Flexible Writing Pad based on a Piezoresistive Thin Film Sensor Matrix

Digital multimedia tools are becoming increasingly important as the world slowly shifts to an online mode. In this work, we present a low-cost, flexible writing pad that uses a $16 \times 16$ pressure sensing matrix based on the piezoresistive thin film of velostat. The writing area is $5 \mathrm{~cm} \times 5 \mathrm{~cm}$ with an effective pixel area of $0.06 \mathrm{~mm}^{2}$. A read out circuit is designed to detect the change in resistance of the velostat pixel using a voltage divider. A microprocessor raster scans through the sensor pixel matrix to obtain a data frame of 256 integers. This data is processed using techniques like squaring and normalising (S&N), Gaussian blurring, and adaptive thresholding to generate a more readable output. The writing pad is able to resolve characters larger than $2 \mathrm{~cm}$ in length. The flexible writing pad produces legible output while flexed at bending radius of upto $4 \mathrm{~cm}$. Such flexibility promises to enhance the usability and portability of the writing pad significantly.