V0.4 - 68d9c07

Main new features:

• SpO2: cleaner pulses with new red / IR defaults (current 32 = 25 mA, 64 us pulses instead of 8 / 24 at 128 us): 1.2x (red) and 1.4x (IR) the pulse-to-noise ratio, measured in a sweep on 5 rings worn at once
• SpO2 fix: the IR stayed off when the heart-rate LED was switched off ("Off" in the app), so the SpO2 IR graph showed only noise (the web app also works around it on V0.3 rings)
• Heart Rate, HRV & SpO2 app: red / IR brightness and pulse length controls, a spectral SpO2 estimate (red / IR amplitudes at the heart rate over 20 s, more robust than the time-domain one), and a recorder to label the signal good / medium / bad and download the raw data
• display mode over BLE: [0xF4, mode] on the PPG characteristic (0 fluid, 1 ring ID, 2 off), e.g. to switch the displays off before optical measurements

Extra:

• light leak calibration: stored per ring and per pulse length, turns Auto on and gives up after 30 s (it could hang); recalibrate each ring once in the dark with the new defaults
• camera check: makes the IR LED visible through a phone camera; brightness slider up to 255
• SpO2 tips in the app: sensor on the palm side, hand palm up (palm down gave ~2.5x weaker pulses), nothing pressing on the sensor
• make flash with several rings: fixed the final check
• the PPG driver is now HX3918.c (the actual sensor)
• rings already running this firmware (V0.1 or later) accept any of the 2 binaries, whatever their model. Stock rings only accept the binary of their product ID:
  • LibRing_68d9c07_0x4C_2026_rings.bin: 2026 rings (product ID 0x4C)
  • LibRing_68d9c07_0x17_older_rings.bin: original SR08 rings (product ID 0x17)

Notes:

• SpO2 is experimental and not a medical measurement (not calibrated against a reference oximeter; the red / IR ratio differs between rings)
• a stock ring that was used with the vendor app refuses the computer: flash it once with the Android SUOTA app