Unknown voltage in IR pulse sensor circuit
Yuck,  That's a mess of a schematic.  But for a start the photodiode (PD) D2 is put in backwards.  Right now it's always forward biased.. so you get ~0.5V there, and no response for the light.
Flip it around and you'll get V(PD) = 5V- I(PD) * 39 k ohm.  (I'd typically put the resistor R4 to ground, then you have zero signal with no light.)