Pulse oximetry measures the relative concentration of arterial oxygenated hemoglobin and reduced hemoglobin. This paper presents the design of a pulse oximeter, which measures the presence of a third substance, carboxyhemoglobin (HbCO), by unfiltered LEDs at 660 nm, 810 nm, and 940 nm. Background information is given on the theory of pulse oximetry. Theoretical approximations and calculations which are implemented in our instrument are presented. Sensor design and a flowchart, showing the signal processing routine that accomplishes the required analysis is included. The problem of noise occurrence during the signal processing routine is also discussed.