--- title: "A small system" subtitle: "Arduino · Real hardware · Lesson 7" --- # A small system **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Your plant-water alarm works on your desk: it blinks when you twist the knob down. Does it meet the brief? How would you know, without watching it for a week? :::teacher-note **In one sentence:** Students run a plant-water alarm against a test table that passes but misses the threshold, learn to choose test cases (either side of the threshold, the threshold itself, the ends), fix an off-by-one, add a steady state, build it on real hardware, and meet a three-band fridge alarm brief against six tests. **Students should be able to say:** I can choose test cases for a brief, including the boundary. I can fix a sketch until every test passes. I can explain what passing tests does and doesn't show. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **The sketch says if (level < 300). The brief says 300 or below. Which test reading would show the mistake?** ☐ 0 ☐ 150 ☐ 300 ☐ 800 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 300: the brief wants it to blink, but 300 < 300 is false, so it stays off. Any reading away from 300 behaves the same either way. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: choose the tests :::keep-together ### A test table 1. $\text{brief: blink at } \le 300$ — Find the threshold and which side it belongs to. 2. $299, \; 300 \to \text{blink}$ — Just below, and the threshold itself (the brief includes it). 3. $301 \to \text{off}$ — Just above. 4. $0 \to \text{blink}, \; 1023 \to \text{off}$ — The two ends of the range. 5. $\text{5 cases}$ — Each one could catch a different slip. Now pick the best set. **Which set of readings tests this brief best?** ☐ 0, 299, 300, 301, 1023 ☐ 300 only ☐ 0, 1023 ☐ 100, 200, 400, 500 ::: :::teacher-note **Answer:** 0, 299, 300, 301, 1023 Both sides of 300, 300 itself, and the two ends. ::: :::keep-together ### Find the mistake **Sam's alarm passed both tests, but stays off at a reading of 300. Click the line where Sam's plan first goes wrong.** ``` 1 The brief: the alarm blinks when the soil reads 300 or below. 2 Test 1: reading 0, the alarm should blink. 3 Test 2: reading 1023, the alarm should be off. 4 Two tests pass, so the alarm meets the brief. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 4 Line 4: two far-away tests can't show a slip at the threshold. Sam needs cases at 299, 300 and 301. ::: :::typical ## Vary: change the brief **Fix the boundary.** **Three states.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: :::typical ## Build it for real A real sensor's reading flickers by a few numbers, so a reading near the threshold can make the alarm flicker too. The fix (a gap between switching on and off) is a good stretch. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Tests :::keep-together ### A fan **A fan must turn on when the temperature reading is above 600. Which three readings test the threshold?** ☐ 600 only ☐ 0, 600 and 1023 ☐ 100, 600 and 900 ☐ 599, 600 and 601 ::: :::teacher-note **Answer:** 599, 600 and 601 Just below, at, and just above: 600 itself should be off (above, not at). ::: :::keep-together ### All pass **Every test passes. What can you say?** ☐ The sketch has no mistakes ☐ The tests are right ☐ The sketch does what those tests check: no more ☐ The sketch meets the brief ::: :::teacher-note **Answer:** The sketch does what those tests check: no more Tests show what they check. A good table covers the boundaries and the ends. ::: ## Make: a fridge door alarm Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **A brief says a buzzer sounds when the reading is 500 or more. Write three test readings for the threshold and what each should do.** {{lines-3}} ::: :::teacher-note **Answer:** 499 silent, 500 sounds, 501 sounds. :::