Computer is a very personal thing. Probably, you don't want to everybody see what you have in it. You can use password, but what if it's not enough? Maybe you don't want to anybody turn on your computer? Then, it's great instructable for you.
You can use RFID cards like power button but button can click everyone, and RFID cards not. If you will make this project only persons who have your card can turn on computer(power button won't work).
Step 1: Parts
All you need to do this, are:
Arduino or replacement,
USB cable for Arduino,
power supply for Arduino.
Step 2: Check Card ID
Firstly, you have to check your card ID. Wiring:
RFID Pins --> Arduino Pins
VCC --> +3.3V
RST --> 9
GND --> GND
MISO --> 12
MOSI --> 11
SCK --> 13
NSS --> 10
To use RFID you need to download RFID library. This library contain no examples, but I've got one made by Henri de Jong. If you get card close to RFID it will print out its number on Serial Monitor(Ctrl+Shift+m to open Serial Monitor). Save this number(only decimal).
Step 3: In Computer
Secondly, you have to take off cover from your PC and find where goes wires from power button. There should be pins(probably on right-bottom corner) named reset(or rst) and power(or pw). Disconnect power button pins and connect yours. Ok, almost done :)
Step 4: All Together
Let's back to the Arduino. You've already connected RFID to it. Now, you will connect relay.
Relay pins --> Arduino Pins
S(steering) --> 8
VCC --> +5V
GND --> GND
Download the code, change variable called cardNumber, to your card number and send it to Arduino.
If you've done that, disconnect USB cable from Arduino(it won't be useful now) and connect relay pins - NO(normal open) and MAIN(it should be the middle pin) to pins which you connected to computer in previous step. Connect power to Arduino. You can insert Arduino and relay to plastic to avoid short circuit. RFID attach to PC cover. In my cover there was a hole on the bottom so I could easily get wires from Arduino to RFID in front of PC. But if you don't have a hole on the bottom or in front and you don't want to make one you can attach RFID on side and get wires through hole at fan, or attach RFID on back(which could be not very practical but if you don't have any option you could do that).
That's it! It was my first instructable so thank you to done it. Now you can turn on your computer using card and no one else would open it :)
If you have any questions - ask.