Ryan (co-founder of the MAKE NYC group) made a cool keyless entry hack for his apartment awhile back, he writes –
Now don’t tell the co-op board, but I crafted a plan to ditch my Mul-T-Lock key for good. My apartment building, of course, uses an intercom system to remotely unlock the front door for guests, deliveries, etc. This keychain upgrade gets me in the front door… keylessly.
The mechanism is simple enough, just a momentary button on the panel in my apartment, but the mess of wires in the wall is a little unsettling. I took some meter readings of the terminals and sorted out the door button wiring. You can see where the door button is soldered through the PCB, which made it easier to identify the proper terminals. This panel has a “Door”, “Talk” and “Listen” button. The “Door” button is normally-open and the circuit is about 22V. In the following steps we’re going to effectively bypass the button with our own circuit.
Keyless entry hack – Link.