The big idea: An output device turns an electrical signal back into something in the world.
The useful way to sort them is by how the user perceives them: seen, heard, felt, done, or kept. Choosing the group first and the device second is how an output is actually specified.
The guide's list of output devices, sorted by the sense each one reaches — and then how they are driven.
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| Group | Devices | What it buys, and what it costs |
|---|---|---|
| Seen | LED, lamp, LCD, braille display | Silent, so it disturbs nobody, and an LCD carries far more information than any other output. Useless to a user who is not looking — and a braille display is the output that carries the same information by touch |
| Heard | Buzzer, speaker, headphones | Reaches a user who is not looking, which is why every alarm uses it. It also reaches everyone else, so it is wrong in a library or a ward at night, and useless to a deaf user |
| Felt | Haptic motor, vibration alert | Private, and it works in noise and darkness. It needs contact, so it only works for something worn or held |
| Done | Motor, relay | Acts on the world rather than telling somebody something. Both usually need more current than a chip can supply directly |
| Kept | Printer, plotter | The output outlives the moment: it can be taken away, filed, signed or checked later, which is why both survive in medicine, surveying and manufacturing |
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A chip pin cannot drive most of these: A microcontroller pin supplies a few tens of milliamps — enough for an LED with a resistor, and nothing else.
A motor, a lamp or a mains load needs a transistor or a relay between the pin and the device. Connecting one directly destroys the pin.
Specifying the drive
Work out the current
An LED is about 20 mA. A small motor is hundreds of milliamps. A mains heater is several amps.
Choose transistor or relay
A transistor for a DC load within its rating. A relay when the load is mains, or when the two circuits must be electrically separate.
Add the flyback diode
Across any coil — a relay or a motor — so the spike when the current stops has somewhere to go instead of destroying the transistor.
Check the supply can deliver it
A motor starting draws several times its running current, and a supply that sags will reset the microcontroller at the same moment.
How this is tested — outlining analogue and digital output components. It comes up two ways:
Paper 1 — multiple choice
- Identify the output device suited to a stated situation.
- Choose what must sit between a chip pin and a motor.
Paper 2 — analysing a product
- Justify the output device chosen for a named product.
- Explain how an output device is driven from a control circuit.
The trap: Choosing an output without asking who has to perceive it, where, and whether they can. A buzzer in a hospital ward at night, or a screen for a blind user, is a failed specification however good the circuit is.
A hospital infusion pump must alert staff when a line is blocked. Justify the output devices you would specify.
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