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How are output transducers driven and protected by the process subsystem?

Output transducers (LED, lamp, buzzer, motor, relay, solenoid), current-limiting resistors and driver/interface circuits.

A CCEA A-Level Technology and Design answer on output transducers such as LEDs, lamps, buzzers, motors, relays and solenoids, calculating the LED current-limiting resistor, and using transistor and relay driver circuits to interface high-power loads to logic.

Generated by Claude Opus 4.812 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

CCEA expects you to know common output transducers (LED, lamp, buzzer, motor, relay, solenoid), to calculate an LED current-limiting resistor, and to interface high-power loads to a low-power process stage with driver circuits (transistor, relay). The LED resistor calculation and the role of a relay are standard questions.

The answer

Output transducers

The LED current-limiting resistor

Driver and interface circuits

Worked example: driving a buzzer and an indicator LED

Examples in context

Example 1. Appliance standby LED. A single resistor sets the indicator LED current from the internal supply, the simplest output stage and the one most often examined.

Example 2. Washing-machine valve. A microcontroller drives a transistor that energises a solenoid valve (with a flyback diode); a relay would switch the mains heater, showing transistor and relay drivers used side by side for different loads.

Try this

Q1. An LED (forward voltage 2.0 V, current 10 mA) runs from a 5 V supply. Find the series resistor. [2 marks]

  • Cue. R=(5−2.0)/0.010=300 ΩR = (5 - 2.0)/0.010 = 300\ \Omega (choose 330 ohm).

Q2. State two reasons a relay is used to switch a mains load from a low-voltage circuit. [2 marks]

  • Cue. It provides electrical isolation between control and load sides, and its contacts can handle the high mains voltage/current (and AC).

Q3. Why does an LED need a series resistor? [1 mark]

  • Cue. It has a fixed forward voltage and almost no internal current limiting, so a resistor is needed to limit the current to a safe value and prevent burnout.

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 20196 marksAn LED with a forward voltage of 2.0 V and a recommended current of 20 mA is to be driven from a 9 V supply. Calculate the value of the series current-limiting resistor required, and explain why the resistor is necessary.
Show worked answer →

The resistor must drop the difference between the supply and the LED forward voltage, at the chosen current. Voltage across the resistor:

VR=Vs−VLED=9.0−2.0=7.0 V.V_R = V_s - V_{LED} = 9.0 - 2.0 = 7.0\ \text{V}.

By Ohm's law, with I=20 mA=0.020 AI = 20\ \text{mA} = 0.020\ \text{A}:

R=VRI=7.00.020=350 Ω.R = \frac{V_R}{I} = \frac{7.0}{0.020} = 350\ \Omega.

So a 350 ohm resistor is needed (the nearest preferred value, 390 ohm, would be chosen to be safe, slightly reducing the current).

The resistor is necessary because an LED has a very low and roughly fixed forward voltage; without a series resistor the current would be limited only by the LED's tiny internal resistance and would rise to a destructive level, burning out the LED. The resistor limits the current to the safe recommended value.

Markers reward VR=Vs−VLEDV_R = V_s - V_{LED}, the correct R=VR/I=350 ΩR = V_R/I = 350\ \Omega, and a clear reason (limits current to protect the LED, which has no useful internal current limiting).

CCEA 20214 marksExplain why a relay is used to switch a mains lamp from a low-voltage electronic circuit, and state one disadvantage of a relay compared with a transistor.
Show worked answer →

A relay is an electromagnetic switch: a small current through its coil closes (or opens) separate contacts that can carry a much larger current at a different voltage. It is used to switch a mains lamp from a low-voltage circuit because it provides electrical isolation between the low-voltage control side and the dangerous mains side (the coil and contacts are not electrically connected), and its contacts can handle the high mains voltage and current that a small signal transistor could not, and can switch AC.

One disadvantage compared with a transistor: a relay has moving mechanical contacts, so it is slower, wears out (limited operations), is bulkier, can arc/bounce, and needs more drive power; a transistor/MOSFET switches faster, silently and indefinitely.

Markers want the isolation and high-voltage/current handling as the reasons, and a valid mechanical disadvantage (slow/wear/bulky/arcing).

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