ER / door-lock

Let's imagine. After finishing busy day, You arrive in front of your house and try to get in but the digital door-lock doesn't work. You can see the sign there is not enough battery and even the sign isn't clear due to low battery.
There is a tiny mark of lightning on doorknob. If you look closely, the mark is placed on switch where you grab and turn with a thumb and index finger.
Therefore I suggest a door-lock system ‘ER(Electronic Resuscitation)' which can produce electricity urgently when the digital door-lock is no longer working because of no battery.