project technical description
TRANSCRIPT
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
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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
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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.
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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:
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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-
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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
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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
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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
instrumentation amplifier. This circuit is constructed from a buffered amplifier stage with three
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).
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
is the most common used
instrumentation amplifier. This circuit is constructed from a buffered amplifier stage with three
, 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).
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
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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
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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.
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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.
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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.
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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.
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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
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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)
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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
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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
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Figure : The full control and display panel circuit after soldered on the strip board
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