Why use an LED as a sensor when there are devices designed for this purpose?
I was thinking that I only need something cheap and basic, something that will work with a fixed light source and stable ambient conditions, and I am not familiar with other solutions.
You are looking for something called a "phototransistor" or "photodiode".
Thanks. Do you have any models that you would recommend?
...without scaling using an op amp you will probably not like it.
Forgive my ignorance, but what is an op amp?