Basic Electronic Components

Lessons

Senior 3 · Physics

Basic Electronic Components

4 lessons

Lesson 1 of 4

Introduction to Electronics: Components, Symbols and Resistors

30 min

What is inside a phone charger?

Open the back of an old radio or look at a broken phone charger (with an adult, and never while it is plugged in) and you will see a small green board covered in tiny parts: striped cylinders, little drums, black boxes with three legs, and shiny lines of copper joining them. These parts are electronic components. In S1 and in unit 7 you used cells, lamps, switches and resistors in circuits that carry energy to appliances. Electronics uses small currents to control things and to process information: to switch on a light at dusk, sound an alarm, or turn a weak radio signal into music.

Definition

Electronics: the branch of physics and technology that uses components such as diodes and transistors to control small electric currents, in order to process signals and information. Example: the circuit that switches a street lamp on at dusk.

Passive component: a component that cannot amplify or control a current with another signal; it can only resist, store or release energy. Examples: resistors, capacitors, inductors.

Active component: a component that can control a current using a separate, smaller signal, or amplify. Examples: transistors and integrated circuits. (Diodes, which conduct one way only, are made of the same semiconductor materials and are often listed with active components.)

Circuit symbols

Rwanda, like most of the world, uses the symbols of the International Electrotechnical Commission (IEC). A resistor is a plain rectangle, not a zig-zag.

Component	Active or passive	What it does
Resistor	Passive	Limits current; shares voltage
Variable resistor	Passive	Resistance adjusted by hand, as in a volume control
LDR	Passive	Resistance falls when light falls on it
Thermistor	Passive	Resistance changes with temperature
Capacitor	Passive	Stores charge
Diode, LED	Semiconductor	Lets current through in one direction only; an LED also gives light
Transistor	Active	Switches or amplifies a current using a small base current

Resistors and the colour code

A resistor is made to have a chosen resistance. Small resistors are too tiny to print numbers on, so their value is shown by coloured bands. On a four-band resistor, the first two bands are the first two digits, the third band is the multiplier (the power of ten), and the fourth band, set a little apart, is the tolerance: how far the real value may differ from the marked one.

Colour	Digit	Multiplier	Tolerance
Black	0	$\times 1$
Brown	1	$\times 10$	± 1%
Red	2	$\times 100$	± 2%
Orange	3	$\times 10^{3}$
Yellow	4	$\times 10^{4}$
Green	5	$\times 10^{5}$
Blue	6	$\times 10^{6}$
Violet	7
Grey	8
White	9
Gold		$\times 0.1$	± 5%
Silver		$\times 0.01$	± 10%

Worked example 1: reading the code

Find the values of resistors banded (a) brown, black, red, gold, and (b) yellow, violet, orange, gold.

(a) Brown = 1, black = 0, red = $\times 100$ : $10 \times 100 = 1000\,\Omega = 1\,\mathrm{k\Omega}$ , gold = ± 5%.

(b) Yellow = 4, violet = 7, orange = $\times 10^{3}$ : $47 \times 1000 = 47\,000\,\Omega = 47\,\mathrm{k\Omega}$ , ± 5%.

Worked example 2: tolerance

Between which values can a 1 kΩ ± 5% resistor really lie?

5\%\ \text{of}\ 1000\,\Omega = 50\,\Omega \qquad 1000 - 50 = 950\,\Omega \quad\text{to}\quad 1000 + 50 = 1050\,\Omega

Worked example 3: from value to colours

What are the bands of a 220 Ω ± 5% resistor, and of a 4.7 kΩ ± 5% resistor?

220 = 22 × 10: red, red, brown, gold. 4700 = 47 × 100: yellow, violet, red, gold.

Choosing a resistor for an LED

An LED (lesson 2) needs only about 2 V and is destroyed by a large current, so it is always used with a series resistor.

Worked example 4: an LED on a 9 V battery

A red LED needs 2.0 V and a current of 20 mA. Find the series resistor for a 9.0 V battery.

V_R = 9.0 - 2.0 = 7.0\,\mathrm{V} \qquad R = \frac{V_R}{I} = \frac{7.0}{0.020} = 350\,\Omega

Resistors are made only in standard ("preferred") values; the common series runs 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 times a power of ten. Choose the next value above 350 Ω, which is 390 Ω (orange, white, brown), so the current is a little under 20 mA: $\frac{7.0}{390} = 0.018\,\mathrm{A} = 18\,\mathrm{mA}$ .

Warning

Safety: mains electricity and electronic projects

Build school electronics only on batteries or a low-voltage supply provided by your teacher. Never connect a home-made circuit to a mains socket.
Never open a charger, television or radio that is plugged in. Even after unplugging, capacitors inside a mains device can hold a dangerous charge for some time.
If someone receives an electric shock, do not touch them while they may still be in contact with the supply: switch off at the socket or the main switch first, or push the source away with something dry that does not conduct, such as a wooden broom handle. Then call for help (in Rwanda, the emergency medical number is 912) and give first aid.

Tip

Remember the colours in order: "Black Bananas Rot Only Yellow Gets Brown Very Gradually Wasting" is one way; better still, make up a sentence of your own about Rwandan food. Remember too: the third band is the number of zeros to add.

✦

Practice

Draw the IEC symbols for a resistor, a capacitor, a diode, an LED and an LDR.
Classify as active or passive: transistor, resistor, capacitor, thermistor.
Find the value and tolerance of a resistor banded red, violet, yellow, silver.
Give the colour bands of a 330 Ω ± 5% resistor and of a 10 kΩ ± 5% resistor.
A resistor is marked 680 Ω ± 10%. Find the lowest and highest values it may really have.
A green LED needs 2.2 V and 15 mA. Calculate the series resistor needed with a 6.0 V supply, and choose a preferred value from the series in worked example 4.
Why must the next preferred value above the calculated value be chosen, rather than the one below?

Show answersHide answers

Resistor: rectangle; capacitor: two parallel plates; diode: triangle against a bar; LED: diode symbol with two arrows pointing out; LDR: rectangle with two arrows pointing in.
Transistor: active. Resistor, capacitor and thermistor: passive.
Red 2, violet 7, yellow $\times 10^{4}$ : $27 \times 10^{4} = 270\,000\,\Omega = 270\,\mathrm{k\Omega}$ , ± 10%.
330 Ω: orange, orange, brown, gold. 10 kΩ: brown, black, orange, gold.
10% of 680 = 68 Ω, so 612 Ω to 748 Ω.
$R = \frac{6.0 - 2.2}{0.015} = \frac{3.8}{0.015} = 253\,\Omega$ ; choose 270 Ω.
A larger resistance gives a slightly smaller current, which is safe for the LED. A smaller resistance would let more current through than the LED is designed for, and could damage it.

Key points

Electronics uses components to control small currents and process information.
Passive components (resistors, capacitors) cannot amplify; active components (transistors) can control a current with a small signal.
Four-band code: digit, digit, multiplier, tolerance. Example: brown, black, red, gold = 1 kΩ ± 5%.
An LED always needs a series resistor: $R = \frac{V_{\text{supply}} - V_{\text{LED}}}{I}$ , rounded up to a preferred value.

Sources and credits

Emergency number: Contact Us, Rwanda Biomedical Centre, which lists 912 as the toll-free SAMU ambulance number (page read 1 October 2026).

Written by Ganzaa: the lesson text, the symbol chart, the resistor and circuit diagrams, the tables, worked examples and practice exercises. The colour code and preferred values follow the international standards for resistor marking (IEC 60062 and IEC 60063).

1What is inside a phone charger?

2Circuit symbols

3Resistors and the colour code

4Choosing a resistor for an LED

What is inside a phone charger?

Circuit symbols

Resistors and the colour code

Choosing a resistor for an LED