Read and React to Obstacles

Sessions

1. Session 1 · 75 min

   Where should the sensor sit on the rover?

   Mounting the HC-SR04 at the front, level, with nothing in front of its two transducers; distance from the sensor's face vs from the rover's front edge (an offset); how wide it sees one box, found by experiment.

   In the studio

   • Read a sensor

   Hands-on

   In Read a sensor, set the sensor 3 cm behind the ruler's zero and see what happens to every reading. On the rover, with the motor battery switched off and unplugged and USB unplugged, mount the sensor at the front and wire it as the guide says (VCC 5V, GND, Trig D9, Echo D10). Upload Read a sensor's sketch and print the distance. With a box straight ahead, compare the reading with a ruler from the rover's front edge at three distances, and work out the offset. Then slide the box sideways at 40 cm until it is no longer seen, and mark that edge on both sides.

   Starting code: Read a sensor's sketch (Copy code in the lab's code panel): it reads with pulseIn(echoPin, HIGH, 30000) and prints the distance, or “no echo” for 0.

   Pins

   D9

   HC-SR04 Trig

   D10

   HC-SR04 Echo

   Evidence to collect

   The offset worked out from three readings, and a floor sketch of where the rover sees this box at 40 cm (both edges marked; a smaller or angled box gives other edges).

   Predict

   Will the reading be more or less than the ruler measured from the rover's front edge?

   Debug

   If every reading is “no echo”, unplug USB and check Trig and Echo against the guide.

   Reflect

   Where in front of the rover would a box be missed?

   ⚠ Adult checks before power-on: Sensor VCC to 5V and GND to GND, not reversed, before USB goes in; Trig on D9 and Echo on D10, with no other part on those pins. The motor battery's power-off switch is off and its holder unplugged: no motor turns in this session. No wire is moved, pulled or plugged in while anything is powered: power-off switch off and battery unplugged first, then USB.

   Works towards: Stop rules

2. Session 2 · 90 min

   How far before the box must the rover decide to stop?

   The stop rule and its margin: the rover keeps moving between two readings (at least 60 ms apart) and while it brakes, so it decides at stopCm and comes to rest nearer. Brake with the driver's short brake.

   In the studio

   • Build a project: the Rescue Rover

   Hands-on

   In Build a project, run the five trials at speed 128 and at 255 with the same stopCm, and compare where the model rover comes to rest (255 stays in the model). On the rover, start from your R13-U1 session 5 sketch and paste in the reading lines from Read a sensor's sketch: read, and if the reading is real and under stopCm, brake; otherwise drive forward. Upload and test it lifted first, wheels in the air, with a box moved by hand. Then on the floor drive at a box three times at the lowest value that starts both wheels and measure where it comes to rest; repeat at a higher value, never above 160 of 255. Choose a stopCm that leaves at least your margin (say 15 cm) in every run.

   Starting code: The learner's own sketch from R13-U1 session 5, plus the reading lines from Read a sensor's sketch (Copy code in the lab's code panel).

   Evidence to collect

   A rest-distance table (speed × stopCm × three runs), the chosen stopCm and margin, and a sentence on why the faster rover rests nearer.

   Predict

   How much nearer than stopCm will the rover rest at each speed?

   Debug

   If it never brakes, print each reading to the Serial Monitor before changing the rule.

   Reflect

   Why does the faster rover need a bigger stopCm?

   ⚠ Adult checks before power-on: The motor battery reaches the driver's VM only, through its power-off switch; the board's 5V goes to the driver's VCC only; every GND is joined. Lifted for the first run; then on the floor at the lowest value that starts both wheels, never above 160 of 255, the power-off switch in reach, a soft box and a clear area. Lift the rover (wheels in the air) before switching the motor battery back on: the sketch keeps running on USB, so the wheels take its last command at once.

   Works towards: Stop rules

3. Session 3 · 90 min

   What should the rover do when there is no echo?

   With pulseIn(echoPin, HIGH, 30000), as in Read a sensor, no echo gives 0. pulseIn()'s default timeout is one second, and with no echo the sensor's Echo pin stays HIGH for 38 ms (datasheet version 2). In the UNO's core, one loop budget covers both the wait for the pulse and the pulse itself, so a 38 ms pulse never fits in 30000 µs and pulseIn() gives 0; with the default timeout it would return about 38000 µs, read as about 655 cm. That 0 is no reading, not 0 cm. Then: one stray close reading; two close readings in a row; and silence, a run of 0s that may mean a loose sensor.

   In the studio

   • Read a sensor

   Hands-on

   In Read a sensor, try the rover's stop rule in Apply: does 0 count as close, or is it skipped? One close reading, or two? Find the rule that passes all three runs, including open space, where it must never stop. On the rover, lifted with its wheels in the air, upload your session 2 sketch, point the sensor at open space and at a soft cushion, and print each reading. Change the rule: skip a 0, and brake only after two close readings in a row. Then try a silence rule: brake and wait after one second with no reading. It catches a loose sensor, but it also stops the rover in open space, which the Apply rule never does: keep it only for a course with walls always in range. To fake a loose sensor, switch the motor battery off, unplug it and USB, and take out the Trig wire (never the Echo wire: a floating D10 can read HIGH or LOW at random). Then plug USB in again and watch: with no trigger the sensor never starts a reading, Echo stays LOW, and every pulseIn() gives 0 after its timeout.

   Starting code: The learner's session 2 sketch.

   Pins

   D9

   HC-SR04 Trig

   D10

   HC-SR04 Echo

   Evidence to collect

   A decision table: a sequence of readings (real, 0, one stray close one, a second of 0s in open space, Trig out) → what each rule did and what it should do, checked on the lifted rover, with the silence rule's open-space trade-off written down.

   Predict

   Before each test, predict what the rule should do: box, open space, cushion, Trig out.

   Debug

   If it brakes in open space within a second, print the readings: a 0 counted as close is the fault. After a full second of 0s, braking is the silence rule at work.

   Reflect

   When is stopping on silence worth also stopping in open space?

   ⚠ Adult checks before power-on: The motor battery reaches the driver's VM only, through its power-off switch; the board's 5V goes to the driver's VCC only; every GND is joined. No wire is moved, pulled or plugged in while anything is powered: power-off switch off and battery unplugged first, then USB. The motor battery stays off and unplugged while the Trig wire is out; lifted, wheels in the air, for every test in this session. Lift the rover (wheels in the air) before switching the motor battery back on: the sketch keeps running on USB, so the wheels take its last command at once.

   Works towards: Stop rules

4. Session 4 · 75 min

   What did the rover see just before it stopped?

   Logging: one Serial.println() per reading, with the time from millis() and the decision, while the rover stays on its USB tether; reading the log against what was seen.

   In the studio

   • Build a project: the Rescue Rover

   Hands-on

   In Build a project, look at how the sketch prints one line per reading and what the Serial Monitor shows. Start from your session 3 sketch and print each reading as one line: the time from millis(), the distance (or “no reading”) and the decision. Upload, then open the Serial Monitor before the motor battery goes on (opening it restarts the UNO). With the USB cable held by the adult, drive at a box on the floor. Find in the log the reading that made it brake, and work out how far the rover went after it.

   Starting code: The learner's session 3 sketch.

   Evidence to collect

   A short log copied from the Serial Monitor, annotated: the braking reading, the readings before it, and any 0 or stray reading and what the rule did with it.

   Predict

   How many readings come between “far” and the brake?

   Debug

   If the Serial Monitor shows nothing readable, check its baud rate against Serial.begin() in the sketch.

   Reflect

   What does the log show that watching the rover did not?

   ⚠ Adult checks before power-on: The motor battery reaches the driver's VM only, through its power-off switch; the board's 5V goes to the driver's VCC only; every GND is joined. The USB cable is held clear of the wheels by the adult; on the floor at the lowest value that starts both wheels, never above 160 of 255; the power-off switch in reach. Lift the rover (wheels in the air) before switching the motor battery back on: the sketch keeps running on USB, so the wheels take its last command at once.

   Works towards: Stop rules

5. Session 5 · 90 min

   Does it stop every time?

   Repeatable trials: the same start line, speed and box each time, varied on purpose (two distances, a box at an angle, a soft target); every trial logged; a trial that rests inside the margin is a failure, even once.

   In the studio

   No studio lab: this session is off-screen.

   Hands-on

   Start from your session 4 sketch and do not change it during the trials. Mark a start line and two box positions with tape. Run five trials at each position, the same way each time, logged. Add one trial with the box at an angle and one with a cushion: twelve trials in all. Record where it rests each time, and write down any failure and what the log showed.

   Starting code: The learner's session 4 sketch, unchanged during the trials.

   Evidence to collect

   The unit's evidence: repeatable stop decisions with logged readings. A trial table of the ten straight-on trials and the two harder cases (rest distance, pass or fail): a trial passes only if the rover rests at least the chosen margin from the box, and the unit passes only when all twelve pass. The log of any failure.

   Predict

   How many of the twelve trials will rest outside your margin?

   Debug

   For a failure, find the reading in the log that came too late or was missing.

   Reflect

   Is the stop decision repeatable, and what one change would you test next?

   ⚠ Adult checks before power-on: The motor battery reaches the driver's VM only, through its power-off switch; the board's 5V goes to the driver's VCC only; every GND is joined. The power-off switch tested before the first trial; on the floor at the lowest value that starts both wheels, never above 160 of 255; the course clear of people's feet and anything fragile. Lift the rover (wheels in the air) before switching the motor battery back on: the sketch keeps running on USB, so the wheels take its last command at once.

   Works towards: Stop rules