This door bell is great. There is an option to screw the doorbell to the door or stick it with very sticky pads. I have chosen to stick mine as I didn't want screw holes in my new door frame and it is really secure.
The door bell receiver has many different tunes to choose from and the sound can be increased to a high volume so even the had of hearing can hear it. The receiver is also portable so you can plug it in to any room.