The hardware part is super easy, it's not even worth to draw a circuit diagram. The whole thing is controller by Arduino Micro, it uses 433MHz RF receiver, same frequency key fob and a little buzzer for audio feedback. Advice if you want to make something similar: buy good radio freq receiver, I've previously tried cheap one which costs under 1$ with RC resonator and it was total crap, so I've bought a good one with super heterodyne (RX-MID-3V) which is great.
Firmware is not complicated at all. RC receiver data pin is connected to external interrupt pin and I use rc-switch library ( to read keyfob values. You can print those in serial monitor and then configure define directives. Or even go even further to implement key fob binding by storing values in eeprom, I went the easier way. When program detects known key fob data value it locks/unlocks pc by simulating keyboard key presses (using Arduino built in keyboard lib). This is important to use Arduino Micro or Leonardo or any other that has USB support.
Password is stored in eeprom memory and it can be changed in serial terminal. You need to send command like "set-password <your_password>" and it saves new password to eeprom.