Arduino Uno R3: pins, power and limits

A small computer with no screen. You give it instructions once, over USB, and it runs them every time it has power, forever, until you give it different ones. That is the whole idea. It cannot do two things at once and it has no memory of yesterday.

Every pin this car uses

An Uno has 14 digital pins and 6 analog ones. This build uses nine of them, and knowing which is which turns the wiring from a diagram you copy into something you can debug.

Pin	Goes to	Job
0 (RX)	HC-06 TXD	the Bluetooth link
1 (TX)	HC-06 RXD
2	buzzer signal	the horn and the reversing beep
5	L298N ENA	speed, left and right
6	L298N ENB
9	L298N IN4	direction
10	L298N IN3
11	L298N IN2
12	L298N IN1
Vin / GND	battery pack	power, and the shared ground

Pins 0 and 1 are not ordinary pins

This is the one thing on this page worth memorising.

Pins 0 and 1 are the hardware serial port — the same one the USB cable uses to load your sketch. When you upload, the bootloader is listening on that port for a specific sequence of bytes. If the Bluetooth module is also sitting on those pins, it is talking over the top of the conversation, and the bootloader hears noise where it expected its instructions.

Two devices, one wire, and no way for either to know the other is there.

What a pin can and cannot drive

A pin can signal, but it cannot push. One supplies roughly 20 milliamps. A gear motor wants a few hundred. Wire a motor straight to a pin and you do not get a slow car — you get a dead Arduino.

That single number is the entire reason the L298N motor driver exists in this build. The Arduino never touches motor current; it only ever says which way and how hard, over six thin signal wires carrying almost nothing.

The same limit applies to the 3.3V pin, which is rated for 50 mA total — not per device, the whole rail. That is why the HC-06 goes on 5V instead.

Why the speed pins are 5 and 6

A digital pin is normally just on or off — 5 volts or nothing. There is no half on. PWM fakes it by switching on and off very fast and varying how long it stays on each cycle: on 60% of the time reads, to a motor, as roughly 60% power.

Only some pins can do that. On an Uno they are 3, 5, 6, 9, 10 and 11, and it is why the two enable wires sit on 5 and 6 rather than anywhere convenient. Put them on a non-PWM pin and speed control silently stops working — every setting behaves as full speed, and the sketch gets blamed.

Powering it

Over USB while you are programming. From the battery pack once the car is loose — the same two leads that feed the motor driver also go to Vin and GND, which powers the board and gives the two of them a shared ground at the same time.

Genuine or clone

A clone is fine here, and cheaper — around $11 with a USB cable against roughly three times that for an official board. It runs the same code, has the same pins and the same limits. The differences that matter to some projects — the USB-serial chip, the quality of the regulator, supporting the people who designed it — do not change anything in this build.

The chip underneath

The board is a carrier for an ATmega328P microcontroller: 16 MHz, 14 digital I/O pins of which 6 do PWM, 6 analog inputs, a USB connection, a power jack, an ICSP header and a reset button.

Worth knowing that on a socketed board the chip lifts out. The worst thing that usually happens when you experiment is that you kill the ATmega and not the board, and a replacement is a couple of dollars.

Download the ATmega328P datasheet if you want the manufacturer’s own document.

Where this fits

The Uno makes the decisions, the L298N does the pushing, and the HC-06 listens to your phone. Getting the sketch onto the board is step 4 of the build guide — the step worth having an adult at — and every part has a page.

Every part of the car