A Self-Driven and Low-Cost Resistance-to-Frequency Converter Circuit With Wireless Integrations for Wearable Physiological Monitoring Applications

Wearable physiological monitoring systems increasingly require continuous, low-power sensing interfaces capable of seamless integration into flexible Internet of Things (IoT) platforms. Conventional architectures rely on analog-to-digital converter (ADC)-based front-ends and rigid electronics, which elevate system power, increase design complexity, and limit textile-level conformity. To address these restraints, this article presents a self-driven, fully integrated resistance-to-frequency converter system for continuous, and low-power wearable physiological monitoring. The proposed architecture eliminates conventional ADCs and high-gain amplification stages by directly converting resistive strain variations into a frequency-modulated signal. The system comprises a flexible carbon-PDMS-based flexible sensor, exhibiting a gauge factor of approximately 80. The fabricated resistive sensor is embedded in a Wheatstone bridge front-end, which drives a low-complexity BJT-based push-pull astable oscillator operating in the 1-3 kHz band with a sensitivity of 50 Hz/kΩ. The BJT-based oscillator leverages a hybrid RC-LC topology, where an integrated inductor (L₁) enhances the quality factor from 5 to 15, reducing phase noise by over 15 dB and directly improving start-up reliability and temperature stability. The overall system is realized on a flexible PCB with an integrated mini ATmega16U4 microcontroller for frequency counting and an nRF52840 Bluetooth module for wireless transmission. Experimental validation demonstrates robust tracking of respiratory and body movement patterns with subhertz resolution, mean absolute error (MAE) below 0.8 breaths per minute, and motion repeatability error under 3%. The design achieves an ultralow duty-cycled power consumption, with a time-averaged current of 75μA, enabling extended time operation from a 90 mAh battery. Finally, the system is fully textile-integrated using hand-knitting technique, ensuring user comfort and mechanical conformity by retaining >98% of its baseline electrical performance after 20 gentle machine-wash cycles.