It is based on http://www.keelog.com/diy.html and from there you can download the software to get the keystrokes.
Warning! I hold no responsability with what you do with this keylogger.
Step 1: The items
- Atmel AT89C2051 microcontroller (or AT89C1051, AT89C4051)
- AT24C512 serial EEPROM chip (or compatible)
- 12 MHz crystal
- 2 x 33p capacitor
- 10 uF capacitor
- 10 k resistor
- small push-button