The pulse oximeter needs a high resolution ADC and the Arduino ADC is useless.
This pulse oximeter measures transmitted light by sweeping the LED current and measuring the time until the output of the phototransistor reaches a specific voltage.
As a result, it does not require an ADC or dedicated module, and can be assembled with only cheap and general parts that will not run out of stock even in special COVID19 demand.
I explained the operation principle in detail in youtube videos.
|2. General Pulse Oximeter Principles|
|3. Circuit Operation|
|4. Program explanation|
|5. Assembling Probe|
|6. Assembling Controller|