I got my hands on a Masimo LNCS Adtx single-use adult SpO2 sensor and was curious to see what’s in there and why it’s a $20 part.
It’s very simple. There’s a red LED:
And a photodiode that ends up on the opposite side of the finger. The forward voltage of the photodiode is 0.65V in my testing.
Also interesting is the shielding over the photodiode. Even the top of the photodiode is shielded with thin copper wires cut out of the copper tape.
The shielding that surrounds the wires in the cable is soldered onto the back of the copper tape.