HC-06 Bluetooth module for Arduino

A Bluetooth radio that pretends to be a wire. Your phone connects to it, the app sends a single letter, and the HC-06 hands that letter to the Arduino as though someone had typed it. Press forward and the phone sends F. That is the entire protocol, and once you know it the code stops being mysterious.

Read this before you buy one

This is the single most expensive mistake in the subject, because it cannot be fixed after the fact and because it does not look like a hardware problem. Your code is fine. Your wiring is fine. The phone simply cannot see the module, and never will.

Which of the three you actually want

	HC-06	HC-05	HM-10
Bluetooth	Classic 2.0	Classic 2.0	BLE 4.0
Android	yes	yes	yes
iPhone	no	no	yes
Role	slave only	slave or master	slave or master
Configuring	barely needed	full AT set	full AT set
Price	cheapest	a little more	~1.2–1.8× the HC-06

This car uses the HC-06, and the simplicity is a real feature: there is almost nothing to configure, so there is almost nothing to configure wrongly. What you give up is initiating. It is a slave and only a slave — it sits there advertising and something else connects to it. An HC-06 can never be the one that reaches out.

Four wires, and one of them crosses

Module pin	Goes to	Why
VCC	Arduino 5V	Not 3.3V — see below
GND	Arduino GND	Both grounds must be the same ground
TXD	Arduino RX	Its talking pin feeds your listening pin
RXD	Arduino TX, through a divider	Its listening pin, fed by your talking pin

The crossing is the part people get wrong. In plain language rather than as a rule to memorise: TX means "this pin talks." Two things that both talk, wired together, produce silence. So the module’s talking pin has to reach the Arduino’s listening pin, and the other way round.

Wire it straight through — TX to TX — and nothing breaks. You simply get nothing, for ever, with no error to explain it.

Why the 3.3V pin betrays you

Here is a mistake that looks like sound reasoning. The module is a 3.3V part. The Arduino has a 3.3V pin. Therefore the module goes on the 3.3V pin.

It does not, and the numbers say why:

	Current
Uno’s 3.3V pin, maximum draw	50 mA — the whole rail, for everything
Module while pairing or searching	under 40 mA
Module while actually sending	about 20 mA
Module paired but idle	under 8 mA

Forty against fifty is not headroom. It is a rail running at eighty percent of its rating at exactly the moment the module needs it most — during pairing, the very thing you are trying to get to work.

The divider question, and why both camps are right

This is the argument the internet cannot settle, and the reason is that the answer is different in each direction.

Module TXD → Arduino RX: connect it directly. The Arduino’s input needs at least 3.0V to read a HIGH, and the module drives 3.3V. Three point three clears three. No resistors, nothing to buy.

Arduino TX → module RXD: fit the divider. Now the Arduino drives a full 5V into an input designed for 3.3V. You are not comfortably inside the spec, you are past it, on every bit you transmit.

Plenty of people will tell you they skipped it and their module is fine. They are describing a real experience — most breakout boards put some protection on that input, and many modules survive 5V indefinitely. What you do not see are the ones that did not, because a module killed this way does not announce itself. It stops answering, the owner concludes it was faulty, buys another, and often gets away with it the second time. Two resistors cost a few cents. Fit them.

Baud rate and AT commands

The HC-06 runs at 9600 for data, and unlike the HC-05 it has no separate AT baud rate — one number does both jobs. If your Serial Monitor is showing symbols instead of words, that is a baud disagreement, not a broken module.

It accepts AT commands whenever it is powered and not connected to anything. There is no button and no mode to enter. The trap is the second half of that sentence: the moment a phone connects, the HC-06 stops listening to you and starts passing your bytes through to the phone. If your AT commands are being ignored, check whether something is paired to it.

The numbers

From the manufacturer’s datasheet, for reference rather than reading:

Spec	Value
Protocol	Bluetooth V2.0
Band	2.40 – 2.48 GHz, ISM
Power level	Class 2 (+6 dBm)
Receiver sensitivity	−85 dBm
Operating voltage	+3.3V to +6V
Operating current	40 mA
Temperature range	−20 °C to +55 °C

Download the HC-06 datasheet if you want the manufacturer’s own document.

Where this fits

The Arduino Uno makes the decisions, the L298N does the pushing, and the HC-06 listens to your phone. Wiring it is step 3 of the build guide, and every part has a page.

When the link itself misbehaves — it never appears, it pairs and drops, the characters are garbage — the Arduino Bluetooth fixes index lists every symptom with the test that identifies it.

Every part of the car