← IGCSE Computer Science Paper 1

Infra-red touch screen

A screen with a grid of infra-red beams across it. A touch breaks beams, and the broken x and y beams give the touch position. Works with any object.

Part of: IGCSE Computer Science 0478 — 3.2 Input and Output Devices — Module 13

Understanding Infra-red touch screen in IGCSE Computer Science Paper 1

What it does: The broken vertical beam gives the x position and the broken horizontal beam gives the y position. The processor combines them to find the touch point. Infra-red touch screens detect a blocked beam, not pressure or electrical charge, so any object that breaks the beams will register a touch. Each beam travels across the screen from an emitter to a receiver opposite it, so they must be on opposite sides. Two flexible layers pressed together describes a resistive screen. An infra-red screen uses a beam grid.

Common mistakes: The receivers must be on the sides opposite the emitters so the beams cross the screen. They should be on the right and bottom, not the same sides as the emitters. The touch position is the broken x beam and the broken y beam together. The second value should be y_broken, not x_broken again.

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This module's key terms are also in a printable crossword (Modules 252–261) and word search (Modules 240–261).

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Learning
Duo
How does an infra-red touch screen detect where you touch?
A touch changes the electric charge on a glass layerA touch breaks infra-red beams in a grid across the screen
Matches
Infra-red screens only work with a bare finger
↓
Wrong: any object that breaks the beams works, such as a gloved hand or a pen. A bare finger is not needed.
Applying
Meanings
Term
infra-red touch screen
Meaning
A screen with a grid of infra-red beams across it. A touch breaks beams, and the broken x and y beams give the touch position. Works with any object.
Practice
IF x_beam_broken AND y_beam_broken THEN
Testing
Gaps
An touch screen uses a beam grid
Check Answer
Spots
1emitters ← left AND top sides
2receivers ← left AND top

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Quick Questions

1. How does an infra-red touch screen detect where you touch?
A. A touch breaks infra-red beams in a grid across the screen ✓
B. A touch changes the electric charge on a glass layer
The infra-red grid of beams is interrupted by the touch. Changing charge on the glass is how a capacitive screen works.
2. Where are the infra-red emitters and receivers placed?
A. Emitters on two sides, receivers on the opposite sides ✓
B. Emitters and receivers both on the same side of the screen
Each beam travels across the screen from an emitter to a receiver opposite it, so they must be on opposite sides.
3. How is the position of a touch worked out?
A. The screen measures how hard the finger presses down
B. The broken horizontal and vertical beams give the x and y position ✓
Where the broken x beam and broken y beam cross is the touch point. Pressure measurement belongs to resistive screens.
4. Which objects can be used on an infra-red touch screen?
A. Any object that breaks the beams, such as a gloved finger or a pen ✓
B. Only a bare finger, because it needs to conduct electricity
Infra-red screens only need a beam to be blocked, so any object works. Only capacitive screens need a conductive touch.
5. Which use is an infra-red touch screen well suited to?
A. A tiny smartwatch screen only
B. A large interactive whiteboard or information kiosk ✓
Infra-red screens suit large screens because the beam grid is easy to scale up in size.

Match the Pairs

infra-red touch screen → A screen with a grid of infra-red beams across it. A touch breaks beams, and the broken x and y beams give the touch position.
emitters on two sides, receivers opposite → The infra-red LEDs are placed along two edges of the screen. Receivers sit on the opposite edges to detect each beam.
A touch breaks one beam running across and one beam running down. Together they give the x and y position. → broken beams: x = 5, y = 3 -> touch at (5, 3)
It works with any object that blocks the beams, not just a finger, so gloves or a pen can be used. → touch_object = finger OR glove OR pen
Infra-red screens only work with a bare finger → Wrong: any object that breaks the beams works, such as a gloved hand or a pen. A bare finger is not needed.
Dust on the screen cannot affect an infra-red screen → Wrong: dust or dirt can block the beams and cause false touches, so the screen needs to be kept clean.

Key Term

infra-red touch screen
A screen with a grid of infra-red beams across it. A touch breaks beams, and the broken x and y beams give the touch position. Works with any object.

Fill in the Blanks

1. An infra-red touch screen uses a beam grid
Hint: Type the kind of invisible light the beams in the grid are made of.
2. Emitters send beams to receivers opposite
Hint: Type the name of the parts that detect the beams on the opposite side.
3. A touch breaks beams in the grid
Hint: Type the verb for what a finger or object does to the beams.
4. Broken x and y beams give the touch position
Hint: Type the word for the location on the screen that is worked out from the broken beams.
5. Dust or dirt can cause a false touch
Hint: Type the word meaning a touch that was not real.

Spot the Error

Question 1
A emitters ← left AND top sides
B receivers ← left AND top
C beams ← grid of IR light
Line B — The receivers must be on the sides opposite the emitters so the beams cross the screen. They should be on the right and bottom, not the same sides as the emitters.
Question 2
A x_broken ← 6
B y_broken ← 4
C pos ← (x_broken, x_broken)
Line C — The touch position is the broken x beam and the broken y beam together. The second value should be y_broken, not x_broken again.
Question 3
A WHILE screen_on
B scan all IR beams
C IF any beam is broken THEN
D measure finger pressure
E ENDWHILE
Line D — An infra-red screen does not measure pressure. It works out the position from which x and y beams are broken. The line should find the x and y position.
Question 4
A object ← glove
B beam_broken ← FALSE
C OUTPUT 'Touch detected'
Line B — Any object, including a gloved finger, breaks the infra-red beams. So beam_broken should be TRUE and the touch is detected.
Question 5
A screen_size ← LARGE
B suits ← phone AND smartwatch
C technology ← infra-red
Line B — Infra-red touch screens suit large screens such as kiosks and interactive whiteboards. They are not used on small devices like phones and smartwatches.

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