RFID tags are all over the place. They're used in building access control systems, passports, inventory tracking. This instructable will show how you can use an Arduino and a few simple components (wire coil, transistor, capacitor, resistor) to make a device that can spoof an 125 KHz (low frequency) RFID tag. This is version 1, so there are many enhancements that can be made, but this version is stupid simple, yet it works.