Need a bit of help with a door lock project

[electrophile]

Hi, I'm trying to build a door lock system that is interfaced with a fingerprint scanner. The fingerprint gives out an acknowledgement when the fingerprint is accepted or denied and this is read by the MCU which in turn switches on the power supply that opens the door lock or keeps it off (door lock is normally engaged when no power is given and opens when power is applied). I've attached a small block diagram of the whole system. Now, I want this system to run even when there is no power. Enter battery backup. My question is if there is no mains power the MCU would shut down and the whole system would fail. Can the Atmel series of 8-bit microcontrollers monitor its own power source? If not, how do I get the MCU to switch to a battery back up so that it remains operational even in a power outage. The door lock operates on 12V DC and the MCU has a 5V power supply. Any help would be appreciated!

 

[zipius]

maybe you can connect a battery with a diode to MCU voltage on MCU power source. the power drained by MCU should be low, but if you want to save the batt you can also connect a NC contact of a relay to that battery circuit, connecting the relay coil to 12v

 

[jayanth.devarayanadurga]

The power supply should be such that when mains fail a relay should (which was active) connect battery to MCU power. You can use opamp comparator circuit which monitors under/over voltage and operates a relay. When mains fail relay gets deactivated and connects battery to MCU power supply. MCU should hold the last state of locking system. You can also ass a 25000 uF capacitor at 7805 o/p which connects to MCU.

 

[electrophile]

Thanks folks. Can you tell me, if I can use a solid state relay used for switching AC loads for DC applications?

 

[mathespbe]

this might help you...
http://solidstaterelaystore.com/