## Title
HC-05 Bluetooth Serial Module

![HC-05 Bluetooth serial module](../../Images/hc05_module_.jpg)

![HC-05 Bluetooth serial module course pinout](../../Images/hc05_pinout_course.svg)

## What It Is
The `HC-05` is a Bluetooth serial module that acts like a wireless bridge for UART communication.

## What It Does In This Project
- gives the course project optional wireless control
- lets the Android app communicate with the firmware
- reuses the same command system as wired serial

## What Bluetooth Is
Bluetooth is a short-range wireless communication system designed to let devices exchange data without a cable.

In this project, it makes the firmware feel like it has a wireless serial cable connected to it.

## Origins And Background
Bluetooth was created in the 1990s as a cable-replacement technology. One major goal was to remove short messy cables between nearby devices like phones, headsets, keyboards, and computers.

The name `Bluetooth` comes from Harald Bluetooth, a Danish king who was famous for uniting groups of people. The technology name was chosen as a symbol of uniting different devices with one standard.

The HC-05 became a hobby classic because it made short-range wireless serial communication cheap and easy. For many students, it is their first experience of turning a wired UART link into a wireless one.

## Frequency Band
Bluetooth operates in the `2.4 GHz` ISM band.

That is an unlicensed industrial, scientific, and medical radio band used by many technologies, including Wi-Fi, Bluetooth, and some cordless devices.

This is a good chance to explain that wireless systems do not just send “invisible magic.” They use real radio frequencies in shared bands.

## How It Communicates
- talks to the MCU using `UART`
- talks over the air using Bluetooth Classic serial profile behaviour
- often uses AT commands for configuration

## Pinout
Typical HC-05 breakout pins:

- `VCC` : module power
- `GND` : ground
- `TXD` : UART transmit from the module
- `RXD` : UART receive into the module
- `KEY` or `EN` : enters or helps hold AT-command mode on some boards
- `STATE` : optional status output on some versions

## Course Wiring
In this course setup the important pins are:

- `TXD` -> Bluepill `PA3` (`USART2_RX`)
- `RXD` -> Bluepill `PA2` (`USART2_TX`)
- `VCC` -> module supply rail
- `GND` -> common ground

The key teaching point is that the Bluetooth link still looks like ordinary UART from the MCU side.

## Why UART Fits Well Here
UART is simple and common. The HC-05 lets students see that wireless links often still look like ordinary serial data from the MCU’s point of view.

## Physical Layer Notes
Between the MCU and the module, it is just electrical UART TX/RX signalling. Over the air, the radio handles the wireless side using the Bluetooth radio system in the `2.4 GHz` band.

## Why It Matters
This device is a great bridge between embedded systems and mobile apps. It helps students see that the same command protocol can travel over different physical links.

## Teaching Focus
- UART basics
- wired vs wireless links
- AT command configuration
- radio frequency bands
- protocol reuse across different transport layers

## Datasheet Navigation Tips
Look for:
- voltage levels
- baud rate settings
- command mode behaviour
- pin descriptions such as `KEY`
- pairing defaults

## Interesting Detail
From the firmware’s point of view, Bluetooth often looks surprisingly ordinary. The “magic” is mostly handled by the radio module.

## Good Questions To Ask Students
- Why can the same commands work over USB and Bluetooth?
- What part is the protocol and what part is the transport?
- Why is UART still useful in modern systems?
- Why do lots of wireless devices share the same `2.4 GHz` band?