# Trimodal Watch-Type Wearable Health Monitoring Device
## 2 System Design
### Hardware Development  
The system's control and sensor data processing are managed by the 'CY8C5888 LIT-LP097' microcontroller unit (MCU) from Cypress Semiconductor (San Jose, CA, USA), selected for its compact size, low power consumption, and cost-effectiveness. For gaseous phase detection, the 'BME680' sensor from Robert Bosch (Gerlingen, Germany) is utilized to identify various VOCs present in exhaled breath. This sensor is distinguished by its extremely low power consumption of 12 \(\mu\)A, affordability, rapid response time of 0.75 s, and compact form factor. The measurement of physiological parameters such as dopamine levels in human sweat is conducted using screen-printed electrode sensors (SPESs). The output current for dopamine detection with SPES typically ranges from several \(\mu\)A to approximately 100 \(\mu\)A, depending on the specific electrode material used [20; 21; 22]. Consequently, a target measurement range of \(-\)100 \(\mu\)A to +100 \(\mu\)A with a resolution of 0.05 \(\mu\)A is set for this study. Optical phase detection is performed using the 'TMD26353' infrared (IR) proximity sensor from ams OSRAM (Munich, Germany), which operates with a low current consumption of 340 \(\mu\)A. The analog-to-digital converter (ADC) associated with the IR sensor is calibrated to account for variations in body temperature.  
To facilitate wireless communication, the system incorporates the 'SPBTLE-1S' Bluetooth Low Energy (BLE) module from STMicroelectronics (Geneva, Switzerland), which includes an integrated onboard chip antenna. This module operates with a maximum transmission current of 15 mA at a transmit power of +4 dBm, and exhibits a minimal standby current of 500 nA. The device can be powered through either a rechargeable lithium polymer (LiPo) battery or a USB connection. With a peak power consumption of 33 mA, a 500 mAh rechargeable LiPo battery enables the device to function for approximately 15 h per charge. Charging is facilitated via a wired connection using the 'MCP73831' charger from Microchip (Chandler, AZ, USA) with a micro-USB interface, or wirelessly through the 'BQ51013 BRHLR' wireless charging integrated circuit (IC) from Texas Instruments (Dallas, TX, USA) in conjunction with the '760308101303' wireless charging coil from Wirth Elektronik (Niedermhall, Germany). To provide stable power to the components, a low drop output (LDO) voltage regulator has been utilized. A level shifter is implemented for I\({}^{2}\)C communication between the MCU and IR sensor, since the MCU is operating at 3.3 V while the IR sensor's operating voltage is 1.8 V.  
The entire system is mounted on a printed circuit board (PCB) with dimensions of 3 cm \(\times\) 3 cm, as depicted in Figure 2. This figure also illustrates how the device is housed within a watch-type case worn on the wrist, thereby validating the proof of concept. The top layer of the PCB includes the BLE module, USB port, battery connector, wired and  
Figure 1: System block diagram.  
wireless charging modules, and the 'BME680' sensor, Conversely, the bottom layer of the PCB accommodates the MCU, 'TMD26353' sensor, and SPES connector.