Lab
Things you can poke at
Serial is easier to believe once you can change a number and watch the result. Each of these runs entirely in your browser — nothing to install, no board needed.
What 8N1 actually means
Take a UART frame apart. Change the data bits, parity and stop bits and watch the waveform change with them.
Baud mismatch playground
Set two rates, type any text, and watch exactly which bytes arrive, which are rejected, and why the garbage looks the way it does.
How close is close enough?
Drag the clock error and watch where the receiver starts sampling the wrong bit — always at the stop bit, never at the start.
Which baud rate do I need?
Four questions that tell you whether the baud rate is your problem at all, and which number both ends should be set to.
Factory baud rates by module
What each module ships at — and the separate AT-mode rate that catches most people out.
Will it brown out?
Move one wire and watch the verdict flip. The Uno's 3.3V pin gives 50 mA in total; an HC-05 wants close to 40 of it at the exact moment you are trying to pair.
Why every AT command returns ERROR
Pick your module, pick the dropdown, send a bare AT. Watch the same command answer OK, return ERROR, or say nothing at all — with nothing else on the bench changing.
What the Arduino actually receives
Type what your app sends, pick its line ending, and see every byte as a number, the dispatcher step it reaches, and what the car does.
Which Bluetooth module should I buy?
Say what has to talk to it: an iPhone, a web page, another Arduino. See which of the HC-06, HC-05 and HM-10 that means, and the row that decides it.
Which Bluetooth module do I actually have?
Count the pins, look for the button, send one bare AT. Three things you can check in a minute settle which of the four modules is on your desk — and therefore which settings are yours.
Change baud without locking yourself out
Build AT+UART= correctly, see the exact line and its terminator bytes, and find out before you send it whether you will be able to get back in afterwards.
Which fault is this?
Twenty Arduino Bluetooth faults in a new order every time. Name the box, pick the one test, and see the lure named when you fall for it.
The H-bridge: four switches, one motor
Flip IN1, IN2 and ENA on an L298N bridge and watch the current's path, the motor's direction, the brake and the coast. Then the car's five letters.
PWM: speed is a trick
Slide the duty cycle and watch the square wave, the average voltage the motor feels, and why a speed wire on pin 13 gives one speed.
A web-page Bluetooth remote for the HM-10
Connect to any HM-10 from a web page and drive: hold an arrow to send its letter, let go to stop, and read what your Arduino sends back.
These accompany the written fix for garbage in the Serial Monitor. Start there if you are stuck right now; come back here for why it works.