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.

Youtube Video List
1. Introduction
2. General Pulse Oximeter Principles
3. Circuit Operation
4. Program explanation
5. Assembling Probe
6. Assembling Controller

Download List Probe STL files Case STL files
Schematic_Rev4_1.pdf Schematic
PartsList_Rev4_1.pdf Parts List Main board gerber files which can mount Arduino Nano Main board gerber files which can't mount Arduino Nano Probe board gerber files
POM_v0_1_1.ino Pupse Oximeter basic program