HC-05 6 Pin Wireless Bluetooth RF Transceiver Module for Arduino BT Serial Pass-Through Module HC05 Wireless Serial    Module parameters:     1.1 Little common sense (very important):  TXD: The sender is generally represented as its own sender. Normal communication must be connected to the RXD of another device.  RXD: The receiving end is generally represented as its own receiving end, and normal communication must be connected to the TXD of another device.  In normal communication, the TXD of its own is always connected to the RXD of the device  Self-receiving: RXD is connected to the TXD of other devices during normal communication. Therefore, if you want to receive the data sent by yourself as the name implies, that is, you receive the data sent by yourself, that is, its own TXD is directly connected to RXD, which is used to test its own transmission and reception. Whether it is normal or not is the fastest and easiest test method. When a problem occurs, the test is first performed to determine whether the product is faulty. Also known as loopback testing.    1.2 Level Logic:  TTL level: Usually the data indicates binary. The specified +5V is equivalent to logic "1". 0V is equivalent to logic "0". It is called TTL signal system and is positive logic.  RS232 level: -12V to -3V, equivalent to logic "0", +3V to +12V logic level, equivalent to logic "1", is negative logic    1.3 Product Features:  1. The core module uses the HC-05 slave module. The lead-out interface includes VCC, GND, TXD, RXD, KEY pin, Bluetooth connection status pin (STATE), unconnected output is low, and the output is high after connection.  2, led indicates the Bluetooth connection status, flashing indicates no Bluetooth connection, slow flashing means entering the AT mode, double flashing indicates that the Bluetooth is connected and the port is opened  3, the bottom plate is set with anti-reverse diode, with 3.3V LDO, the input voltage is 3.6~6V, the current is about 30mA when not paired, about 10mA after pairing, the input voltage is prohibited to exceed 7V  4, the interface level 3.3V, can be directly connected to a variety of microcontrollers (51, AVR, PIC, ARM, MSP430, etc.), 5V microcontroller can also be directly connected, without MAX232 can not go through MAX232  5, the effective distance of the open space is 10 meters (power level is CLASS 2), more than 10 meters is also possible, but the connection quality of this distance is not guaranteed.  6. After pairing, when using the full-duplex serial port, you do not need to know any Bluetooth protocol, support 8-bit data bit, 1-bit stop bit, and can set the parity communication format. This is also the most common communication format, and does not support other formats.