Build a Car With Your Kid · Chapter 12

If the car doesn't move

When the car doesn’t do what it should, the question isn’t “what’s wrong with it?” It’s which box is it in? Each stage test can fail only for a reason inside its own box. So run them in order, until one fails.

It’s almost never the sketch: the sketch is the same on every car, and it passed its tests before the book was printed. Every time, use the loop from Chapter 3: look, which box, change one thing, test again, say what you found.

Run the tests in order

Stage A · the board
Plug in the USB cable and open the Serial Monitor at 9600. Does the board say Car ready? (Press RESET if the screen is empty.)
Stage B · waits for the one before it
Test C1 · waits for the one before it
Test C2 · waits for the one before it
Stage D · waits for the one before it

The full table: symptom, test, cause and fix

The same table as the book, grouped by the test that catches each symptom, wiring causes first. Step numbers (B37, C8 and so on) are the book’s build steps.

Stage A · the board

ON light lit, no port on the laptop: Most likely: Charge-only USB cable; clone needs the CH340 driver. Fix: Another cable; install the driver (Chapter 6).
L light blinks slowly forever, monitor empty: Most likely: The sketch never uploaded. Fix: Upload again; wait for “Done uploading”.
L light on, but no Car ready even after RESET: Most likely: Nothing is proved: the L light can be on for other reasons. Fix: Upload again (A3).
Strange symbols in the Serial Monitor: Most likely: Serial Monitor not at 9600. Fix: Choose 9600; press RESET.
Upload error (“not in sync”, “port busy”): Most likely: Wrong board or port; another program using the port. Fix: Tools → Board: Arduino Uno; pick the right port; replug.

Stage B · the wheels

No light anywhere when you switch on: Most likely: Cells backwards or not seated; battery lead loose. Fix: Cells out (parent), check + marks; tug the leads.
No horn on V, but Got V from USB: Most likely: Buzzer wires. Fix: Check B21–B25.
Motor driver’s light on, board’s ON light off (USB out): Most likely: The Vin or GND jumper is missing or loose. Fix: Check B37–B44; tug both jumpers.
Got F from USB, but no wheel turns: Most likely: Switch off; battery leads or jumpers loose; signal wires on wrong pins. Fix: Switch on; check B37–B44; check B29–B34.
All four wheels turn backwards on F: Most likely: Every motor is the other way round for this sketch (normal). Fix: Swap red and black on all four; type F again.
One side turns backwards; the car spins: Most likely: That side’s motors are the other way round. Fix: Swap that side’s red and black (both motors); type F again.
One wheel fights the other on its side: Most likely: That one motor’s wires are the other way round. Fix: Swap red and black on the one turning backwards; type F again.
F is right, but L turns right: Most likely: The left motors are on OUT3/OUT4, the right ones on OUT1/OUT2. Fix: Move the left motors to OUT1/OUT2 and the right to OUT3/OUT4; type F again (expect all four backwards, then the row above).
Every speed the same: Most likely: Cap still on ENA or ENB; speed wire on a pin without ~. Fix: Check B18–B19 (caps), B29–B30 (pins 5, 6).
Wheels twitch and stop; monitor shows (newline): Most likely: Line ending isn’t “No line ending”. Fix: Choose No Line Ending.
ON light goes out when the USB cable comes out: Most likely: The jumper from +12V to Vin, or from GND to GND, is missing or loose. Fix: Redo B37–B44.
Motors hum but don’t turn on the floor(B / D): Most likely: Speed too low for the floor, or tired cells. Fix: Try speed 9; charge the cells.

Test C1 · module power

Module light off: Most likely: Module VCC or GND not in its column; board’s 5V/GND jumpers out. Fix: Check C5, C6, B21, B22.
ON light dims or goes out when you switch on with the module: Most likely: A short: 5V joined to GND. Fix: Switch off at once; find the wire in the wrong column.

Test C2 · phone and module

HM-10: “no devices found” (Android): Most likely: Location off, or Chrome lacks Nearby devices. Fix: Location on; allow Nearby devices for Chrome.
HM-10: nothing happens on Connect (iPhone): Most likely: You’re in Safari. Fix: Open the remote in Bluefy.
HM-10: the remote can’t find it after pairing in Settings: Most likely: The phone is holding the module. Fix: Settings → the module → Forget.
HC-06: never appears in the scan: Most likely: No power, or connected to another phone. Fix: Check C1; turn off the other phone’s Bluetooth.
HC-06: PIN refused: Most likely: The module’s code isn’t 1234. Fix: Try 0000.

Stage D · the round trip

Connected, but no Got F from phone: Most likely: TXD not on pin 2; TXD and RXD swapped. Fix: Check C7, C8.
Got F from phone, but no Car heard: Most likely: The 1 kΩ, the pin 3 wire or the RXD wire. Fix: Check C2, C4, C8 against the Chapter 10 figure. (The 2 kΩ can’t show up in any test: C9 checks it by eye).
HC-06: the car jerks and stops after each letter: Most likely: The app sends a newline. Fix: Settings → Send → Newline: None.
Beeping won’t stop: Most likely: It’s the reversing beep, or the horn. Fix: Send S.
The car ignores you while the tune plays: Most likely: Happy Birthday blocks everything for about 16 s, then repeats. Fix: Wait for the end, then send S. Stop before X next time.

Power · tired cells

It stops and restarts when it starts moving: Most likely: Tired cells (the board’s power dips). Fix: Charge the cells.

Before you start

A locked school or work laptop won’t install the IDE: Most likely: No permission to install software. Fix: Use a laptop you’re allowed to install on (Chapter 1).

Still stuck

Run the loop from Chapter 3 once more, out loud, and write down exactly what you see at each stage test.

Then write to [email protected]. Say which stage test fails and what you see, and you’ll get help. If it turns out the book was wrong, it gets a dated correction.

Why a side turns backwards: the H-bridge lab →
Why every speed can be the same: the PWM lab →
Ten broken cars, to practise on →
The free book this page comes from: Build a Car With Your Kid →