What output devices does a system use, and how does a transistor switch a large load from a small signal?
Output devices: lamps, buzzers, motors and relays, and the transistor used as an electronic switch to control a larger current from a small input.
A CCEA GCSE Technology and Design answer on output devices: lamps, buzzers, motors and relays, and how a transistor acts as an electronic switch, turning a small input current into the switching of a much larger output current, with a protection diode for inductive loads.
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What this dot point is asking
CCEA wants you to know the main output devices - lamps, buzzers, motors and relays - and to explain the transistor used as a switch, which lets a small input signal control a much larger output current. You should also know why a relay and a protection diode are used.
The answer
Output devices
The problem: small signal, large load
A sensor or logic output usually provides only a tiny current - not enough to drive a motor or a bright lamp directly. A transistor solves this by acting as an electronic switch.
This is why a transistor sits between the process stage and a high-current output: it amplifies the switching capability so a small signal controls a large load.
The relay: switching even larger or mains loads
A transistor often switches the relay coil, and the relay then switches the heavy or mains load - combining both devices.
The protection (flyback) diode
Worked example: driving a motor from a sensor
Examples in context
- Example 1. An automatic night light
- A weak dark-sensing signal turns on a transistor, which drives a bright lamp the sensor could never power directly.
- Example 2. A mains heater under low-voltage control
- A transistor switches a relay coil, and the relay contacts switch the mains heater, keeping the dangerous mains circuit isolated from the control electronics.
- Example 3. A toy with a motor
- A transistor switches the motor from a small control signal, with a flyback diode across the motor to protect the transistor when it switches off.
Being able to explain the transistor switch and the relay (with the protection diode) lets you answer both the "how does a transistor switch" and "why use a relay" questions.
Try this
Q1. Name two output devices and the action each produces. [2 marks]
- Cue. Any two: lamp/LED (light), buzzer (sound), motor (movement), heater (heat).
Q2. What turns a transistor on when it is used as a switch? [1 mark]
- Cue. A small current (or voltage) applied to its base.
Q3. Why can a small sensor signal not drive a motor directly? [2 marks]
- Cue. The sensor provides too little current; the motor needs a much larger current, so a transistor switch is needed.
Q4. What is a relay? [1 mark]
- Cue. An electrically operated switch: a coil current opens or closes separate contacts.
Q5. Why is a protection diode fitted across a motor driven by a transistor? [2 marks]
- Cue. To safely conduct the high voltage produced when the inductive load switches off, protecting the transistor.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksExplain how a transistor is used as a switch in an electronic circuit.Show worked answer →
A transistor turns on when a small current (or voltage) is applied to its base (1). When it is on, it allows a much larger current to flow through it from collector to emitter (1).
So a small input signal, for example from a sensor, switches a larger output current on or off (1). This lets a weak signal control a device such as a motor or lamp that needs more current than the input can supply (1).
CCEA style3 marksWhy is a relay used to control a mains-powered output from a low-voltage circuit?Show worked answer →
A relay is an electrically operated switch: a small current through its coil closes (or opens) a separate set of contacts (1). The contacts can switch a high-power or mains circuit while the coil is driven by the low-voltage control circuit (1).
This keeps the high-voltage circuit electrically separate (isolated) from the low-voltage control circuit, which is safer (1).
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Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)