Required Parts:

• Pro Trinket or similar tiny 5V Arduino.
• A 5V power source. I'm using two tiny LiPo RC helicopter batteries and an LM2940 5V regulator.
• An RFID reader of some sort. The smaller the better (if you can make one from scratch that can fit inside a PVC pipe post the instructions and I'll use it!). For this build I used an ID Innovations ID-2 which is an older version of the ID-3V. The ID-12 or ID-20 would work too but you'll need to use a larger diameter PVC pipe to fit it inside. You could even use the Parallax RFID reader if you don't mind it being duck-taped all ugly-like to the side of your Ninja Tool =D .
• An RFID Antenna for reading tags. You could wrap your own but I recommend getting a professionally-made one since they're cheap. I'm using this one from Coilcraft.
• A 10k resistor; for the transmit (TX) antenna.
• A 10nF (0.01 uF) capacitor; for the transmit (TX) antenna.
  
• An NPN-type transistor (I'm using a 2N3904); for the transmit (TX) antenna.
• A small prototyping board. I'm using a 2cm x 8cm board which fits easily into the PVC pipe.
• 26-34awg magnet wire; for the transmit (TX) antenna.
• At least 10cm (~4 inches) of SDR-21 (the thin wall kind) 3/4-inch PVC pipe. You *must* use the thin-walled kind or your parts won't have enough room inside the pipe (just ask the hardware store to help you select the right kind--Home Depot and Lowes stock it). If you're feeling adventurous the Pro Trinket *does* just barely fit inside the 1/2-inch thin-walled PVC but fitting the battery might be a problem. If you're using the ID-12 or ID-20 you'll need 1-inch PVC (it will still fit in your pocket just a bit bulkier).
• A toggle switch of some sort to turn it off and on.

Optional Parts:

• A 0.96 (or similar size) OLED display.
• Wire wrap tool and some wire wrap wire (if you don't want to solder directly to your RFID reader).
• Copper tape for improved touch-sensitive input (it beats tapping a wire).
• A 100k and an extra 10k resistor for adding voltmeter capabilities.
• Some bright LEDs to make a flashlight.
• An extra toggle switch to turn the flashlight on/off.
• A temperature sensor for making a thermometer.