2.1 · Sensors & Devices

Goal: describe how programs read sensors and control simple actuators.

What a sensor does

A sensor converts a physical quantity (light, temperature, motion, sound) into an electrical signal that a microcontroller can read.

Sensor	Measures	Example use
Light (LDR)	Brightness	Auto-on street lamps
Temperature	Heat	Smart thermostat
Accelerometer	Motion / tilt	Phone screen rotation
Gyroscope	Rotation	Drone stabilisation
Magnetometer	Magnetic field	Compass
Microphone	Sound	Voice assistants
Ultrasonic	Distance	Reverse-park sensors
GPS	Location	Maps
Heart-rate	Pulse	Fitness wearables

Common platforms

• micro:bit — BBC's educational board, beginner-friendly.
• Arduino — Wide ecosystem; C/C++ programming.
• Raspberry Pi — Full Linux computer; Python easy.
• ESP32 / ESP8266 — Wi-Fi-enabled microcontrollers.

Reading a sensor on a micro:bit

# micro:bit (MicroPython)
from microbit import display, accelerometer, button_a
import time

while True:
    if button_a.was_pressed():
        x = accelerometer.get_x()    # -1024 to +1024
        display.scroll(str(x))
    time.sleep(0.1)

Controlling an actuator

Actuator	Action
LED	Light on/off, brightness
Buzzer	Beep
Servo motor	Rotate to angle
DC motor	Drive a wheel
Stepper motor	Precise rotation
Display	Show text or images

from microbit import pin0, display
pin0.write_digital(1)   # turn on connected LED
display.show("OK")

Sense → think → act pattern

read sensor → make decision → drive actuator

from microbit import display, accelerometer
import time

while True:
    tilt = accelerometer.get_x()
    if tilt > 200:
        display.show(">")
    elif tilt < -200:
        display.show("<")
    else:
        display.show("o")
    time.sleep(0.1)

Hong Kong context

• Schools use micro:bit and Arduino in STEM lessons.
• HKSTP and Cyberport run maker spaces.
• Smart city initiatives (smart lampposts) use environmental sensors.

Common student mistakes

• Sampling too fast → wastes battery.
• Treating analog readings as exact (they have noise).
• Forgetting to power the sensor correctly.

Exam-style question

Q (5 marks): Describe how a program can use a light sensor to switch on an LED when the room becomes dark. Sketch the pseudocode.

Sample answer:

The program reads the light sensor periodically, compares the reading to a threshold, and drives the LED accordingly.

LOOP forever
    light ← read light sensor
    IF light < THRESHOLD THEN
        turn LED ON
    ELSE
        turn LED OFF
    END IF
    WAIT 100 ms
END LOOP

The threshold should be tuned to the room — too high and the LED stays on all the time, too low and it never comes on. Battery life can be extended by sampling less often (e.g. every second).

Key takeaways

• Sensors capture the physical world; actuators change it.
• Sense → think → act pattern.
• Programs run in loops, sampling and reacting.

➡️ Next: 2.2 Event-driven Programs