This instructable assumes that you have basic knowledge of electronics and a cursory understanding of C++.
NOTE: This device is useless unless you already know the contents of a particular magnetic stripe card that you want to emulate. Thus, you can't use this device to do Something You're Not Supposed To Do unless you've already done something naughty to obtain the data from a magnetic stripe card you're not supposed to have.
Don't do naughty things. Your mother wouldn't like it.