I just started to build a second version of a pulse oximeter. If you don't know what a pulse oximeter is, here is a simple but detailed description what a pulse oximeter is and how it works.
The first thing I wanted to do is to be able to adjust the brightness of the red and IR LED by code. This is very important for calibration of the pulse oximeter. If the LEDs are too bright, it will blind the photo diode. If the brightness is too low, no signal can be obtained as well. This is achieved with a MCP4921, which is a 12 bit DAC with an SPI interface and a constant current drive circuitry (op amp, NPN transistor and two resistors, see circuit diagram below). The first video shows the brightness adjustment of a red LED.
Reference: Arduino & MCP4921 (German)
I will update this blog as soon as progress is made.
https://www.youtube.com/watch?v=KIfo6P4nJgg