Project Technical Description

Project Technical Description (Hardwares & Softwares)

In this design project, Toilet Seat Integrated with Weight Scale, both the hardwares and

softwares are equally important towards making the project a success. Without the hardwares,

the softwares will not be at any use and similarly, without the softwares, the hardwares will not

be able to complete to project by itself. A complete listing of hardwares and components

required in this project are stated clearly in Chapter 5, Section 5.2.1 Final Production Cost (page

xx). From the final design block diagram shown in Chapter 2, Section 2.5.1 Block Diagram of

Final Design Solution (page xx), basically the upper half of the design process are dealing with

the hardware parts while the lower half of the design process involved more softwares and

programming. In this chapter, all the efforts and works done relating the hardwares and softwares

are explained in details.

Hardware Required in The Project

The hardwares required in this project are referring to the circuitry involved in the whole

project. The hardwares in this project can be generally divided into 3 parts which are the:

a) Strain Gauge Sensors

2 Strain gauges per sensor (4 strain gauge sensors used)

Resistors for Wheat Stone Bridge configuration with the strain gauge sensors

b) Weighing Circuit

Power Supply (Voltage Regulators, Batteries, Capacitors)

Instrumentation amplifiers (Operational Amplifier LM358)

Summing amplifier (Operational Amplifier LM358)

Inverting amplifier (Operational Amplifier LM358)

Simple Filter and Rectifier (Capacitor and Diode)

c) Control and Display Panel Circuit

Power Supply (Voltage Regulator, Batteries, Capacitors)

Liquid Crystal Display (LCD) JHD162A

4 x 3 Matrix Keypad

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Microcontroller (Microchip PIC16F877A)

The Strain Gauge Sensors

The strain gauge sensors used in this project are ready-made sensors which are taken from

a digital weight scale itself. Inside each of the sensors there are 2 strain gauges attached together

back-to-back. Each strain gauge has 2 legs and one of the legs of the 2 strain gauges are tied

together to form a common node. Then these strain gauges are mounted on a piece of nicely weld

metal forming a complete strain gauge sensor. From the appearance of the sensor, it appeared to

be a strain gauge sensor with 3 legs. These strain gauge sensors work best with Wheatstone

Bridge configuration to provide a difference output voltage when force or stress is applied on

them.

Figure : Bottom view of the sensor Figure : Top view of the sensor

General strain gauge sensor with

Wheatstone bridge configuration

Theoretically,

When ଵோଵ ൌ ଶோଶ , the output voltage, ο = 0V.

But, when ଵோଵ ଶோଶ, the output voltage, ο = certain value depending on the





force applied on the sensor.

Figure : Wheatstone bridge of a

typical strain gauge

Figure : Configuration of the strain

gauge sensor with the Wheatstone

bridge

How the strain gauge sensor work?

In one sensor, there are actually 2 strain gauges embedded in it. The 2 strain gauges will

act as 2 different resistive devices and to be connected in the Wheatstone bridge connection.

When force is applied on the sensor, one strain gauge will experience expansion while the other

counterpart will experience compression. Due to this expansion and compression, the resistance

of the strain gauges will vary or change. Strain gauge that experience expansion (the bottom

strain gauge), its resistance will decrease while strain gauge that undergoes compression, its

resistance will increase. These changes in resistance will result in an imbalance bridge and thus a

difference output will be produced. Theoretically, the output voltage of the strain gauge sensor

should be directly proportional to the force (users weight) applied on it.

The Power Supply

In this project, the power sources used are 9V batteries which are easily available in the

market. However, 9V supply most of the time are just too high for most electrical devices or

component to work with as it will supply a lot of current to the device which might in turn spoil





the device such as the Microchip PIC16F877A. Details about the voltage and current rating of

the microcontroller are explained later in the Section 3.1.5 The Microchip PIC16F877A.

Figure : The schematic of the

power supply circuit which

regulate a 9V supply into 5V

output source to the overall

circuits of the project.

Since the microcontroller used in this project works best in 5V power supply condition,

the whole circuitries of this project are running under 5V source. In order to regulate a 9V supply

from batteries to 5V, the 7805 voltage regulators are used. The connections and schematic of the

voltage regulator with the 9V supply are shown as in Figure xxxxx. Generally, the input voltage

must be at least 2V higher than the desired output voltage, so a 7805 regulator would require

about 7V to work properly. The voltage regulator has a metal plate with a hole in it which is

intended for attaching a heat sink to it if the input voltage is very high (much higher than 9V).

However, for our project, the input voltage of 9V is just fine for the voltage regulator to work

with, without need to attach any heat sink to it. 2 capacitors are connected across the terminals of

the voltage regulator to increase its stability and transient response.





Figure : 9V battery supply Figure : 7805 Voltage regulator with

2 capacitors across its terminal

The Operational Amplifier LM358

Op-amp LM 358 is a low power dual operational amplifier. This circuit consists of two

independent, high gain, internally frequency compensated which were designed specifically to

operate from a single power supply over a wide range of voltages. The low power supply drain is

independent of the magnitude of the power supply voltage.

One of the reasons LM358 is preferred over uA741 is because LM358 uses CMOS

technology while the uA741 (which was proposed in the proposal) is only a TTL operational

amplifier. TTL devices consume substantially more power than equivalent CMOS devices at

rest. Moreover, at low voltage supply (approximately 5V), the uA741 cannot function at its best.

The uA741 requires higher voltage supply which is approximately 9V to work properly.

However, the power source of the overall circuit is only 5V, therefore, LM358 which can work

well at low voltage power supply is used. Moreover, the LM358 has higher internally frequency

compensation compare to uA741. In general, the LM358 can operates from the range 3V to 32V

voltage supply which suit this project just fine. The range of temperatures where this op-amp can

operates is from 0°C to 70°C as stated in its datasheet.

This LM358 also has very low input offset voltage (approximately 2 mV) and low input

offset current (approximately 2 nA). The input common-mode voltage range includes ground and

the differential input voltage range equal to the power supply voltage. Maximum power

dissipation rating of the LM358 is approximately 500mW

In this project, the op-amp LM358 is used to build the instrumentation amplifier, the

summing amplifier and the inverting amplifier which will be explained later in this report.

Therefore, the LM358 plays a major role in the weighing circuit of this project. Any

malfuctionality will bring failure to the project.

Pin Connections of LM358:





AN001

Figure 20: General Op-Amp LM358

The LCD JHD162A

The Model JHD 162A Series LCD is the typical standard HD44780 type of LCD with 16

characters x 2 row LCD module that can be easily found in the market. Since this project, Toilet

Seat Integrated with Weight Scale requires the final product to display the weight of the user,

therefore, a LCD module is necessary. LCD module is preferred over seven-segment display is

because it can display more information (can display more characters) than typical seven-

segment-display and does not require complex software interfacing.

The LCD is used to display all the information that is needed to show the user such as

step-by-step guide that teaches the user on how to operate the product accordingly. This LCD

module is also another essential device or hardware of this project which cannot be left out.

The ratings of this LCD model is VDD = 5V, Vss = 0V and operating temperature is

approximately 25°C. Therefore, this is one of the reasons why the operating voltage of 5V is

chosen in this project. The ideal operating condition for the LCD is VDD = 5V, IDD = 1.5mA.

Pin Configuration of the LCD: 1 2 3 4 5 6 7 8 9 0 11 12 13 14 15 1

6

Vs

s

Vc

c

Ve

e

R

S

R/

W

E

N

DB

0

DB

1

DB

2

DB

3

DB

4

DB

5

DB

6

DB

7

L

+

L-





Figure : The LCD JHD162A used as display in this project

The Keypad

A keypad is simply an array of push buttons connected in rows and columns, so that each

can be tested for closure with minimum number of connections. There are 12 keys on a phone

type pad (0-9, #,*), arranged in a 3×4 matrix. The columns are labeled 1, 2, 3 and the rows A, B,

C, D, If we assume that all the rows and columns are initially high, a keystroke can be detected

by setting each row in turn and checking each column for zero.

In this project, the keypad is used as an input device

which enables the user to key in his or her height into the system

so that after his or her weight is measured, the BMI (body mass

index) can be calculated. Some of the buttons of the keypad is

modified to suit usage of our design such as the * key is used

as the CANCEL button while the # key is used as the

ENTER button.

Figure : 4x3 matrix keypad used in this project

The Microchip PIC16F877A

The microcontroller used for this project is the 40-pin Microchip PIC16F877A which has

the built-in ADC capability. Basically the PIC16F877A is the core of the software part of the

project. Without this microcontroller, the program for the whole project cannot be executed. One

of the functions of this microcontroller is used to convert analog input from the weighing circuit

into digital values (ADC) so that the weight of the user can be calculated. This microcontroller is

also the main link that connects the LCD, keypad and the weighing circuit together forming the

final product of our design.

All the processing and the calculation parts of the project are basically done at the

microcontroller section. At the microcontroller, it will receive height input from user, process his





or her weight based from output of the weighing circuit, calculate the BMI and send data to the

LCD for display purposes.

Below are the absolute maximum ratings of the PIC16F877A:

a) Total power dissipation ---------- 1.0W b) Maximum current out of VSS ---------- 300 mA c) Maximum current into VDD pin ---------- 250 mA d) Maximum output current sunk by any I/O pin ---------- 25 mA e) Maximum output current sourced by any I/O ---------- 25 mA f) Maximum current sunk or sourced by PORTA, PORTB and PORTE ---------- 200

mA g) Maximum current sunk or sourced by PORTC and PORTD ---------- 200 mA

The PIC16F877A operating voltage range is from 4.0V to 5.5V and the operating

frequency range is from 1Hz to 20MHz (optimum). Meanwhile, the operating temperature range

for microcontroller is -40°C to 85°C.

Figure : The 40-pin Microchip PIC16F877A

Hardware Design Implementations

The Instrumentation Amplifier





Figure

The instrumentation amplifier in

instrumentation amplifier. This circuit is constructed from a buffered amplifier stage with three

resistors linking the two buffer circuits together. If consider R

feedback of the upper left op-amp causes the

Likewise, the voltage at the point below R

across Rgain is same as the voltage difference between V

current through Rgain, and since the feedback loops of the two input op

that same amount if current through R

produces the voltage drop between both R

side of the circuit then takes this voltage drop, and amplifies it by a gain of 1 if R

This will come out the equation as below.

The voltage gain can be easily changed by alternating the R

The resistors R2 and R3 are used to calculate the common mode rejection ratio (CMMR).

The common mode rejection ratio (CMMR) is refined as the ratio of the powers of the

differential gain over the common mode gain, measured in positive decibels. The CMMR

Figure : General instrumentation amplifier

The instrumentation amplifier in the Figure above is the most


resistors linking the two buffer circuits together. If consider R3 is equal to R2, then the negative

amp causes the voltage at the point at the top of Rgain

Likewise, the voltage at the point below Rgain is equal to V2. This establishes a voltage drop

is same as the voltage difference between V1 and V2. The voltage drop causes a

, and since the feedback loops of the two input op-amps draw no current,

mount if current through Rgain must be going through the R2 above and below it. This

produces the voltage drop between both R2. The regular differential amplifier on the right

side of the circuit then takes this voltage drop, and amplifies it by a gain of 1 if R

This will come out the equation as below.

The voltage gain can be easily changed by alternating the Rgain . The gain is calculated as below.

are used to calculate the common mode rejection ratio (CMMR).



is the most common used


, then the negative

gain is same as V1.

. This establishes a voltage drop

. The voltage drop causes a

amps draw no current,

above and below it. This

r on the right-hand

side of the circuit then takes this voltage drop, and amplifies it by a gain of 1 if R3 is same as R2.

. The gain is calculated as below.

are used to calculate the common mode rejection ratio (CMMR).







measures the tendency of the device to reject input signals common to both input paths. High

CMMR is important for the precise measurements such as the voltage interest is represented by a

small voltage fluctuation superimposed on a voltage offset. To obtain good common mode

rejection ratios, it is necessary that the ratio R3 to R2 at the above part match the ratio R3 to R2 at

the bottom. For example, if the resistors in the circuit shown in Figure 24 had a total mismatch of

0.1%, the common mode rejection would be 60 dB times the closed loop gain, or 100 dB.

The weighing circuit has four sensors. Each sensor (strain gauge) is connected in a

Wheatstone bridge connection as shown in Figure 25 below. The connections of the other

sensors are the same. The output of voltage from Wheatstone bridge is then connected to the

instrumentation amplifier as shown in Figure 26 to detect the voltage difference produced by

each sensor. This voltage difference is directly proportional to the weight of the user and thus is

the direct indication of the users weight. The reason instrumentation amplifier is used instead of

just directly use the difference amplifier is because instrumentation amplifier has higher

common-mode rejection ratio (CMRR) compare to ordinary difference amplifier. The value of

resistors used throughout the instrumentation amplifier is 3.9kΩ to avoid any amplification at

this stage of the design.

Figure : Output from the strain gauge sensor





Figure : Output from one sensor is fed into an instrumentation amplifier

The Summing and Inverting Amplifier

The output voltage from each of the instrumentation amplifier is then fed into a summing

amplifier to obtain the sum of the difference voltage produced by each and every sensor due to

the weight or load applied on them. The sum of the difference voltage will be directly

proportional to the total weight of the user who sits on the seat. However, the output of the

summing amplifier is a negative value and the microcontroller cannot detect negative voltages,

thus, the output from the summing amplifier is then fed into an inverting amplifier to obtain a

positive voltage. At the same time, the inverting amplifier is also used to amplifier the final

signal before sending it to the microcontroller for analog to digital conversion.

The value of resistors used in the summing amplifier is all 6.2kΩ so that a unity gain can

be obtained (without doing any amplification on the output of the summing amplifier). Then in

the inverting amplifier, the value of input resistance is 3.6kΩ while the feedback resistance is

110kΩ which give an approximately output voltage amplification of 30.5 times the input voltage.

Finally, the output voltage of the inverting amplifier is fed into the ADC port of the

microcontroller, RA0 for ADC purposes.





Figure : Output from the instrumentation amplifiers are fed into a summing amplifier

followed by an inverting amplifier

The Simple Filter and Rectifier

The output of the strain gauge sensors are not perfectly dc voltage. Although the whole circuit is powered

by dc 9V batteries, still the output of the sensor is oscillating at the end of its dc level as shown :

Figure : Oscillation of the output signal of the weighing circuit at the end of its dc level

However, the oscillation of the output is unwanted needed to be taken care of because the

ADC converter function in PIC16F877A an only read constant dc voltage. When this signal was

sent to the ADC converter port of the microcontroller, RA0, the value after the ADC keeps

varying non-stop. This is not the desired case. What are needed from the signal is only the dc

component. Therefore, a simple filter and rectifier has been built at the end of the weighing

circuit to filter off all the unwanted oscillation and flatten out the signal to provide a constant dc

voltage for the ADC converter to read.

A diode is connected in series with the output of the inverting amplifier while a 2.2µF

capacitor is connected in parallel across the output and its ground. The usage of the diode is to

rectify the output signal, filtering the negative side of the oscillation while the capacitor is used

to smoothen out (flatten out) the output signal by discharging nature. This constant dc output is

then fed into the ADC converter for conversion into digital value for processing later.





Figure : The output of the weighing circuit is flatten out to produce an almost

constant dc output voltage

Interfacing the LCD with the Keypad Using the PIC16F877A

Another hardware implementation done was combining and interfacing the LCD and the

keypad through the microcontroller PIC16F877A. Initially, each of the devices was interfaced

and programmed with the microcontroller separately. The LCD was programmed to display the

desire message that wanted to be displayed while the keypad is programmed so that the

microcontroller is able to receive the inputs by user through the keypad.

Before we can display text on the LCD screen, the LCD module must be first configured

accordingly. Configuring the LCD module is very important as this is the initialization step to

start up the LCD module. Initialization is sort of like telling the LCD what we want the LCD

module to do. The following the schematic diagram for interfacing the LCD with the

PIC16F877A.





RA0/AN02

RA1/AN13

RA2/AN2/VREF-/CVREF4

RA4/T0CKI/C1OUT6

RA5/AN4/SS/C2OUT7

RE0/AN5/RD8

RE1/AN6/WR9

RE2/AN7/CS10

OSC1/CLKIN13

OSC2/CLKOUT14

RC1/T1OSI/CCP2 16

RC2/CCP1 17

RC3/SCK/SCL 18

RD0/PSP0 19

RD1/PSP1 20

RB7/PGD 40RB6/PGC 39

RB5 38RB4 37

RB3/PGM 36RB2 35RB1 34

RB0/INT 33

RD7/PSP7 30RD6/PSP6 29RD5/PSP5 28RD4/PSP4 27RD3/PSP3 22RD2/PSP2 21

RC7/RX/DT 26RC6/TX/CK 25

RC5/SDO 24RC4/SDI/SDA 23

RA3/AN3/VREF+5

RC0/T1OSO/T1CKI 15

MCLR/Vpp/THV1

U1

PIC16F877APROGRAM=..\..\..\..\LCD TESTING.HEX

D7

14D

613

D5

12D

411

D3

10D

29

D1

8D

07

E6

RW

5R

S4

VS

S1

VD

D2

VE

E3

LCD1LM016L

B15V

X1CRYSTAL

R1

4k7

Figure : The schematic diagram for interfacing the LCD with the microcontroller

Figure : After interfacing the LCD with the

microcontroller, it is able to display whatever

messages that are desired.





When interfacing the keypad, simple arrays of LEDs are used to indicate whether the

microcontroller has received the input from the keypad or not. The following Figure 31 is the

relevant schematic on how to test and interface the keypad with the microcontroller. 3 pull-up

resistors are required to be connected to the 3 columns of the keypad and that is all required for

interfacing a simple keypad to the microcontroller.

Figure : The rows and columns of the keypad are directly connected to the pin of the

microcontroller





Figure : The keypad is interfaced with the microcontroller using LEDs indications on the

breadboard.

The Final Weighing Circuit

Figure : The first part of the schematic of the final weighing circuit (consisting the

instrumentation amplifier for one sensor only)





Figure : The second part of the schematic of the final weighing circuit (consisting the

summing amplifier and the inverting amplifier)

Figure : The full weighing circuit after soldered on the strip board





The final weighing circuit is comprising of mainly 3 types of amplifiers which are the

instrumentation amplifier, summing amplifier and finally the inverting amplifier. All together

there are a total of 14 operational amplifiers required. These 14 operational amplifiers can be

accomplished by using seven LM358 op-amps since there are 2 op-amps each in the LM358. In

each of the four instrumentation amplifiers in the weighing circuit, there are 3 operational

amplifiers required (2 of them are the used as voltage followers while the other is used as

difference amplifier)

Outputs from each of the instrumentation amplifiers (a total of 4) are then fed into the

summing amplifier to sum up all the voltage difference produced by each sensor. Next, the

output of the summing amplifier is inverted and amplified at the inverting amplifier. Finally, the

simple filter and rectifier is used to smoothen out the output signal to give a constant dc output

voltage. This final signal will then be fed into ADC port of the microcontroller which is located

in the control and display panel circuit.

The Final Control and Display Panel Circuit

Figure: Schematic of the final control and display panel circuit consisting the keypad and

the LCD





Figure : The full control and display panel circuit after soldered on the strip board





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Project Technical Description