Ready Bluetooth solutions are often quite expensive. The cheapest Bluetooth modules that are currently commercially available, there are from the Chinese manufacturer Rayson . The modules BTM-112 (Class 2) and BTM-222 (Class 1) each cost around 15 €. Since the modules are designed for 3.3V supply voltage is required for connection to a 5V system in addition to a 3.3V voltage regulator for 2 level converter (Level Shifter) for the UART signals. In addition, you do not want to give up a status display with LED safely. The price of a module is still all in all about 20 €. The circuit The circuit of the Bluetooth module is designed to operate on a 5V system. As a voltage regulator, a controllable LM317L is used.Similarly, a 3.3V voltage regulator such as the LF33CV could be used. Level converter A level converter (English Level Shifter) is used to adjust signal levels of different voltage levels. A typical example of a level converter is for example a RS232 converter. This converts the 1...