How does a small sensor signal switch on a large output device like a motor or lamp?
Output devices and transistor switching: common output transducers and using a transistor as an electronic switch driven by a sensing circuit, including a protective diode.
An SQA National 5 Engineering Science answer on output devices and transistor switching, covering output transducers such as the lamp, LED, buzzer and motor, how a transistor acts as an electronic switch turned on by a small base voltage, the use of a series resistor for an LED, and a protective diode across a motor coil.
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What this key area is asking
The SQA wants you to know the common output transducers and to explain how a transistor acts as an electronic switch that lets a small sensor signal control a much larger current to a lamp, motor or buzzer.
Output transducers
An LED (light-emitting diode) only conducts one way round and is easily damaged by too much current, so it is always used with a series resistor to limit the current to a safe value.
The transistor as a switch
A sensing circuit (a voltage divider with an LDR or thermistor) produces only a small current that cannot drive a motor or lamp directly. The transistor solves this: it uses the small signal to control a much larger current.
This is exactly the process stage of the systems diagram: the input (sensor and divider) feeds the base, the transistor switches, and the output (lamp, motor, buzzer) responds.
Protecting the circuit
When the output is a motor or a relay coil (an inductive load), switching it off suddenly produces a large reverse voltage spike that can damage the transistor. A protective diode (also called a flyback or back-emf diode) is connected across the coil, the "wrong way" round, so it safely conducts this spike away and protects the transistor.
Why this matters
Transistor switching is the bridge between sensing and doing. Every automatic circuit in the course - the security light, the frost alarm, the cooling fan - relies on a transistor (or a comparator driving one) to turn the small sensor signal into enough current to operate a real output device.
Try this
Q1. State the approximate base voltage at which a transistor switches on. [1 mark]
- Cue. About .
Q2. Why is a series resistor always used with an LED? [1 mark]
- Cue. To limit the current through the LED to a safe value and stop it being damaged.
Q3. Explain why a diode is connected across a motor switched by a transistor. [2 marks]
- Cue. When the motor is switched off it produces a reverse voltage spike; the protective diode conducts this away safely and protects the transistor.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksA transistor is used to switch a 12 V motor on when a light sensor goes dark. Describe how the transistor acts as a switch in this circuit.Show worked answer →
Explain the on/off action in terms of the base voltage.
The sensing circuit (a voltage divider with an LDR) feeds a voltage to the base of the transistor. When it is dark, this base voltage rises above the transistor's switch-on voltage (about 0.7 V).
Once the base voltage is high enough, the transistor turns on and conducts, allowing current to flow through the motor connected to its collector, so the motor runs.
When it is bright the base voltage is too low, the transistor is off, no current flows and the motor stops.
Markers reward linking a rising base voltage to the transistor turning on, current flowing through the output, and the reverse when the base voltage is low.
SQA N5 style2 marksState two output transducers used in electronic systems and the form of energy each produces.Show worked answer →
Markers want two devices, each with its output energy form.
A lamp or LED produces light energy.
A buzzer produces sound energy. (A motor producing kinetic energy is also acceptable.)
Markers reward two valid output transducers each correctly paired with the energy it produces.
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