D & T Design Lesson
MacPherson Secondary School
Technological Areas -
Electronics
Sec 3B
By: MdmLee Kah Gek
1

Technological Areas
Structures
Mechanisms
Electronics
2

3

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
4

• AC rating ( 230V, 110V, 415V)
• AC to DC power supply
• DC to DC power supply
• Batteries
5

• Samples of switches
6

 Resist electricity
 Used to supply the correct voltage to the various components in an electric circuit.
Variable resistor
7

• Write down the colour code
• Check the table
• State the resistor value
8

A
R
1
R
2
Three resistors in series.
Total Resistance AB is
R
T

R
1

R
2

R
3
R
3 B
R
1
P
R
2
Two resistors in parallel
Q
Total resistance PQ is
1
R
T

1
R
1

1
R
2
9

Exercise 1: Find the total resistance
Q1.
A
30 K Ω 6.67 K Ω
B 40 K Ω
P
Q2.
P
Total resistance
= R
1
+ R
2
= 30 K Ω + 6.67K Ω
= 36.67 k Ω
10 K Ω 30 K Ω
Q
60 K Ω
Q
1
R

1
R
1

1
R
2
1
R

1
40000

1
60000
R


1
24000
24000


24 K

20 K Ω 40 K
Total top series resistance
Ω
= 10 K Ω + 30 K Ω
= 40 K Ω
Total bottom series resistance
= 20 K Ω + 40 K Ω
= 60 K Ω 10

V
1
I
R
1
Va
R
2
Voltage can be stepped down from
9 V to 3 V and 6V.
Va

V
1
R
1
R
2

R
2
Or Va = IR
2
50K
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.
Voltage across resistor
Device
A = 3 – 2 V
3V
= 1 V
R
V=IR
1 = 0.004R
R = 250 Ω 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 k Ω (ii) 40 k Ω
A
10 k
Voltage divider or potential divider
( i ) Vb

6

5
5

10

2 V
B
C
( ii ) Vb

6

40
40

10

4 .
8 V
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 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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 Transistors have many uses including switching, voltage/current regulation, and amplification
 Two types NPN and PNP
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 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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 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.
16

Identify the electronics symbols
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