Demo

Vid. 1 Use the werable photoplethysmogram (PPG) device to collect PPG signal from the wrist, and display the signal on mobile phone.

Project Objective

A PPG is an optically obtained plethysmogram that can be used to detect blood volume changes in the microvascular bed of tissue. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption.

In this project, I developed a wearable PPG device to collect the pulse wave, using reflective mode.

Fig. 1 Principle of PPG: (a) reflective mode; (b) transmitting mode; (c) example of PPG signal.

System Architecture

The wearable PPG device mainly includes 3 modules:

  1. Pulse-oximeter signal acquisition module to emit red and infrared ray, detect the lights reflected back from the skin and collect the PPG data.
  2. MCU module to control the operation flow and transmit BLE signal.
  3. Power management module to regulate the voltage, control the power consumption and support wireless charging.
    Fig. 2 System architecture. The blue part represents pulse-oximeter signal acquisition module; The green part represents MCU module; The yellow part represents power management module.

PC and mobile phone are both available to receive and display the PPG signal.

Hardware

  1. Use AFE4490 for pulse-oximeter signal acquisition, with sample accuracy with 22-bit and adjustable LED current. Write the register to set proper current, and read the register to get the analog output voltage of the receiver
  2. Use DCM04 as dual emitter and photo detector to emit dual red and IR and detect the lights reflected back from the skin.
  3. nRF52832 MCU.
    • Use the SPI to communicate with the AFE4490, initializing registers, writing registers, and reading register according to the register map and data format.
    • Set the timer and trigger event to sample per 10 ms.
    • Declare service, characteristic and handles. Every time after sampling 2 times, encapsulate a BLE packet and update the characteristic value to transmit ECG data to the master.
    • Sleeping mode: Set BLE slave’s advertising interval 5 s and disable all peripheral functions to decrease power consumption, and the current consumption is about 6 μA.
    • Operating mode: When the BLE slave is scanned and connected by a master, pull up the GPIO which controls the load switch, then the load circuit can be powered. Also enable ADC and timer, then start to collect and transmit data. The current consumption is about 10 mA.
  4. Power Management
    • Use TPS7A022 to regulate voltage to 3.3 V
    • Use TPS22919DCKR as load switch to control the power supply for load circuit.
Fig. 3 (a) FPC of PPG device with the size of 1.7 cm × 3 cm; (b) encapsulated PPG device