The Lockey M210DC works the same way as the M210 shown below.
EITHER REPLACE YOUR EXISTING LOCK...
The Lockey M210 Keypad Deadbolt Lock can be used to completely replace an existing deadbolt, latchbolt, or just a plain door knob. Just take off your existing door hardware and install this one in its place. It has been designed to easily retrofit onto standard doors.
OR USE WITH YOUR EXISTING LOCK...
You can also leave your existing lock in place and install this keyless home lock either above or below it for added security and convenience. If you do this, people who know your secret code will only be able to enter when both locks are unlocked, perhaps, during the day, for example.
On the inside or outside, just turn the knob (or lever or thumbturn). You don't need to enter the code.
On the inside, just turn the turn knob or thumb turn to retract the deadbolt. You don't need to enter the combination.
On the outside, press the "C" button first (or turn the thumb turn as far as it will go). This will clear out any previous button presses. Then enter in the code and turn the knob (or lever or thumbturn).