Hi ProFPGA,
Here is a rearrangement of you circuit which is simpler and should provide an improved performance.
spec
Issue 2 of 2016_08_17
NOTES
(1) The value of R13 is notional and defines the red LED on current.
(2) The value of R14 is notional and defines the IR LED on current.
(3) The value of R9 is notional and defines the opto receiver diode speed and voltage gain.
(4) I would imagine that the output from the opto receiver diode would need to be amplified with an opamp.
(5) If the microcontroller operates from a 3.3V supply line, I would imagine that a potential divider will be required between the oximeter and GP103.
(6) The sense of GP104 is now reversed (high turns the red LED on).
(7) The sense of GP105 is now reversed (high turns the IR LED on).
(8) R11 is included to speed the red LED turn-off.
(9) R12 is included to speed the IR LED turn-off.
(10) C1 is ceramic X7R dielectiric, +- 10% or better, through hole (not surface mount), 18V minimum. A higher capacitance can be fitted.
(11) C1 should be mounted as close as possible to the terminals of the voltage regulator.
(12) C4 is ceramic X7R dielectiric, +- 10% or better, through hole (not surface mount), 10V minimum. A higher capacitance can be fitted.
(13) C4 should be mounted as close as possible to the terminals of the voltage regulator.
(14) C7 is aluminum dielectric, +- 20% or better, 10V minimum. A higher capacitance can be fitted.
(15) All resistors are metal film, 5% or better, 250mW minimum (unless otherwise stated).