Features:  1.The button supports up to 20PCS, and the receiver supports up to 100pcs.  2.The transmitter without screw chuck design to achieve waterproof and dustproof.  3.Eye-catching backlight flashes when someone pushes the doorbell. Particularly suitable for the elderly, deaf.  4.Uses machine-learning to prevent interference from other wireless transmitters.  5.The transmitter USE a 23A12V alkaline battery (not include battery).  6.The receiver is available in a variety of plugs.  7.Receiving distance:300 meters in open area.  8.The wireless signal will be affected by obstacles. The more obstacles, the more serious the signal loss.  9.Receiver has 4 levels(can be mute) 0-80db,38 music to choose from.  10.Size:Transmitter:69x51x20mm,Receiver:90x61x31mm.     NOTE:The button cannot be placed on an iron door because it will interfere with the signal.  How to pair signals: Press and hold the volume button of the receiver and the transmitter for about 8 seconds to successfully pair.     Package includeds:  1 x Receiver.  1 x Transmitter.  1 x User Manual