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.
I purchased a relatively cheap RF relay kit from Carl's Electronics. The HD2Combo 2-channel RF relay kit. These things are fun and just $30, get one even if you don't think you'll ever use it. Don't forget a 12V supply for the board. This is a rolling code transmitter, so no -- not just anyone with a remote can activate the relay.
Okay so, kit and intercom meet. I connected one of the normally-open relay circuits on the RF board in parallel to the "Door" button with wires marked "To Relay" and "From Door" in the photos above. This configuration allows either the relay or the button to operate the door. If your particular application uses a normally-closed circuit to open the door, wire the button in series and connect it to the normally-closed circuit on your relay.
Also show in the above photo are the 12v DC supply wires (black) and the antenna wire (also black).
My apartment is actually directly over the building entry, so range on the transmitter fob is not nearly an issue. I can activate the relay from my bedroom or from in front of the building. It's not the most impressive hack, but it sure makes my life easier.
And there you go. A keyless entry. So what's the second button do? Nothing. At least, not yet.
TODO: Hide the receiver and wire.
This is one of those things where I might caution that you not try at home unless you're very snuggly with your landlord.