Circuit Design

Handheld Device for Communication with
Speech Impaired Patients/Automatic Outdoor
Light Controller
School of Electrical and Electronic Engineering (NTU)
Done by: Darius
Marcus
Emily
Idris
Bernard

Thursday, June 30, 2011

Project Supervisor and Advisors

Supervised by Dr. Arindam Basu
Advised by Mdm Wong, Steven and Li Fei


Objectives of Project

At the end of this project, we should be able to:

Be familiar with the concepts of voltage, current and resistance, and also know how to
measure these values.

Understand the change of resistance for series and parallel connections

Understand the use of a transistor as a voltage controlled switch

Combine concepts learnt to make a circuit whose property depends on a sensor whose
resistance changes with applied force/ambient light


Charge

Represented by ‘Q’
SI Unit is Coulomb
Also known as Electronic Charge


Voltage

Represented by ‘V’
Measure in Volts (V)
Power source of the electric current in the circuit


Current

Represented by ‘I’
Measured in Amperes (A)
Deﬁned as the rate of ﬂow of charges


Resistance

Represented by ‘R’
Measured by Ohms (Ω)
It’s the property of a material that measures how easy or difﬁcult it is for the
charge to ﬂow through


A simple representation...

Plumbing Pipes
Voltage is the same as water pressure
Current is the same as ﬂow rate
Resistance is the same as the pipe size.


Ohm’s Law

V = I × R (Voltage = Current × Resistance)
Rearrange the elements to find the formula for current and resistance
I = V/R (Current = Voltage ÷ Resistance)
R = V/I (Resistance = Voltage ÷ Current)
To find the overall resistance in a parallel arrangement, use
(r₁ × r₂) ÷ (r₁ + r₂) (Only applies for 2 resistors in parallel arrangement)
To find overall resistance in a series arrangement, just add the resistance of both resistors.


Relationship between the 3

High voltage = High current
High resistance = Low current (vice-versa)
High current = High voltage OR Low resistance


Multimeter

Also known as multi-tester or volt/ohm meter (VOM)
Electronic measuring instrument that has many measurement functions in a single unit
Most units are able to measure voltage, current and resistance.
Used as a bench instrument and to troubleshoot electrical problems in various industri
and household devices.


Resistor

Component of circuit that resist ﬂow of electrical current.
Mainly used to create and maintain the safe current within a component.
Electricity passes through 2 terminals of the resistor, and the voltage of the current will be
dropped as electricity passes from one to another.


Transistor

Semiconductor device used to amplify and switch electronic signals.
At least 3 terminals for connection to external circuit.
Can be used as a voltage/current switch
Connects circuits from “Collect” (C) terminal to the “Emitter” (E) terminal. Depending on
whether or not the voltage is high enough, the switch will turn on (if high voltage) and
off (if low voltage). When switch is on, currents can pass through, allowing the transistor
to act as a voltage switch


Light Dependent Resistor (LDR)

A resistor in which resistance decreases with increasing light intensity
Also known as photoconductor


Force Sensitive Resistor (FSR)

A resistor in which resistance changes when force or pressure is applied
Resistance is inversely proportional to force applied


Potentiometer

Also known as Rheostat
A three-terminal resistor with a sliding contact, which essentially forms an adjustable
voltage divider.
When only 2 terminals are used, it acts as a variable resistor/rheostat.
Commonly used in volume controls.


Light Emitting Diode (LED)

A special semiconductor that emits light without the use of ﬁlament of glass tubes
Light is converted from the electrons from the electricity passing through the LED
More efﬁcient at producing light then incandescent globes and also have more intense
light output, produces very minimal heat, uses less energy and also lasts longer.


Breadboard

Construction base for electronic circuit
Used for temporary prototypes and experimenting with circuit design
Very easily reused
Each breadboard has columns of 5


Breadboard (Parallel)

Resistors are placed horizontally, one above another.
Both resistors have terminals on the same column.
The arrangement is as shown above


Breadboard (Series)
Resistors are placed vertically
The arrangement is as shown


Experiment 1

In this experiment, we had to set the supply to provide certain amount of
electricity.
Through this experiment, we learnt how to use the power source, how to
provide certain amounts of voltage and how to ﬁne-tune it.


Experiment 2

In this experiment, we were to try and find the value of each resistor by using
the colored bands and referencing it to the color codes.
We had to use the multimeter to confirm our answers.
We learnt how to find the resistance value and also how to use the multimeter.


Experiment 3

Here, we had to use one of the resistors and form a simple circuit using the resistors.
Then, we were to provide different voltages of current to power the circuit and, using the
multimeter, find out how much current is flowing through the circuit.
We noticed that there was lesser current flowing through the circuit due to the resistors,
and that the increase of the voltage given is directly proportional to the increase in current.
Different resistors have different resistance, thus the final amount of Amperes left in the
circuit will be different when different resistors are used. This also means that different
resistors would give a different increase ratio.


Experiment 4 (1)

Here, we had to use different sets of resistors and arrange them in series in the
circuit.
We learnt how to calculate the ﬁnal amount of amperes left in the circuit, by
taking the total voltage provided and deducting the total resistance.


Experiment 4 (2)

In part 2, we had to repeat what we did previously, but this time arranging the
resistors in parallel.
Here, we learnt about another formula, how to derive with the ﬁnal amperes in
the circuit with parallel resistors.
(r₁ × r₂) ÷ (r₁ + r₂)


Experiment 5

In this experiment, we had to connect an LED with a rheostat in series and
slowly turn the knob of the rheostat until the LED is turned on. After that, we
had to find out the voltage across and the current through the LED.We were
then asked to find out how much resistance there was in the rheostat when
the LED was turned on.
We learnt about the use of the rheostat and how useful it can be, being able to
allow certain amount of current to flow by adjusting the resistance.


Experiment 6 (1)

Measured resistance of the C-E Terminals of a transistor through a series of
resistance 1kΩ.
Learned that a transistor is able to work as a switch.


Experiment 6 (2)

A transistor is a three-terminal-switch, repeated experiment with a diode - a
Two-Terminal-Switch


Experiment 7

We were asked to measure the resistance of the FSR with and without force
applied.
We noticed that the resistance changed with the force applied, more force
applied, less resistance
Less resistance will mean more current ﬂow, and thus more of the original
voltage will get back to ground.


Experiment 8

Now, we were asked to repeat the previous experiment, but this time using the
LDR.
We noticed that the results were very similar to the previous experiment, the
greater the amount of light, the lesser the resistance.
Thus, there will be greater amount of the original voltage that will get back to
ground.


Grand Challenges


Automatic Outdoor Light controller
Challenge
We want to make a circuit that can be used in “smart” street lamps. It
can sense the ambient light and turn itself “ON” when the light is less
(after sunset) and turn itself “OFF” when it is bright (daytime). This will
save unnecessary power loss by turning “OFF” the light when it is not
needed. Assume that the LED is the street lamp. Now combine all
your earlier experiments to make a circuit that can turn ON the LED
when the ambient light reduces.


Prototype Circuit Diagram (Light
Sensor)


Voltage Divider theory
5v Power source

Resistor

V out

R sense
R sense
V out = 5 x
Ro + R sense


5v Power source

Resistor
When R sense is large

V out

R sense
R sense
V out = 5 x
Ro + R sense

= 5 (approx)


5v Power source

Resistor
When R sense is small

V out

R sense
R sense
V out = 5 x
Ro + R sense

= 0 (approx)


Result


Handheld Device for Communication
with Speech Impaired Patients
Effective communication is a difﬁculty faced by many people who
have impaired speech along with a lack of hand control, which
prevents them from writing or typing. We can give them a device
which they can press with different degrees of strength to mean
different things. We will do a simple circuit where the person can say
“yes” or “no” by pressing on the FSR which will light a LED if pressed.
Make a circuit that can turn ON the LED when the FSR is pressed.


Prototype Circuit Diagram (Force
Sensor)


Experience and Reﬂections

It helps spark the interest in electronics to all of us.
Teaches us some Basics of Electronics
Learn about


Thank You! :D


Acknowledgments
Especially Professor Arindam Basu

Mdm Wong

Steven (Student helper)

Li Fei (Student helper)

Miss Teo

Miss Peck

Mr Yeo

Circuit Design

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