Are there any iPhone/Android + Doorknob solutions out there? I don't want keys at all! I would love to replace my doorknob and just have my phone transmit a key

Are there any iPhone/Android + Doorknob solutions out there? I don't want keys at all! I would love to replace my doorknob and just have my phone transmit a key to unlock the door as I approach. One problem could be if the phone ran out of battery. Perhaps a small usb charger near the door for emergencies? Man I would love that.