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D & T Design Lesson Technological Areas Electronics Sec 3B By: MdmLee Kah Gek MacPherson Secondary School 1 Technological Areas Electronics Structures Mechanisms 2 Electronics Module 3 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 4 Power Supply • AC rating ( 230V, 110V, 415V) • AC to DC power supply • DC to DC power supply • Batteries 5 Switches • Samples of switches 6 1. Resistor (fixed and variable) Resist electricity Used to supply the correct voltage to the various components in an electric circuit. Variable resistor 7 Task 1: Read the resistor value • Write down the colour code • Check the table • State the resistor value 8 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 9 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 24K 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 10 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. 11 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 65 2V 5 10 B C 6 40 (ii )Vb 4.8V 40 10 12 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 13 3. Transistors Transistors have many uses including switching, voltage/current regulation, and amplification Two types NPN and PNP 14 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) 15 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. 16 Identify the electronics symbols 17