain

The diagram below shows how to link

74192-3 up/down counters with

separate up and down clock inputs,

notice how carry and borrow are

connected to

the up clock and down clock inputs

respectively of the next counter.

7447 Bcd To 7-Segment Display Driver

The appropriate outputs a-g become low to display the BCD (binary coded

decimal) number supplied on inputs A-D. The 7447 has open collector outputs a-g

which can sink up to 40mA. The 7-segment display segments must be connected

between +Vs and the outputs with a resistor in series (330 with a 5V supply).

A common anode display is required.

Display test and blank input are active-low so they should be high for normal

operation. When display test is low all the display segments should light (showing

number 8).

If the blank input is low the display will be blank when the count input is zero

(0000). This can be used to blank leading zeros when there are several display digits

driven by a chain of counters. To achieve this blank output should be connected

to blank input of the next display down the chain (the next most significant digit).

The 7447 is intended for BCD (binary coded decimal) which is input values 0

to 9 (0000 to 1001 in binary). Inputs from 10 to 15 (1010 to 1111 in binary) will light

odd display segments but will do no harm.

7474 - Dual D Flip-Flop with Preset and Clear

Pin Number Description

1 Clear 1 Input

2 D1 Input

3 Clock 1 Input

4 Preset 1 Input

5 Q1 Output

6 Complement Q1 Output

7 Ground

8 Complement Q2 Output

9 Q2 Output

10 Preset 2 Input

11 Clock 2 Input

12 D2 Input

13 Clear 2 Input

14 Positive Supply

D flip-flop

The D flip-flop is the most common flip-flop in use today. It is better known

as data or delay flip-flop (as its output Q looks like a delay of input D).

Truth table:

Clock D Q Qprev

Rising edge 0 0 X

Rising edge 1 1 X

Non-Rising X Qprev

('X' denotes a Don't care condition, meaning the signal is irrelevant)

The Q output takes on the state of the D input at the moment of a positive edge at the

clock pin (or negative edge if the clock input is active low).[23] It is called the D flip-flop

for this reason, since the output takes the value of the D input or data input,

and delays it by one clock cycle. The D flip-flop can be interpreted as a primitive

memory cell, zero-order hold, or delay line. Whenever the clock pulses, the value of

Qnext is D and Qprev otherwise.

Most D-type flip-flops in ICs have the capability to be forced to the set or reset state

(which ignores the D and clock inputs), much like an SR flip-flop. Usually, the illegal S =

R = 1 condition is resolved in D-type flip-flops. By setting S = R = 0, the flip-flop can be

used as described above.

Inputs Outputs

S R D > Q Q'

0 1 X X 0 1

1 0 X X 1 0

1 1 X X 1 1

555 Timer IC

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse

generation and oscillator applications. The part is still in widespread use, thanks to its

ease of use, low price and good stability. As of 2003, it is estimated that 1 billion units

are manufactured every year.

Pin Name Purpose

1 GND Ground, low level (0 V)

2 TRIG OUT rises, and interval starts, when this input falls below 1/3 VCC.

3 OUT This output is driven to + V CC or GND.

4 RESET A timing interval may be interrupted by driving this input to GND.

5 CTRL "Control" access to the internal voltage divider (by default, 2/3 VCC).

6 THR The interval ends when the voltage at THR is greater than at CTRL.

7 DIS Open collector output; may discharge a capacitor between intervals.

8 V+, VCC Positive supply voltage is usually between 3 and 15 V.

The 555 has three operating modes:

1. Monostable

Monostable mode: in this mode, the 555 functions as a "one-shot" pulse

generator. Applications include timers, missing pulse detection, bouncefree switches,

touch switches, frequency divider, capacitance measurement, pulse-width

modulation (PWM) and so on.

In the monostable mode, the 555 timer acts as a “one-shot” pulse generator.

The pulse begins when the 555 timer receives a signal at the trigger input that falls

below a third of the voltage supply. The width of the output pulse is determined by

the time constant of an RC network, which consists of a capacitor (C) and a resistor (R).

The output pulse ends when the voltage on the capacitor equals 2/3 of the supply

voltage. The output pulse width can be lengthened or shortened to the need of the

specific application by adjusting the values of R and C.[5]

The output pulse width of time t, which is the time it takes to charge C to 2/3

of the supply voltage, is given by

where t is in seconds, R is in ohms and C is in farads. See RC circuit for an explanation

of this effect.

While using the timer IC as a monostable the main disadvantage is that the

time span between the two triggering pulses must be greater than the RC time

constant.

2. Astable

Astable – free running mode: the 555 can operate as an oscillator. Uses

include LED and lamp flashers, pulse generation, logic clocks, tone generation, security

alarms, pulse position modulation and so on. Selecting a thermistor as timing resistor

allows the use of the 555 in a temperature sensor: the period of the output pulse is

determined by the temperature. The use of a microprocessor based circuit can then

convert the pulse period to temperature, linearize it and even provide calibration

means.

In astable mode, the 555 timer puts out a continuous stream of rectangular

pulses having a specified frequency. Resistor R1 is connected between VCC and the

discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin

(pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node.

Hence the capacitor is charged through R1 and R2, and discharged only through R2,

since pin 7 has low impedance to ground during output low intervals of the cycle,

therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1,

R2 and C:

The high time from each pulse is given by

and the low time from each pulse is given by

where R1 and R2 are the values of the resistors in ohms and C is the value of the

capacitor in farads.

Note 1: The power capability of R1 must be greater than .

Note 2: particularly with bipolar 555s, it is essential to avoid low values of

R1 as the discharge current capability of the IC is limited. If the limit is reached, the

discharging output DIS will not be saturated (ie not essentially at GND as assumed for

the timing calculations), so the capacitor voltage will not discharge from

0.67*Vcc towards 0, and the output low time will be greater than as given in the above

equation.

To achieve a duty cycle of less than 50% a diode can be added in parallel with

R2 towards the capacitor. This bypasses R2 during the high part of the cycle so that the

high interval depends only on R1 and C.

3. Bistable

Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS

pin is not connected and no capacitor is used. Uses include bounce free latched

switches.

In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset

inputs (pins 2 and 4 respectively on a 555) are held high via Pull-up resistors while the

threshold input (pin 6) is simply grounded. Thus configured, pulling the trigger

momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to Vcc (high

state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin

to ground (low state). No capacitors are required in a bistable configuration. Pin 5

(control) is connected to ground via a small-value capacitor (usually 0.01 to 0.1 uF); pin

7 (discharge) is left floating.

7805 (5 Volt), 7809 (9 Volt) Voltage Regulator

The 78xx (sometimes LM78xx) is a family of self-contained fixed linear

voltage regulator integrated circuits. The 78xx family is commonly used in electronic

circuits requiring a regulated power supply due to their ease-of-use and low cost. For

ICs within the family, the xx is replaced with two digits, indicating the

output voltage (for example, the 7805 has a 5 volt output, while the 7812 produces 12

volts). The 78xx line are positive voltage regulators: they produce a voltage that is

positive relative to a common ground. There is a related line of 79xx devices which are

complementary negative voltage regulators. 78xx and 79xx ICs can be used in

combination to provide positive and negative supply voltages in the same circuit.

78xx ICs have three terminals and are commonly found in the TO220 form

factor, although smaller surface-mount and larger TO3 packages are available. These

devices support an input voltage anywhere from a couple of volts over the intended

output voltage, up to a maximum of 35 or 40 volts, and typically provide 1 or

1.5 amps of current (though smaller or larger packages may have a lower or higher

current rating).

There are common configurations for 78xx ICs, including 7805

(5 volt), 7806 (6 volt), 7808 (8 volt), 7809 (9 volt), 7810 (10 volt), 7812 (12 volt), 7815

(15 volt), 7818 (18 volt), and 7824 (24 volt) versions. The 7805 is common, as its

regulated 5 volt supply provides a convenient power source for most TTL components.

V. PROCEDURES

SIMULATION

1. Design a circuit that would produce an adjustable countdown timer.

2. Simulate the circuit using multisim to verify its operation.

3. If the circuit is not running well, troubleshoot problems until the circuit is running.

PCB DESIGN

1. Do the PCB circuit design using Express PCB or manually design the PCB using marker or

tape.

2. Be sure that the connection during simulation is identical to the PCB design.

3. If the circuit is done, print the circuit in acetate for exposure.

4. Place the acetate on the top of the presensitized PCB and exposed the circuit using a

lamp (incandescent lamp preferably) for 10 minutes.

5. After exposing, dilute the developer to the water (2-3 parts) and put the exposed PCB in

the solution for 6 t0 8 minutes or until the circuit design is visible.

6. Then, place the presensitized PCB in a ferric chloride solution for etching purposes until

the remaining copper that is not part of the circuit is dissolved.

7. Wash the presensitized PCB with water.

COMPONENTS PLACEMENT

1. Drill the PCB design suitable for the components size. The component must fit the holes

so proper drill bit must be used.

2. Place the components on the PCB in a manner that is identical to the circuit design.

Observe polarity and double check if the component is mounted on the right place.

3. Using soldering iron and lead solder the components in the PCB.

4. Use multimeter to check if the circuit is correctly connected. Check if coppers which

should be connected to copper but is not connected or connected coppers but should

not be connected.

CIRCUIT CONNECTION AND CHASSIS PLACEMENT

1. Components outside the PCB or unmounted components should be connected including

its subparts, power supplies, etc.

2. When everything is connected run the whole circuit to see if there is a possible problem.

3. Place the whole circuit in a chassis and again try to run the whole circuit.

VI. CONCLUSION

Whether you have a year to prepare for an exam or a week to cram and

study, having an countdown timer during examinations can help students be reminded

of how much time is only left. Seeing the timer approaches to zero triggers a student’s

adrenaline rush which aids to finish the work he/she is doing more quickly and more

eagerly. Being unaware of the time left might put anyone to a comfortable zone, not

doing his/her task quickly, not knowing that time is running out; or it might put anyone

to a cram situation, doing his/her task too rapidly without focus, not knowing that there

is still time left. Having a timer, time management will indeed be better. This project

helps not only the students but also examiners as well.

We have implemented the appropriate logic circuit for proper timer

operation. We also have verified the theory presented. We have been familiarized with

pin configurations and proper application of specific integrated circuits related to this

project. We have also practiced troubleshooting logic circuits.

We conclude that this project is indeed suitable for Electronics Engineering

students, especially to those taking up Digital Logic Circuits and Switching Theory. By

accomplishing this project, we now appreciate the theory more. We have now seen an

exact useful application of our lessons such as digital logic gates, logic circuit designing,

digital timers, BCD to Seven Segment decoding, and a lot more. Being able to construct a

useful electronic device is indeed fascinating.

VII. RECOMMENDATION

Though this project primarily focuses on examination timer as its title implies, modification of this project can lead to other apllications as well. A parking meter for example. Inserting a coin starts the internal timer counting down from an initial preset

point. When the time has elapsed, the “EXPIRED” flag is raised. One type of timer used extensively in industry is the process control timer.

Another example, when a product is made, it may need to be heat treated for a specific length of time. In an automated manufacturing system, the process control timer determines the amount of time that the item is heated. In some applications, such as integrated circuit manufacturing, the timing process can be critical for proper operation. Process control timers are also used in many different types of laboratory environments. Calibration laboratories use timers to calibrate units such as radiation detectors, where they regulate the amount of time the detector is exposed to the radiation source. Any uncertainty in the time of exposure directly influences the uncertainty of the detector calibration.

Timers are also used in the medical field. For example, medical laboratories use process control timers when specimen cultures are grown. Hospitals use timers to regulate the amount of medication given to patients intravenously

Many types of timing devices are used every day in commercial applications. Parking meters, automatic car wash facilities, taxicab meters, and commercial parking lots are examples of entities that either charge a certain amount for a specified period or provide a certain period of service for a specified amount.

For further studies or any modification on how the timer should be used in a system, the proponents suggest that this project will serve as a backbone for further studies that is related to timing circuits

VIII. REFERENCES

1. http://www.kpsec.freeuk.com/components/74series.htm#7447

2. http://www.futurlec.com/IC7474.html

3. http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/nand5.gif

4. http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/gate2.gif

5. http://www.electrokits.com/downloads/pdf/7805-datasheet-fairchild.pdf

6. http://www.wikipedia.org/

7. http://www.google.com.ph/