Input and output devices

What you need to be able to do

“Describe the principal operation” wants the sequence of steps the device goes through, in order, each one a separate point, using the technical names (diaphragm, coil, drum, toner, extruder, coordinates). Count the marks: 4 marks means four distinct steps.

Laser printer

1. The drum is given an electrostatic charge (positive) by a charge roller / corona wire.
2. A laser beam scans across the rotating drum, guided by a mirror; where it shines it removes the charge, leaving an electrostatic image of the page.
3. The drum rotates past the toner (charged powder); toner sticks only to the areas forming the image.
4. The paper is given a charge that pulls the toner off the drum onto the paper as it passes.
5. The paper passes through the fuser: heated rollers melt the toner and fuse it to the paper.
6. A discharge lamp removes the remaining charge from the drum, ready for the next page.

3D printer

1. A digital 3D model is made with CAD (computer-aided design) / 3D modelling software (or a 3D scan) and sent to the printer.
2. Software slices the model into thin horizontal layers.
3. The printer builds the object one layer at a time, starting from the bottom, using x, y and z coordinates: this is additive manufacturing.
4. Each layer is fused / cured onto the one below; the process repeats for every layer; some materials need time to cool / set.

Sensors inside: a temperature sensor checks the nozzle / material is hot enough but not overheating, so the printer can adjust the heater.

Microphone

1. Sound waves (vibrating air) reach a thin diaphragm, which vibrates.
2. The vibration moves a coil past a magnet (dynamic microphone) / changes the capacitance between two plates (condenser) / deforms a crystal (crystal / piezo).
3. This produces a small varying electrical signal (an analogue voltage) matching the sound wave.
4. For a computer, the signal is then sampled and converted to digital by an analogue-to-digital converter (ADC).

Speakers

1. The digital sound data is converted to an analogue signal by a digital-to-analogue converter (DAC) and amplified.
2. The varying electric current flows through a coil (voice coil) attached to a cone (diaphragm).
3. The coil sits in the field of a permanent magnet: the changing current makes a changing magnetic field, so the coil is pushed in and out.
4. The cone moves with it, vibrating the air and producing sound waves.

Touchscreens

Either way, a microprocessor calculates the x, y coordinates of the touch.

Other types also credited: infra-red (beams across the screen are broken by the finger), optical imaging (a shadow), acoustic pulse. To turn a touch into a selection, the software matches the coordinates to the item displayed there (e.g. a menu item is added to the order).

Virtual reality (VR) headset

1. One or two (LCD / OLED) displays, seen through lenses, show a slightly different image to each eye, giving a 3D view.
2. Speakers / headphones give surround sound for a realistic experience.
3. Head movements are detected by sensors: a gyroscope / accelerometer (and magnetometer).
4. A microprocessor analyses the sensor data to find the direction and speed of the movement and updates the image to match, very quickly.
5. Some headsets use cameras to track the user's eye movements.

Mark-scheme focus

Complete the description of touchscreens (5 marks)

Credited

• Resistive … circuit … capacitive … change (increase / decrease) … coordinates B5

Not credited

• the two types swapped
• vague words instead of “coordinates” (e.g. “place”, “area”)

From: 9618/13/M/J/23 Q3(a)

Describe the principal operation of a microphone (3 marks)

Credited

• The microphone has a diaphragm / ribbon B1
• Incoming sound waves cause vibrations of the diaphragm B1
• … causing a coil to move past a magnet (dynamic) // changing the capacitance (condenser) // deforming a crystal; an electrical signal is produced B1

Not credited

• describing a speaker
• describing sampling

From: 9618/12/O/N/21 Q3(a)

Worked examples

Example 1 · 3D printing and the temperature sensor (Paper 1, 5 marks) ★ Easy

1 Read it with me what is actually happening?

(a) generic steps (model, layers, bottom up) plus one method-specific step. (b) why the printer measures temperature.

2 What is it asking for? the target, part by part

• (a)3 steps3
• (b)purpose2

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

1. (a) Additive manufacturing from a digital 3D model / CAD file B1; builds the model one layer at a time, starting from the bottom (x, y, z coordinates) B1; e.g. FDM: material heated and pushed through a nozzle / extruder // layers fused together B1
2. (b) To prevent overheating // ensure the material is hot enough B1 … by identifying the temperature of the material / object being printed B1

6 Check it does the answer make sense?

✓

Example 2 · Principal operation of a VR headset (Paper 1, 4 marks) ★ Easy

1 Read it with me what is actually happening?

Four gaps: what shows the picture, what senses head movement, what the processor works out, what watches the eyes.

2 What is it asking for? the target, part by part

• (a)4 words4

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

1. (LCD) displays / screens / lenses
2. gyroscope / accelerometer
3. direction / speed
4. digital cameras B4

6 Check it does the answer make sense?

✓

Example 3 · From a touch to a menu selection (Paper 1, 4 marks) ★★ Medium

1 Read it with me what is actually happening?

Two halves (max 2 each): how the point of touch is detected, then how it becomes a menu item.

2 What is it asking for? the target, part by part

• (b)(i)2 + 24

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

1. Point of touch (max 2): resistive: two layers of material make contact and complete a circuit // capacitive: contact with the screen creates a change in charge // infra-red beams broken B2
2. Selection (max 2): the point of touch is used to determine the x and y coordinates B1; the menu item at those coordinates is recognised and added to the order B1

6 Check it does the answer make sense?

✓

Example 4 · Resistive touchscreen: true or false (Paper 1, 1 mark) ★★ Medium

1 Read it with me what is actually happening?

One mark for all three rows: think about how resistive screens work (pressure on layers).

• alwaysbe suspicious of absolute words

2 What is it asking for? the target, part by part

• (c)3 rows1

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

Why does any object work?

It only needs pressure to push the layers together, not electrical charge.

5 Now solve it have a go on paper first, then open the worked solution

1. Always five layers: false
2. Processor determines the coordinates: true
3. Works if any object touches: true B1

6 Check it does the answer make sense?

The report: the last statement was the one most often wrong ✓.

Summary

Can I do this?

Quiz

Concept checks

Answer each one in your head first, then open it.

Exam-style practice

New questions, not used above. Do them on paper, then mark your work against the official mark scheme.