Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limited to 2.5 milliwatts, giving it a very short range of up to 10 metres (33 ft).

It employs UHF radio waves in the ISM bands, from 2.402 GHz to 2.48 GHz. It is mainly used as an alternative to wired connections to exchange files between nearby portable devices and connect cell phones and music players with wireless headphones.

Bluetooth is managed by the Bluetooth Special Interest Group (SIG), which has more than 35,000 member companies in the areas of telecommunication, computing, networking, and consumer electronics. The IEEE standardized Bluetooth as IEEE 802.15.1 but no longer maintains the standard. The Bluetooth SIG oversees the development of the specification, manages the qualification program, and protects the trademarks. A manufacturer must meet Bluetooth SIG standards to market it as a Bluetooth device.

The name “Bluetooth” was proposed in 1997 by Jim Kardach of Intel, one of the founders of the Bluetooth SIG. The name was inspired by a conversation with Sven Mattisson who related Scandinavian history through tales from Frans G. Bengtsson's The Long Ships, a historical novel about Vikings and the 10th-century Danish king Harald Bluetooth. Upon discovering a picture of the runestone of Harald Bluetooth in the book A History of the Vikings by Gwyn Jones, Kardach proposed Bluetooth as the codename for the short-range wireless program which is now called Bluetooth.

Bluetooth uses a radio technology called frequency-hopping spread spectrum. Bluetooth divides transmitted data into packets, and transmits each packet on one of 79 designated Bluetooth channels. Each channel has a bandwidth of 1 MHz. It usually performs 1600 hops per second, with adaptive frequency-hopping (AFH) enabled. Bluetooth Low Energy uses 2 MHz spacing, which accommodates 40 channels.

Bluetooth is a packet-based protocol with a master/slave architecture. One master may communicate with up to seven slaves in a piconet. All devices within a given piconet use the clock provided by the master as the base for packet exchange. The master clock ticks with a period of 312.5 μs, two clock ticks then make up a slot of 625 μs, and two slots make up a slot pair of 1250 μs.

In the simple case of single-slot packets, the master transmits in even slots and receives in odd slots. The slave, conversely, receives in even slots and transmits in odd slots. Packets may be 1, 3, or 5 slots long, but in all cases, the master's transmission begins in even slots and the slave's in odd slots.

The Bluetooth SIG completed the Bluetooth Core Specification version 4.0 (called Bluetooth Smart) and has been adopted as of 30 June 2010. It includes Classic Bluetooth, Bluetooth high speed and Bluetooth Low Energy BLE protocols. Bluetooth high speed is based on Wi-Fi, and Classic Bluetooth consists of legacy Bluetooth protocols.

Bluetooth Low Energy, previously known as Wibree, is a subset of Bluetooth v4.0 with an entirely new protocol stack for rapid build-up of simple links. As an alternative to the Bluetooth standard protocols that were introduced in Bluetooth v1.0 to v3.0, it is aimed at very low power applications powered by a coin cell. Chip designs allow for two types of implementation, dual-mode, single-mode and enhanced past versions. The provisional names Wibree and Bluetooth ULP (Ultra Low Power) were abandoned and the BLE name was used for a while.

The Bluetooth SIG released the Bluetooth Core Specification Version 5.4 on 7 February 2023.

Bluetooth is defined as a layer protocol architecture consisting of core protocols, cable replacement protocols, telephony control protocols, and adopted protocols. Mandatory protocols for all Bluetooth stacks are LMP (Link Manager Protocol), L2CAP (Logical Link Control and Adaptation Protocol) and SDP (Service Discovery Protocol). In addition, devices that communicate with Bluetooth almost universally can use these protocols: HCI (Host Controller Interface) and RFCOMM (Radio Frequency Communications).

Bluetooth Low Energy, often referred to as Bluetooth® LE, is a wireless communication technology designed for short-range data exchange between electronic devices. It emerged as a response to the need for energy-efficient wireless communication in various applications, especially those where power consumption is a critical concern.

Unlike its predecessor, Bluetooth Classic, which is optimized for continuous and relatively high-data-rate communication, Bluetooth LE focuses on minimizing energy consumption while maintaining connectivity. This makes Bluetooth LE particularly suitable for applications that require long battery life, such as fitness trackers, healthcare devices, smart sensors, and Internet of Things (IoT) devices.

The range of a Bluetooth Low Energy connection can vary depending on several factors, but in typical scenarios, it can extend up to approximately 50 meters (or roughly 164 feet) in a line-of-sight environment.

Bluetooth Low Energy devices operate using different roles and modes that define how devices interact with each other.

A central device in Bluetooth LE is typically a more capable device with features like a higher CPU power, more memory, or a larger battery. Central devices take on the role of initiating connections to peripheral devices. For example, your smartphone is often a central device when connecting to Bluetooth LE peripherals like fitness trackers, smart sensors or an Arduino board.

Peripheral devices are generally resource-constrained compared to central devices (e.g. an Arduino board compared with your smartphone). Peripheral devices advertise their presence and data to central devices. Compared to Bluetooth classic, Bluetooth LE devices don't maintain a continuous connection to the central device to save power.

The ArduinoBLE Library is the main library enabling Bluetooth Low Energy on compatible Arduino boards.

Arduino Bluetooth communication is most often performed by HC-xx type modules. The two basic types of these are HC-05 and HC-06, their comparison:

HC-05	HC-06
- HC-05 module can either be a master or slave - It has a push button - HC-05 has five header pins	- HC-06 module can only be a slave - No push button - HC-06 has only four header pins

Wikipedia: https://en.wikipedia.org/wiki/Bluetooth
Arduino: https://docs.arduino.cc/learn/communication/bluetooth/#frequency-bands-and-range