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D & T Design Lesson

Sec 3B

By: MdmLee Kah Gek

Technological Areas - Electronics

MacPherson Secondary School

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Technological Areas

Structures

Mechanisms

Electronics

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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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Resist electricity

Used to supply the correct voltage to the various components in an electric circuit.

1. Resistor (fixed and variable)

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

21

111

RRRT

R1R1R3R2

R2

Three resistors in series.

Total Resistance AB is

A BP Q

Two resistors in parallel

Total resistance PQ is

321 RRRRT

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Exercise 1: Find the total resistance

30 KΩA B6.67 KΩ

60 KΩ

P Q

40 KΩQ1.

10 KΩ 30 KΩ

20 KΩ 40 KΩ

Q2.

Total resistance

= R1 + R2

= 30 KΩ + 6.67K Ω

= 36.67 k Ω

K

R

R

RRR

24

2400024000

160000

1

40000

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111

21

P Q

Total top series resistance

= 10 K Ω + 30 K Ω

= 40 K Ω

Total bottom series resistance

= 20 K Ω + 40 K Ω

= 60 K Ω

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Voltage Divider

50K

Voltage can be stepped down from 9 V to 3 V and 6V.

By replacing the 10K resistor with a 50K variable resistor, we can vary the stepped down voltage.

Va

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21 RR

RVVa

V1

R1

R2

I

Or Va = IR2

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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

R

3V

Voltage across resistor

= 3 – 2 V

= 1 V

V=IR1 = 0.004RR = 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 kΩ (ii) 40 k Ω

10 k

A

B

CVVbii

VVbi

8.41040

406)(

2105

56)(

Voltage divider or potential divider

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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

2. Relay

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Transistors have many uses including switching, voltage/current regulation, and amplification

Two types NPN and PNP

3. Transistors

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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)

4. Temperature Sensors (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.

5. Light Dependent Resistor (LDR)

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Identify the electronics symbols