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D & T Design Lesson
Technological Areas Electronics
Sec 3B
By: MdmLee Kah Gek
MacPherson Secondary School
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Technological Areas
Electronics
Structures
Mechanisms
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Electronics Module
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Objectives
1. Know basic electronics components and
its function ( Res, var res, relay,
transistor, temp sensor and light sensor)
2. Learn the electrical symbols.
3. Series and Parallel Circuits
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Power Supply
• AC rating ( 230V, 110V, 415V)
• AC to DC power supply
• DC to DC power supply
• Batteries
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Switches
• Samples of switches
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1. Resistor (fixed and variable)
 Resist electricity
 Used to supply the correct
voltage to the various
components in an electric
circuit.
Variable resistor
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Task 1: Read the resistor value
• Write down the colour code
• Check the table
• State the resistor value
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Series and Parallel connections
A
R1
R2
R3
R1
B
P
Three resistors in series.
Total Resistance AB is
RT  R1  R2  R3
Q
R2
Two resistors in parallel
Total resistance PQ is
1
1
1
 
RT R1 R2
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Exercise 1: Find the total resistance
Q1. A
30 KΩ
6.67 KΩ
40 KΩ
B
P
Total resistance
= R1 + R2
Q
60 KΩ
= 30 KΩ + 6.67K Ω
= 36.67 k Ω
Q2.
10 KΩ
30 KΩ
P
Q
1 1
1
 
R R1 R2
1
1
1


R 40000 60000
1

24000
R  24000
 24 K
40 KΩ
20 KΩ
Total top series resistance
Total bottom series resistance
= 10 K Ω + 30 K Ω
= 20 K Ω + 40 K Ω
= 40 K Ω
= 60 K Ω
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Voltage Divider
V1
I
R1
Va
50K
R2
Voltage can be
stepped down from
9 V to 3 V and 6V.
R2
Va  V1
R1  R2
Or Va = IR2
By replacing the 10K
resistor with a 50K
variable resistor, we can
vary the stepped down
voltage.
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Exercise 2:
a) Device A has a voltage drop of 2 V across it and
needs to draw a current of 4 mA. Suggest a
suitable value for resistor R.
Device
A
3V
Voltage across resistor
=3–2V
=1V
V=IR
1 = 0.004R
R = 250 Ω
R
b) What is the circuit arrangement below called? If A and C are
connected across a 6 V battery, calculate the voltage at B if the
variable resistor is set to (i) 5 kΩ (ii) 40 k Ω
A
10 k
Voltage divider or
potential divider
(i )Vb 
65
 2V
5  10
B
C
(ii )Vb 
6  40
 4.8V
40  10
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2. Relay
 Electrically operated switch
 Coil of wire and a metal lever
which changes the position of the
switch.
 Advantage - permits a low voltage
DC circuit to control a completely
separate high-power circuit
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3. Transistors
Transistors have many uses including switching,
voltage/current regulation, and amplification
Two types NPN and PNP
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4. Temperature Sensors (Thermistor)
Decreases as the temperature rises (negative
temperature coefficient or ntc thermistor)
Increase in resistance with temperature. (positive
temperature coefficient, or ptc thermistor)
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5. Light Dependent Resistor
(LDR)
 A LDR varies it resistance according to the amount of light falling on its surface.
 Total darkness -1 MOhm
 In bright light - a couple of kOhm.
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Identify the electronics symbols
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