Practical Circuits
🎯 In this topic you will
- Draw circuit diagrams using symbols for cells, switches, resistors, variable resistors, ammeters, voltmeters, lamps, and buzzers.
- Construct circuits that include some or all of these components.
🧠 Key Words
- dimmer
- fixed resistor
- variable resistor
- volume
Show Definitions
- dimmer: A device that reduces the brightness of a lamp by lowering the current or voltage supplied.
- fixed resistor: An electrical component that limits current with a set resistance value that cannot be changed.
- variable resistor: A resistor whose resistance can be adjusted to control the flow of current in a circuit.
- volume: In electronics, often refers to the loudness of sound output, which can be controlled using a variable resistor (e.g., in speakers).
⚡ Variable resistors in circuits
In many circuits, it is useful to be able to change the resistance. For example, in a dimmer switch for a lamp, or in the volume control of a music player. In this case, the word volume means loudness of sound. The component that is used to change resistance is called a variable resistor.
🔬 Circuit symbols and comparison
The circuit symbol for a variable resistor is similar to that for a fixed resistor, but with an arrow through the symbol. The term fixed resistor is sometimes used for the type of resistors you learnt about in Topic, to avoid confusion with variable resistors.
💡 Effect of resistance on brightness
This diagram shows how a variable resistor is used to control the brightness of a lamp. As the resistance of the variable resistor is increased, the current in the circuit decreases. A smaller current flowing through the lamp makes the lamp dimmer.
As the resistance of the variable resistor is decreased, the current in the circuit increases. A larger current flowing through the lamp makes the lamp brighter.
⚡ Everyday circuits
Electric circuits are used in many homes, schools, cars, bicycles, phones, music systems, watches, computers and lots more things that we use every day.
🔬 Designing useful circuits
Scientists are always designing new circuits, many of which make life easier for us. Here we will look at some examples.
💡 Fridge door light circuit
When opening the door of a refrigerator, a light comes on inside. The circuit for this is shown in the diagram. The switch is opened and closed by the movement of the fridge door. When the door is open, the switch is closed and when the door is closed, the switch is open.
🔔 Warning buzzers
Some equipment is fitted with a buzzer to make a warning sound if something is done incorrectly. Often, the loudness of the sound can be adjusted. This circuit shows how this is done.
🚗 Car buzzer and headlamp circuit
Some cars have a buzzer that makes a sound when the driver opens the door while the headlamps are switched on. The door of the car controls the switch for the buzzer, but the buzzer should only operate when the headlamps are on.
⚡ Parallel headlamp wiring
The headlamps are connected in parallel, so they can be switched on and off together. The buzzer is part of the same branch of the parallel circuit, as shown in the diagram.
🔎 Switch control
Switch S1 controls the headlamps. Switch S2 is controlled by the door. When the door is open, the switch is closed and when the door is closed, the switch is open. Current cannot flow to switch S2, unless switch S1 for the headlamps is closed. That means a buzzer will sound when you open the door while the headlamps are on.
⚡ Uses of 12 V batteries
Equipment that uses 12 V batteries include cars, trucks and some boats. These batteries need to be recharged from time to time. Sometimes, a battery voltage indicator is fitted so the voltage of the battery can be measured.
🔬 Measuring battery voltage
The circuit diagram shows how a switch can be used to display the battery voltage. A voltmeter is connected in parallel across the battery, and a switch is included in this branch of the parallel circuit.
💡 Using lamps of different voltages
Sometimes a lamp with a low voltage rating is required in a circuit with a battery of higher voltage. For example, a 6 V lamp can be operated using a 12 V battery. Resistors are used to produce different voltages.
⚡ Voltage sharing across resistors
If two resistors of equal resistance are connected in series with a battery, then the battery voltage will be shared equally across each resistor. A 6 V lamp can then be safely connected in parallel with one of the resistors, as shown. The lamp will receive a 6 V supply and work properly.
❓ QUESTIONS
1. Draw the circuit symbol for:
a) a fixed resistor
b) a variable resistor
👀 Show answer
b) A rectangle with an arrow drawn through it represents a variable resistor.
2. Look at the circuit diagram. What can this circuit be used for? Write one letter.
A) Changing the loudness of the buzzer, keeping the brightness of the lamp constant.
B) Changing the brightness of the lamp, keeping the sound from the buzzer constant.
C) Changing the loudness of the buzzer and brightness of the lamp at the same time.
D) Changing the loudness of the buzzer and brightness of the lamp separately.
👀 Show answer
3. A circuit will use one cell and two lamps. One switch must turn both lamps on and off at the same time. The brightness of each lamp must be controlled separately. Draw a circuit diagram for this circuit.
👀 Show answer
4. A circuit has two cells, a lamp and a buzzer in series.
a) Draw a circuit diagram to show how the current through the buzzer and the voltage across the buzzer could be measured at the same time.
b) Draw one circuit diagram to show how:
• the lamp and the buzzer can be switched on and off separately
• the brightness of the lamp can be changed without affecting the sound from the buzzer.
👀 Show answer
b) Use separate switches for the lamp and buzzer. Add a variable resistor in series with the lamp only, so its brightness can be controlled independently while the buzzer remains unaffected.
🔬 Think like a Scientist
Designing and building circuits
In this activity, you will draw circuit diagrams and then build the circuits.
Work in pairs.
You will need: cells, lamps, buzzers, variable resistor, voltmeter, ammeter, selection of fixed resistors, wires, connectors.
Method:
- Write a risk assessment for building electric circuits.
- Draw circuit diagrams for the following circuits:
- Circuit 1
• two lamps can be switched on and off separately
• the current through one of the lamps can be measured - Circuit 2
• two lamps can be switched on and off together using one switch
• the brightness of one of the lamps can be varied - Circuit 3
• a fixed resistor and a variable resistor in series
• the voltage across the variable resistor can be measured - Circuit 4
• one lamp can be switched on and off
• a buzzer can only be switched on at the same time the lamp is on
• the brightness of the lamp can be varied without affecting the sound from the buzzer
- Circuit 1
- Ask your teacher to check your risk assessment and circuit diagrams, and then build each of the circuits.
Follow-up Questions:
Show Answers
- 1: Answers will vary – some circuits may not behave exactly as expected due to wiring errors, loose connections, or component limits.
- 2: Possible improvements include using more precise measuring instruments, ensuring secure connections with proper connectors, or replacing components with different resistance ranges to improve control.