Robot Logic in Scratch

Lessons

Senior 3 · ICT

Robot Logic in Scratch

5 lessons

Lesson 3 of 5

Simulating One Sensor: Touch, Colour, Distance, Temperature and Sound

60 min

A robot knows only what it senses

Your simulated robot from the last lesson drives in a straight line until you stop it. It drives straight into walls because it cannot feel them. A robot is only as clever as its sensors. In this lesson you give it one sense at a time, learn what the matching real sensor measures, and write the Scratch logic that reacts to it.

Definition

Digital reading: a sensor value with only two states, such as pressed or not pressed (true or false).

Analogue reading: a sensor value that can take many values in a range, such as a distance of 0 to 255 cm or a loudness of 0 to 100.

Threshold: the value at which a program changes its decision, such as “closer than 60”.

Calibration: measuring a sensor's readings in known conditions (for example on black and on white) to choose good thresholds.

Five sensors and how to simulate them

Sensor	Measures	Real device	Reading	Simulation in Scratch
Touch	Contact with an object	Push switch on a bumper	Digital	<touching [Wall]?>
Colour (light)	Colour or brightness of a surface	Colour sensor or light-dependent resistor, facing the floor	Analogue or a colour	<color [green] is touching [black]?>, where the green spot is part of the robot's costume
Distance	How far away an object is	Ultrasonic sensor (echo of sound) or infrared sensor	Analogue	(distance to [Box])
Temperature	How hot or cold	Thermistor or temperature chip	Analogue	A variable called temperature shown as a slider
Sound	Loudness	Microphone	Analogue	(loudness): uses the computer's real microphone

Diagram drawn by Ganzaa. Draw the robot's costume like this: Scratch can test whether one colour of the costume is touching a colour on the stage.

1. Touch sensor: a bumper

Paint a Wall sprite. The robot drives until it touches the wall, backs off and turns. A real bumper is a switch that closes when pressed: a digital reading.

2. Colour sensor: following a line

Paint a thick black line on the backdrop. A single colour sensor can follow the edge of the line: when the sensor is over black, steer away from the line (left); when it is over the floor, steer back (right). The robot wiggles along the edge, with the line on its right, where the sensor is.

3. Distance sensor

An ultrasonic sensor sends out a short pulse of sound, too high for people to hear, and times its echo. Sound travels about 343 metres per second in air at 20 °C, and the pulse goes there and back, so:

$d = \frac{v \times t}{2}$

In Scratch, distance to [Box] measures from the robot's centre to the box's centre in any direction, so it “sees” behind too. That is good enough here; the next lesson builds a forward-looking sensor.

Worked example 1: the echo

An ultrasonic sensor hears the echo 2.9 milliseconds (0.0029 s) after sending the pulse. How far away is the wall?

$d = \frac{343 \times 0.0029}{2} \approx 0.50 \text{ m}$

The other way round: for a box 1.2 m away, the echo takes $t = \frac{2 \times 1.2}{343} \approx 0.0070$ s, that is about 7.0 ms. Forgetting to divide by 2 is the classic mistake: it would put the wall at 1.0 m, twice as far as it really is.

4. Temperature sensor

Scratch has no thermometer, so a slider stands in for one: make a variable temperature, show its monitor, right-click it → slider, and set the range to 0–50 (right-click → change slider range). Dragging the slider imitates the room warming up. Give the robot (or a fan sprite) two costumes, fan on and fan off:

5. Sound sensor

The loudness block in Sensing reads the computer's microphone (Scratch asks permission the first time). It reports a value from 0 to 100. A clap makes the robot turn back:

Block diagrams drawn by Ganzaa in Scratch 3 colours.

Worked example 2: calibrating a light sensor

A real robot's light sensor reads 8 on the black line and 72 on the white floor. A good threshold is halfway:

$\text{threshold} = \frac{8 + 72}{2} = 40$

The rule becomes: if the reading is less than 40, the sensor is over the line. Later the classroom gets sunnier: black now reads 20 and white 90, so the best threshold is $\frac{20 + 90}{2} = 55$ . With the old threshold of 40, the edge of the line, which reads about 50 in sunlight, would be mistaken for the floor. That is why robots are calibrated in the room where they will run.

Warning

Common simulation mistakes

Colour not exact: color [ ] is touching [ ]? needs exactly the colours in the costume and backdrop. Pick both with the eyedropper in the block.
Sensor spot too big: a large green spot touches the line too early. Keep it small and at the front.
Turning too sharply: with 20° turns, the line follower zig-zags off the line; with 1°, it cannot follow bends. Test values between.
A noisy room: background noise above the loudness threshold makes the robot turn by itself. Watch the loudness monitor first, then choose the threshold.

First_Coding: a line-following robot in Scratch, using a colour in the robot's costume as its sensor.

✦

Hands-on: one sensor at a time

Use your Robot_Sim project. Work through the five sensors; save a copy after each.

Touch: add a Wall sprite and the bumper script. Count, with a variable, how many times the robot bumps in 30 seconds.
Colour: redraw the robot costume with a small green sensor spot at the front. Paint a black oval track on the backdrop. Add the line-follower script; find the smallest turn angle that still follows the bends.
Distance: add a Box sprite and the distance script. Show the distance monitor and find the reading when the robot stops.
Temperature: add a Fan sprite with two costumes and the slider. At what slider value does the fan switch on?
Sound: add the clap script. Watch the loudness monitor in a quiet room and in a noisy one, and choose a threshold that works in both.
For each sensor, write one sentence: what the real sensor measures, and how your simulation imitates it.

Practice

Define a digital reading, an analogue reading and a threshold.
For each sensor (touch, colour, distance, temperature, sound), name what it measures and the Scratch block or method that simulates it.
Why does the line follower turn left when its sensor is on the line and right when it is off it, rather than going straight?
An echo returns after 5.8 ms. Using 343 m/s, find the distance to the object.
A light sensor reads 12 on black and 84 on white. What threshold would you choose? Write the rule.
In the fan script, the temperature slider is at 30. Is the fan on or off? Explain.
A pupil's line-following robot works in the morning but runs off the line every afternoon. Analyse the likely cause and the fix.
A robot cleaner for a school corridor can have only one sensor. Evaluate touch, distance and colour sensors for this job, and recommend one.

Show answersHide answers

Digital: only two states (true or false). Analogue: many values in a range. Threshold: the value at which the program's decision changes.
Touch: contact, <touching [Wall]?>. Colour: colour or brightness of the floor, <color [green] is touching [black]?>. Distance: how far an object is, (distance to [Box]). Temperature: heat, a slider variable. Sound: loudness, the (loudness) block with the microphone.
With one sensor the robot cannot tell which side of the line it is on, so it follows the edge: over black it turns away (left), off black it turns back (right), so it settles on the edge with the line on its right. Going straight, it would leave the line at the first bend.
$d = \frac{343 \times 0.0058}{2} \approx 0.99$ m, about 1 m.
$\frac{12 + 84}{2} = 48$ . Rule: if the reading is less than 48, the sensor is over the black line; otherwise it is over the floor.
Off. The test is temperature > 30; 30 is not greater than 30, so the else branch runs. It switches on from 31 (or any value above 30).
The afternoon sun changes the light readings (both black and white read higher), so the morning threshold no longer separates line from floor. Recalibrate in the afternoon light, shade the sensor, or use a threshold measured at the time of running.
Touch: cheap and reliable, but the robot must hit things to find them, which may hurt people or knock objects. Distance: detects people and walls before contact, safer and smoother. Colour: can follow a line or detect the edge of a mat, but cannot see obstacles. Recommendation: a distance sensor, because a corridor has people and walls to avoid without bumping them.

Sources and credits

Ultrasonic transducer, Speed of sound and Sensor, Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Scratch is a project of the Scratch Foundation; block names and colours follow Scratch 3.
Photo: EV3 Ultrasonic Sensor.jpg, Jim McTurbo, Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: Scratch - Line Following Robot, First_Coding, embedded from YouTube.
The diagram, block diagrams, tables, worked examples, hands-on task and practice items were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A robot knows only what it senses

2Five sensors and how to simulate them

31. Touch sensor: a bumper

42. Colour sensor: following a line

53. Distance sensor

64. Temperature sensor

75. Sound sensor

8Hands-on: one sensor at a time