The crude method is to use a potential divider, so you would have another resistor from the sensor/photon junction to 3.3V and measure the voltage. You have two sensors on 2 different pins, and you treat them as two separate entities.
However this method is not particularly accurate and although the second resistor would prevent a short to 3V, depending on the value you choose and the resolution you desire it could also waste a fair bit of power.
The traditional circuit for measuring resistances is called a Wheatstone bridge https://en.wikipedia.org/wiki/Wheatstone_bridge not exactly a new idea as you can see but still in use today.
There are any number of other approaches to this problem many of which would come up with a quick google but at some level there is almost certainly a divider or bridge circuit involved.
If you wanted to get a good book on electronics fundamentals, almost every student is told to go and buy “The Art of Electronics” by Horowitz and Hill, its expensive but worth it (you could probably get the old 2nd edition pretty cheap).
1 Like