1. No DC isolated power supply: suitable for lithium battery power supply, used to Bluetooth headsets 2. With DC isolated power supply: it is suitable for modifying speakers, power amplifiers, and adding Bluetooth function in the car to solve the common ground current sound and noise 3. AUX audio input, Qualcomm 5.0, /ATPXHD, power on, the default volume is the highest, amp chip, better sound 4. KCOM is the common end of the key, and the other end of the key is respectively connected to +, -, PREV, , PLAY, and the length of the key is within 5cm Key Description: "+": short press volume up, long press volume up "-": short press volume down, long press volume down "": Short press song "PREV": Short press the previous song "PLAY": Short press to play/pause, long press for 3 seconds to force into pairing mode, long press for 6 seconds to clear the pairing list Colour:black Material:PCB type:3.3-4.2V Package Contents: 1 x Bluetooth receiver board Only the above package content, other are not included. Note: Light reflection and different displays may cause the color of the item in the a little different from the real thing. The measurement allowed error is +/- 1-3cm.