Currently, smart bracelets such as Xiaomi Mi Band 2 monitor heart rate through PhotoPlethysmoGraphy (PPG). The principle is simple: blood is red, reflects red light, and absorbs green light. Xiaomi Mi Band 2 combines green LED light and photosensitive photodiodes to detect the amount of blood circulating on the wrist at a specific time to obtain heart rate information. In addition, MiDong health bracelet uses ECG PPG method to obtain more accurate heart rate data.