The big idea: An output device either communicates with the user or controls the environment.
The guide names motors including servos and pumps, an LCD display, a buzzer and a relay. Choose by who has to perceive it and where — or, for control, by what has to be moved and how much current that needs.
Output devices sorted by the sense they reach, then how each is driven.
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| Device | What it does | How it is driven |
|---|---|---|
| Motor | Turns electrical energy into rotation. A plain DC motor spins; a SERVO moves to a commanded ANGLE and holds it; a PUMP moves fluid | Through a transistor or a driver chip, with a flyback diode, and usually on its own supply rail |
| LCD display | Shows text, numbers and symbols — far more information than any other output on this list | From several chip pins or a serial connection, with a backlight often switched separately to save power |
| Buzzer | Makes a sound that reaches a user who is not looking, which is why every alarm uses one | A small one can run from a pin with a resistor; a loud one needs a transistor, and a coil type needs a diode |
| Relay | An electrically operated switch that lets a low-voltage circuit control a mains or high-current one, with the two ELECTRICALLY SEPARATE | A transistor drives the coil, and a flyback diode across it is compulsory |
A servo is not just a motor: A servo contains a motor, a gearbox, a position sensor and a small control circuit, and it moves to the angle a pulse width asks for and holds it there.
So a servo is a small closed-loop system in itself, which is why a design project uses one wherever a precise position matters.
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| Requirement | Choose | Because |
|---|---|---|
| Tell a user something specific | An LCD | It carries text and numbers — a temperature, a countdown, a fault code — which no light or sound can |
| Alert a user who is not looking | A buzzer | Sound needs no line of sight. Use a distinctive pattern so it is distinguishable from other alarms nearby |
| Alert a user privately, in noise | A haptic output | It is felt only by the person holding the product, and it works where sound and sight both fail |
| Move something | A motor, a servo or a pump | A plain motor for continuous rotation, a servo for a held angle, a pump for fluid |
| Switch something large or mains | A relay | It carries the current and keeps the two circuits electrically separate, which is a safety requirement rather than a convenience |
Check the current before anything else: A chip pin supplies a few tens of milliamps: enough for an LED with a resistor, and for almost nothing else.
Every motor, lamp, heater and relay coil on this list needs a transistor or a driver chip between the pin and the device, and every coil needs a diode.
How this is tested — determining appropriate output devices to communicate or control. It comes up two ways:
Paper 1 — multiple choice
- Choose the output device for a stated requirement.
- Identify what must drive a stated load.
Paper 2 — analysing a product
- Justify the output devices for a named product.
- Explain how a named output device is driven from a control circuit.
The trap: Choosing an output without asking who perceives it, where, and whether they can. A buzzer in a ward at night, or a screen for a user who is not looking, is a failed specification.
IB-style questionJustify[6 marks]
An automatic greenhouse controller must open a vent, run a water pump, and tell the grower what it is doing. Justify the output devices.
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