Mobile Robot Hardware

Sessions

1. Session 1 · 90 min

   How can a small pin switch a motor it cannot power?

   The transistor as a switch: a TIP120 on the low side (between the motor and GND), a 1 kΩ base resistor that keeps the pin's current small (a pin is rated 20 mA, 40 mA at most), the 1N4001 flyback diode across the motor, and the motor's own battery (4 × AA alkaline in a holder with a power-off switch) with its − joined to the board's GND. PWM on D9 sets the speed.

   In the studio

   • Control a motor

   Hands-on

   In Control a motor, trace the signal path (D9 → 1 kΩ → base) and the power path (battery + → motor → collector → emitter → GND), then look at its two unsafe faults (no driver; the diode the wrong way round) and say what each would do. On the bench, with the holder switched off and unplugged and USB unplugged, build the same circuit: the TIP120 printed face towards you, legs down (base, collector, emitter from the left); the diode's stripe to the battery +. Clamp the motor so its shaft turns freely. After the adult check, plug in USB, upload, connect the battery last and switch it on; step analogWrite(9, …) up from 0 in tens and note where the motor starts.

   Starting code: Control a motor's sketch (Copy code in the lab's code panel): analogWrite() on pin 9 sets the speed.

   Pins

   D9

   TIP120 base, through the 1 kΩ resistor

   Evidence to collect

   A power-path drawing with the signal path and the power path in two colours, and a table of analogWrite() values: turning or not, with the start value marked and compared with the guide's “about 1/2 its rated voltage”.

   Predict

   Before power, predict the analogWrite() value where the motor starts, from the guide's “about 1/2 its rated voltage”.

   Debug

   If it never turns, switch off and unplug, then check one connection at a time against the drawing, starting with the battery − to GND.

   Reflect

   Why does the pin's current stay small while the motor's is large?

   ⚠ Adult checks before power-on: The motor battery is a separate holder of 4 × AA alkaline cells (1.5 V each, about 6 V fresh) in a holder whose on/off switch is the power-off switch: its power-off switch is off and it is unplugged while wiring, then connected last. The diode's stripe faces the battery +. The TIP120's metal tab is its collector (pin 4 on the datasheet), so it touches no other part or wire; its emitter goes to GND and the battery − to the board's GND; the motor is clamped. Only the 1 kΩ resistor joins D9 to the circuit, and nothing else uses D9. Keep the motors clamped, wheels in the air, before switching the motor battery back on: the sketch keeps running on USB, so they take its last command at once. No wire is moved, pulled or plugged in while anything is powered: power-off switch off and battery unplugged first, then USB.

   Works towards: Motors in code, Power budget

2. Session 2 · 90 min

   What does each pin of a dual motor driver do?

   The TB6612FNG: VCC (logic, from 5V) and VM (the motor battery: 4.5–13.5 V, and at least 5 V for its 1.0 A rating); STBY, which must be HIGH; and for each motor two direction inputs and a PWM input. Its modes: two directions, short brake (both inputs HIGH) and stop (outputs off, the wheel coasts).

   In the studio

   • Steer with two motors

   Hands-on

   In Steer with two motors, predict, then test, what each row of the driver's table does to one wheel. On the bench, with everything unpowered, tape or clamp the two motors to a wooden block so their wheels turn in the air (the chassis comes in session 4). Still unpowered, wire the driver as the lab's “Build it for real” view shows: VCC → 5V, GND → GND, STBY → D3, left motor PWMA D5, AIN1 D4, AIN2 D7, right motor PWMB D6, BIN1 D12, BIN2 D11, then the motor battery through its power-off switch to VM, battery − to GND. Check the breakout's printed labels against the guide. Change the sketch's command to run one mode at a time for two seconds each; upload, then switch the motor battery on and fill in what each wheel did.

   Starting code: Steer with two motors' sketch (Copy code in the lab's code panel): STBY on D3 and one helper per motor.

   Pins

   D3

   driver STBY

   D4

   driver AIN1 (left motor)

   D5

   driver PWMA (left motor)

   D6

   driver PWMB (right motor)

   D7

   driver AIN2 (left motor)

   D11

   driver BIN2 (right motor)

   D12

   driver BIN1 (right motor)

   Evidence to collect

   A driver table in the learner's own words: inputs (IN1, IN2, PWM, STBY) → what the wheel did, for forward, backward, short brake, stop and standby, with the observed difference between braking and coasting.

   Predict

   Before each mode, predict whether the wheel will stop quickly or roll on.

   Debug

   If neither wheel moves, check STBY first: it must be HIGH, or the driver stays in standby.

   Reflect

   When would a rover want to brake rather than coast?

   ⚠ 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 breakout's labels match the guide (if they do not, stop). The motors are fixed to the block, wheels in the air; the power-off switch is off and the battery connected last, after the upload. Keep the motors clamped, wheels in the air, before switching the motor battery back on: the sketch keeps running on USB, so they take its last command at once. No wire is moved, pulled or plugged in while anything is powered: power-off switch off and battery unplugged first, then USB.

   Works towards: Motors in code

3. Session 3 · 75 min

   How much current do the motors need, and for how long?

   A power budget: what each motor draws, its stall current against the driver's 1.0 A per channel, the battery's capacity in mAh, and why the board is not powered from the motor battery. Motors draw the most current when they start or are loaded, much less once running. Cells in series add their volts but keep one cell's capacity.

   In the studio

   • Control a motor

   Hands-on

   In Control a motor, compare a fresh and a run-down battery: what changes at the same speed setting? Then list the loads on the motor battery with the current each data sheet or label gives: each motor's stall current and running current. Check each stall current against the driver's 1.0 A per channel. Estimate the run time in hours: one cell's capacity in mAh ÷ the two motors' running current in mA. Four cells in series keep one cell's mAh, so do not add them; alkaline cells often print no mAh, so the adult gives the figure. The board runs from USB or its own pack, so its current is not in this sum. Then, with the motors still on their block, upload your session 2 sketch with a greeting added in setup(). Open the Serial Monitor first and wait for the greeting (opening it restarts the UNO); only then switch the motor battery on, and count any greeting that appears from then on while both motors start at once.

   Starting code: The learner's session 2 sketch, with Serial.begin(9600) and a greeting printed in setup().

   Evidence to collect

   A power-budget table: each load on the motor battery, its current and where the figure came from, the stall check against 1.0 A, the run time in hours with the working shown, and the number of greetings after the motors started.

   Predict

   Predict the run time in hours, and whether a greeting will appear again when both motors start.

   Debug

   If a greeting appears again after the motors start, the board restarted: check that the motors run only from their own battery and that every GND is joined.

   Reflect

   What would you change to make the batteries last a whole session?

   ⚠ 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 motors stay on their block, wheels in the air; the power-off switch in reach. Keep the motors clamped, wheels in the air, before switching the motor battery back on: the sketch keeps running on USB, so they take its last command at once. If anything gets warm: switch off or unplug at the holder, don't touch it, tell the adult.

   Works towards: Power budget

4. Session 4 · 90 min

   Where does everything go on the rover?

   The chassis: square and firmly fastened; the motor battery holder with its power-off switch where a learner can reach it while the rover moves; the board's own power (USB on a tether, or its own 6 × AA pack in the barrel jack, never the 5V pin); wires tied down so none can be pulled loose; power and signal wires kept apart; the tools used safely.

   In the studio

   • Steer with two motors

   Hands-on

   With every battery unplugged and USB unplugged, move the motors from their block to the chassis and fit the wheels and castor; fix the board, the breadboard with the driver and the motor battery holder; put the power-off switch at the top or the back, in reach. Use the tools as the adult check says, with eye protection on when cutting cable ties. Still unpowered, rewire the driver onto the chassis, following Steer with two motors' “Build it for real” view again, and tie the wires down. Still unpowered, pull each wire gently: none comes loose. Then lift the rover, wheels in the air, upload your session 2 sketch, switch the motor battery on and run it again: both wheels must do what your table says.

   Starting code: The learner's session 2 sketch, unchanged.

   Pins

   D3

   driver STBY

   D4

   driver AIN1 (left motor)

   D5

   driver PWMA (left motor)

   D6

   driver PWMB (right motor)

   D7

   driver AIN2 (left motor)

   D11

   driver BIN2 (right motor)

   D12

   driver BIN1 (right motor)

   Evidence to collect

   A labelled drawing or photo of the chassis (the rover only, no people): battery holder, power-off switch, board, driver and where the power and signal wires run, and session 2's table checked again on the chassis.

   Predict

   Will both wheels still match your session 2 table after the move to the chassis?

   Debug

   If a wheel turns backward for “forward”, switch the motor battery off, unplug it and USB, and swap that motor's two leads.

   Reflect

   Which wire on your chassis is most likely to come loose, and how did you stop it?

   ⚠ 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. Chassis tools: screwdrivers pushed away from hands; side-cutters cut cable ties flush, eye protection on, sharp ends turned in; hot glue low-temperature or adult-only (it burns); soldering or drilling adult-only. The power-off switch is tested off before the rover is powered. 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. No wire is moved, pulled or plugged in while anything is powered: power-off switch off and battery unplugged first, then USB.

   Works towards: Motors in code, Power budget

5. Session 5 · 90 min

   How can one function drive both wheels?

   Functions with parameters, written outside setup() and loop(): a helper per motor that sets its two direction inputs and its PWM, then forward(speed), spinLeft(speed) and a brake function built from them. One command in one place.

   In the studio

   • Steer with two motors

   Hands-on

   In Steer with two motors, read how its sketch drives each motor through a helper with three parameters (in1, in2, speed). Start from that sketch and write forward(speed), spinLeft(speed), spinRight(speed) and a brake function with both inputs HIGH. Call them from loop() in a short sequence (forward 1 s, brake 0.5 s, spin 0.5 s, brake). Upload and run it first with the rover lifted, wheels in the air; then on the floor at the lowest value that starts both wheels, never above 160 of 255, with a learner's hand at the power-off switch.

   Starting code: Steer with two motors' sketch (Copy code in the lab's code panel): its helper per motor.

   Evidence to collect

   The sketch with each function explained in the learner's words (what it takes, what it does), and a command table: function call → what each wheel did, lifted and on the floor.

   Predict

   What changes if you halve the speed parameter of forward(speed)?

   Debug

   If one wheel does the opposite of the command, look at that motor's helper call before touching any wire.

   Reflect

   What would you have had to change without the functions?

   ⚠ 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 of every new sketch; then on the floor at the lowest value that starts both wheels, never above 160 of 255, with the power-off switch in reach 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: Functions, Motors in code

6. Session 6 · 90 min

   Can the rover go forward, turn and stop when we tell it to?

   Demonstrating the rover: forward, a measured turn and a stop, done by the learner without help, after the learner's own pre-power check; the battery reading as a gate; troubleshooting one fault at a time.

   In the studio

   No studio lab: this session is off-screen.

   Hands-on

   Start from your session 5 sketch. Run your pre-power check aloud, in order: (1) the power-off switch is off; (2) the driver's wiring matches the guide (STBY D3; PWMA D5, AIN1 D4, AIN2 D7; PWMB D6, BIN1 D12, BIN2 D11); (3) every GND is joined (board, driver, battery −); (4) still unpowered, no wire comes loose when tugged gently; (5) the rover is lifted, wheels in the air. Then switch the motor battery on and measure it on DC volts at the breadboard rails it feeds: below 5.0 V, replace the cells before any run (the driver's 1.0 A rating needs VM≥5V; a run-down alkaline pack also falls below 5 V). On the floor, find how long a spin must run for a quarter turn at your speed (three tries, take the average). Then demonstrate on your own: drive forward about 50 cm, turn a quarter turn, stop, and switch the motor battery off.

   Starting code: The learner's session 5 sketch, with the turn time found on the floor.

   Pins

   D3

   driver STBY

   D4

   driver AIN1 (left motor)

   D5

   driver PWMA (left motor)

   D6

   driver PWMB (right motor)

   D7

   driver AIN2 (left motor)

   D11

   driver BIN2 (right motor)

   D12

   driver BIN1 (right motor)

   Evidence to collect

   The unit's evidence: the learner independently demonstrates forward, turn and stop, seen by the adult, with the learner's five-step pre-power check, the turn times and the battery readings before and after.

   Predict

   Predict the battery reading after the demonstration, from your power budget.

   Debug

   If something goes wrong, switch the motor battery off, change one thing at a time and say what you changed.

   Reflect

   Measure the battery again: how close was your prediction, and is it still above 5.0 V?

   ⚠ 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. For the reading: red lead in the V/Ω socket, never 10A; probes go on breadboard rows or rails, not at the header pins. The learner runs the five-step pre-power check; the battery reads at least 5.0 V before any run; the power-off switch is tested before the first floor run; on the floor at the lowest value that starts both wheels, never above 160 of 255, a clear floor and no one's feet in the way. 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: Motors in code, Functions, Power budget