Many HR wrist bands use 2-LEDs. However, they are typically matched, positioned symmetrically to the Photodiode centroid, have the same height, and implement optical walls to reduce cross coupling. One external LED can be used with MAX30101 to have two LED solution with one embedded and one external LED. However, matching is not viable as the external LED needs to be exactly matched with the embedded LED. In addition an optical barrier (i.e., light absorbing wall) is recommended between an external LED and the Photodiode.