I am starting a project to implement a "smart" lock for my door using Arduino nano, a solenoid deadbolt and a fingerprint reader. Which on its own works fine, but the whole project will run on a battery and I would require a "failsafe" method to unlock the solenoid deadbolt with a key. How can a solenoid deadbolt be controlled with a key without any current?
The normal way is to not use a solenoid to operate the deadbolt but to open the striker plate on the other side. This is called an electric strike
That way you can use a conventional key operated bolt as a way to open the door when there is no electrical power.
You can buy these strikes for retrofitting to normal types of lock, both mortice locks and surface fitted locks.