Exam practice

• Physical computing: programming a computer that senses the world and acts on it. Example: a night light that switches on when it gets dark.
• Sensor: an input device that measures something in the world by itself. Example: a temperature sensor in a rice cooker; a light sensor.
• Microcontroller: a small computer (processor, memory, storage and pins) on one chip, usually running one program. Example: the chip in a micro:bit, a remote control or a washing machine.
• Threshold: the number a reading is compared with, to decide between two actions. Example: in if light < 50:, 50 is the threshold between dark and light.
• Polling: checking an input again and again in a loop. Example: while True: asking button_a.is_pressed() hundreds of times a second.

• i.Hand dryer. Input: a sensor that notices hands under it. Program: when hands are there, blow; stop a moment after they go. Outputs: the fan’s motor and the heater.
• ii.Rice cooker. Inputs: the buttons (which kind of rice, the time to be ready) and the temperature sensor under the pot. Program: soak, heat hard, then gently, then keep warm, checking the temperature again and again. Outputs: the heater and the display.
• iii.Traffic light. Inputs: the button for people on foot, and at many junctions sensors that notice waiting cars. Program: change the lights in the right order with the right times, and give people a turn to cross after the button is pressed. Outputs: the lamps (and at some crossings a beeping sound for people who can’t see the lamps).

Row by row from the top (9 is lit, 0 is off):

Two eyes and a smiling mouth: a happy face.

from microbit import * while True:    if button_a.is_pressed():        display.show(Image.YES)    elif button_b.is_pressed():        display.show(Image.NO)    else:        display.clear()

is_pressed() is right here, not was_pressed(): the picture must stay for as long as the button is held.

is_pressed() answers whether A is down at this moment: True all the time it is held. was_pressed() answers whether A has been pressed since the program last asked, then forgets that press: one True for each press, even a quick one made during a sleep.

• is_pressed(): a doorbell that rings while the button is held, or a torch that lights while you press.
• was_pressed(): a counter for people coming in, a score in a game, a button that moves to the next picture: anything where each press must count once.

Try it: on the night-light board, move the Light slider back and forth across 50. Then think about using two different numbers.

It will flicker, switching on and off every time the reading crosses 50. Better: use two thresholds. Switch on when the light falls below 40, and switch off only when it rises above 60; between 40 and 60, leave it as it is. To do that, the program remembers whether the light is on in a variable:

from microbit import * on = Falsewhile True:    light = display.read_light_level()    if light < 40:        on = True    elif light > 60:        on = False    if on:        display.show(Image("99999:99999:99999:99999:99999"))    else:        display.clear()    sleep(500)

Thermostats in fridges and water heaters work the same way, with a gap between switching on and switching off, so they don’t click on and off every few seconds.

The need: seedlings can wilt and die on a hot day; the gardener wants a warning in time to open the shade or water them.

Rules: if the temperature is above 35 °C, show a sad face and beep every few seconds; otherwise show a happy face. When A is pressed, scroll the temperature.

from microbit import *import music while True:    if button_a.was_pressed():        display.scroll(str(temperature()) + " C")    if temperature() > 35:        display.show(Image.SAD)        music.pitch(880, 300)        sleep(2000)    else:        display.show(Image.HAPPY)        sleep(500)

Tests (on the Your device board, with the Temperature slider), each written down before running:

The board’s sensor measures its own chip, so before trusting the device, put it beside a real thermometer for a day and compare.

from microbit import *import random while True:    if accelerometer.was_gesture("shake"):        display.show(random.randint(1, 6))

random.randint(1, 6) gives a whole number from 1 to 6, each equally likely, and display.show shows a single digit straight away.

Differences (any three):

• The micro:bit has LEDs, buttons, sensors and a speaker built in; the Arduino Uno has almost nothing built in, and you connect your own parts to its pins.
• The micro:bit is programmed in MicroPython, blocks or JavaScript; the Arduino Uno usually in the Arduino language, based on C++.
• The micro:bit can be programmed from a phone by Bluetooth or from a browser; the Arduino usually from the Arduino IDE on a computer, by USB.
• The micro:bit runs on two AAA cells; the Uno usually on USB, a 9 V battery or an adapter.

The same: each is a board with a microcontroller that runs one program over and over, taking in inputs, deciding, and switching outputs. A program is written on a computer and flashed onto the board, which then runs on its own.

The board’s outputs give only a tiny current at 3 to 5 volts, far too little to run a pump’s motor. A relay is a switch worked by electricity: the board’s small current switches the relay, and the relay switches the pump’s big current. The mains in Nepal is 230 volts, which can kill and start fires. A board and the wires a student builds are made for low voltage only, so anything on the mains must use a relay module made for the job and be built and checked by an adult who knows electrical work.