Looking to build signal level converter to reduce 20V input to 3.3v out. Am I on the right track?
If they are signals to the pi then use a resistor potential divider. A 1k ohm across the input and 0v of the pi and a 9k resistor from the pi input to a high voltage logic signal will reduce that voltage by ten to one. You should be looking for slightly less than a 10:1 reduction. Can you work this bit out yourself? See the link.