Output Devices
1. What Counts as an Output Device
An output device takes binary data from the computer and turns it into a form a human can perceive — an image, a sound, a printed page — or into a physical action. As with input devices, exam questions ask how the device works, so learn the sequence of steps.
2. LCD Displays and LED Backlights
An LCD (liquid crystal display) screen is made of a grid of liquid crystal cells. Applying a voltage twists the crystals so that they either block light or let it through, which is how each pixel is turned dark or bright.
The crucial point: liquid crystals do not produce any light of their own. They only block or pass it. So an LCD screen must have a backlight behind it.
That backlight is what has changed over time:
- Older LCD screens used CCFL (cold cathode fluorescent lamp) backlights.
- Modern LCD screens use LEDs as the backlight instead.
This is why a television sold as an "LED screen" is really an LCD screen with an LED backlight — the picture is still produced by liquid crystals.
| Why LED backlights replaced CCFL |
|---|
| Lower power consumption, so cheaper to run and better for battery devices |
| Brighter, with better contrast and colour |
| Light up instantly — no warm-up time |
| Physically much thinner and lighter |
| Longer working life |
| Contain no mercury, unlike CCFL, so less environmentally harmful to dispose of |
3. Light Projectors
Both types throw an image onto a wall or screen, but they form it in completely different ways.
LCD Projector
- A bright light is shone through a prism or set of mirrors, splitting it into red, green and blue.
- Each colour is passed through its own small LCD panel, which forms that colour's part of the image.
- The three coloured images are recombined and sent out through a lens onto the screen.
DLP Projector
- A DMD chip (digital micromirror device) carries millions of microscopic mirrors — one per pixel.
- Each mirror tilts towards or away from the light source thousands of times per second. The proportion of time it spends tilted towards the light sets that pixel's brightness.
- Colour is added by a spinning colour wheel placed between the lamp and the chip (high-end models use three chips instead).
- The light reflected from the mirrors passes through the lens onto the screen.
| LCD projector | DLP projector |
|---|---|
| Sharper image and richer colour saturation | Higher contrast ratio — deeper blacks |
| Pixels can be visible as a faint grid ("screen door" effect) | Smoother image, no visible pixel grid |
| Filters need cleaning or replacing | More reliable and longer lamp life |
| Contrast is lower | Some viewers see a brief "rainbow effect" from the colour wheel |
4. Inkjet Printer
How it works
- A sheet is fed in and held in position by rollers.
- The print head moves across the paper, spraying extremely small droplets of ink.
- The droplets are produced either by thermal bubble — ink is heated so a bubble forms and forces a droplet out of the nozzle — or by piezoelectric action, where a crystal changes shape when a charge is applied and pushes ink out.
- The rollers advance the paper slightly and the head makes the next pass, until the page is complete.
| Advantages | Disadvantages |
|---|---|
| Excellent quality for photographs and colour images | Slow, especially for long documents |
| Cheap to buy | Cartridges hold little ink and are expensive to replace |
| Small, quiet, suited to home use | Wet ink can smudge, and nozzles clog if unused |
5. Laser Printer
A laser printer prints a whole page at a time using dry powder toner and static electricity.
How it works
- A rotating drum is given a positive electrical charge across its whole surface.
- A laser beam scans the drum, reversing the charge at every point where the image is to appear, leaving an invisible electrostatic image.
- Negatively charged toner is attracted to the reversed-charge areas of the drum, sticking only where the image is.
- The paper is given a charge and passes the drum, pulling the toner off it onto the page.
- A fuser applies heat and pressure, melting the toner so it bonds permanently to the paper.
- The drum's charge is removed, ready for the next page.
| Advantages | Disadvantages |
|---|---|
| Very fast, and faster still on long print runs | Expensive to buy, particularly colour models |
| Low cost per page; toner lasts far longer than ink | Toner cartridges are costly to replace |
| Output is dry and cannot smudge | Produces heat and ozone; bulkier than an inkjet |
6. 3D Printer
A 3D printer performs additive manufacturing: it builds a solid object up in very thin layers, rather than cutting material away.
How it works
- A design is produced in CAD software or captured with a 3D scanner.
- Software slices the model into hundreds or thousands of horizontal layers.
- In direct 3D printing, a print head moves left-right, front-back and up-down, depositing material — often molten plastic — one layer at a time, each layer setting before the next is added.
- In binder 3D printing, the printer makes two passes per layer: it lays down a layer of fine powder, then sprays a binding glue onto just the parts that are to become solid.
Materials include plastics, resin, powdered metal and ceramic powder.
Prosthetic limbsDental and medical implantsPrototypesAerospace and car partsReplacement parts for old machinery| Advantages | Disadvantages |
|---|---|
| Can make shapes that are impossible to cut or mould in one piece | Slow — a single object can take many hours |
| One-off custom items, such as a prosthetic fitted to one patient, become affordable | Equipment and materials are expensive |
| Less waste than machining material away | Limited range of materials, and finished strength can be lower |
| Parts can be made on site instead of shipped | Raises the possibility of manufacturing dangerous or counterfeit items |
7. Speaker
How it works
- A DAC (digital-to-analogue converter) turns the stored binary sound data into a varying electric current.
- The signal is amplified.
- The current passes through a coil of wire attached to a cone, sitting inside a magnetic field.
- The varying current makes the coil — and so the cone — move back and forth.
- The moving cone makes the surrounding air vibrate, producing sound waves.
8. Actuators
An actuator is an output device that produces a physical action rather than an image or a sound — a motor, a valve, a heater, a buzzer. Actuators are how a computer changes something in the physical world, and they are covered in detail in the Sensors and Actuators lesson in this section.
9. Exam Focus
An "LED screen" is an LCD screen. The liquid crystals form the image; the LEDs are only the backlight. If a question asks why LED backlights replaced CCFL, answer on power, thickness, brightness, warm-up time, lifespan and mercury.
Laser printers work by charge, not by heat alone. The sequence is: charge the drum → laser reverses the charge to draw the image → toner sticks to the charged image → paper attracts the toner → the fuser melts it on. Heat only appears at the last step.
Match the printer to the scenario. High-quality photos or occasional home use → inkjet. High volume, speed, low cost per page → laser. A physical object from a design → 3D printer. Justify with the named property.