How can I make a simple analog to digital converter?
What you're asking for is called a "dual comparator", and you can get them as single chips.

However, you should seriously consider using a small microcontroller with a built-in ADC. This would allow you to adjust the two thresholds in software (possibly by remote control), or even to create an adaptive algorithm that doesn't need as much manual adjustment.

If your disabled person is willing to learn Morse Code, your microcontroller could be programmed to decode it and send a much broader range of characters to the Bluetooth interface.