A device (100, 200) for measuring oxygen saturation, the device (100, 200) comprising: a memory (120, 220) configured to store a calibrated difference of forward voltage; circuitry (110, 140; 240, 245, 210) configured to: apply a first current from an anode of a first light emitting diode (460A; 560A; 580A; 582A) to a cathode of the first light emitting diode (460A; 560A; 580A; 582A); while applying the first current, measure a first forward voltage across the anode of the first light emitting diode (460A; 560A; 580A; 582A) and the cathode of the first light emitting diode (460A; 560A; 580A; 582A); apply a second current from an anode of a second light emitting diode (460B; 560B; 580B; 582B) to a cathode of the second light emitting diode (460B; 560B; 580B; 582B); while applying the second current, measure a second forward voltage across the anode of the second light emitting diode (460B; 560B; 580B; 582B) and the cathode of the second light emitting diode (460B; 560B; 580B; 582B); determine a measured difference of forward voltage based on a comparison of the first forward voltage and the second forward voltage; determine that the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) are valid based on the calibrated difference of forward voltage and the measured difference of forward voltage; and in response to the determination that the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) are valid, determine an oxygen saturation level using the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) and output an indication of the oxygen saturation level. a memory (120, 220) configured to store a calibrated difference of forward voltage; circuitry (110, 140; 240, 245, 210) configured to: apply a first current from an anode of a first light emitting diode (460A; 560A; 580A; 582A) to a cathode of the first light emitting diode (460A; 560A; 580A; 582A); while applying the first current, measure a first forward voltage across the anode of the first light emitting diode (460A; 560A; 580A; 582A) and the cathode of the first light emitting diode (460A; 560A; 580A; 582A); apply a second current from an anode of a second light emitting diode (460B; 560B; 580B; 582B) to a cathode of the second light emitting diode (460B; 560B; 580B; 582B); while applying the second current, measure a second forward voltage across the anode of the second light emitting diode (460B; 560B; 580B; 582B) and the cathode of the second light emitting diode (460B; 560B; 580B; 582B); determine a measured difference of forward voltage based on a comparison of the first forward voltage and the second forward voltage; determine that the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) are valid based on the calibrated difference of forward voltage and the measured difference of forward voltage; and in response to the determination that the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) are valid, determine an oxygen saturation level using the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) and output an indication of the oxygen saturation level. apply a first current from an anode of a first light emitting diode (460A; 560A; 580A; 582A) to a cathode of the first light emitting diode (460A; 560A; 580A; 582A); while applying the first current, measure a first forward voltage across the anode of the first light emitting diode (460A; 560A; 580A; 582A) and the cathode of the first light emitting diode (460A; 560A; 580A; 582A); apply a second current from an anode of a second light emitting diode (460B; 560B; 580B; 582B) to a cathode of the second light emitting diode (460B; 560B; 580B; 582B); while applying the second current, measure a second forward voltage across the anode of the second light emitting diode (460B; 560B; 580B; 582B) and the cathode of the second light emitting diode (460B; 560B; 580B; 582B); determine a measured difference of forward voltage based on a comparison of the first forward voltage and the second forward voltage; determine that the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) are valid based on the calibrated difference of forward voltage and the measured difference of forward voltage; and in response to the determination that the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) are valid, determine an oxygen saturation level using the first light emitting diode (460A; 560A; 580A; 582A) and the second light emitting diode (460B; 560B; 580B; 582B) and output an indication of the oxygen saturation level.