The Digispark is a cheap and easy board to build USB devices, but the original circuit suffers from some unlucky design choices:

• The fist one is the LED connected from the pin P1 to the ground (through a resistor, of couse) which bias this pin to ground.
• The second is that the header with I/O pins does not contain a GND pin (neither a VCC line)
• The third one is present on the chinese clones. The pin 5 not programmed as an I/O line but as a RESET pin.

The three issues above makes the life harder when you aim for a minimalist design like in the present project, but let's pour the cards on the table:

As for the resources:

• We have three I/O pins available

And the limitations

• One of the I/O pins is biased to the ground
• No GND at I/O header

Now the demands for this project

• Four keys that might be used in combination (diagonals)
• A potentiometer for selecting sensitivity
• Minimum extra parts involved
• Shall be attached only to the I/O header

Easy Peasy!!