Practice your hex to binary conversion skills! Great for programmers and students.

Tap 'up' to enter one-bits and tap 'down' or 'back' to enter zero-bits. Clear your guess with 'select' if you make a mistake. Hold 'up' or 'down' to increase or decrease level, respectively. At higher levels, you will see each nibble (one hexadecimal digit, four binary digits) on a separate line as you tap it out.

This is free and open-source software. Patches and forks welcome!