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Help with exam project! (keypad + PIC)

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Hello all! ;)

I'm making my last project in el-technique - my exam project. I really, really hope you can help me with this one!

I have chosen to make a burglar alarm. In short, it contains of an emitter and a reciever and when the signal is broken, the alarm starts for 3 minutes.

Now I need to make a codebox, consisting of a keypad and a PIC. The idea is that when you press the 4 digit code followed by "*" the alarm will be deactivated/stop. When you press the code followed by "#" the alarm will be activated.

The PIC will be connected to the reset pin of a monostable multivibrator (making the alarm sound in 3 minutes), and if the deactivation code is typed, it will be GND and if the activation code is typed, then it will be 5V. So I can activate/deactivate the alarm with the codebox.

I have bought this keypad: Velleman Components n.v.
- which is a 3x4 common output keypad.

I have programmed a PIC before, but I'm not good. I imagine that it is possible to connect the 12 buttons to each input pin and then have an output pin which sends GND when deactivation code is typed, and 5V when activation code is typed.

I hope you follow me. Otherwise, please let me explain further.

Which PIC should I use? (I have used PIC16F84A a bit and PIC16F877 a bit lesser.) - I don't know how to make it work exactly, so if you have any ideas, you are more that welcome.

The codebox have to be made the next to days, so I'm in a bit of a hurry.
I really hope you will help me. I will really appreciate it. Thanks in advance.

Best regards
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