Do you need to build a KeyPad for your system? [AverageGuy] did it himself. Here his story:

My old keypad GDO gave up the ghost a while back so I decided I’d undertake a new Arduino based project. I saw there were quite a number of wireless projects already and one wired, but the wired on didn’t go into any detail, so here’s an incomplete implementation. Food for thought if nothing else.

On the [website] there is a full description and the schematics. I really recommend you if you’re interested, because the explanation is complete in every aspects.