A Fully Passive Ten-Digit Numeric Keypad Sensor Using Chipless RFID Technology

A ten-digit chipless radio frequency identification (CRFID)-based numeric keypad that works on the principle of touch event sensing is presented. The tag is designed to operate using the passive frequency-domain (FD) technique. The proposed tag has ten circular resonator rings for each of the keypad's corresponding decimal digits. Each circular ring resonates at a particular frequency within the 400-MHz band from 2.7 to 7.6 GHz to uniquely identify a digit. Identification is performed by decoding radar cross Section (RCS) response of the whole keypad tag. A customized human finger using standard tissue characteristics is modeled to verify the keypad's working mechanism. When the finger is placed on any of the nine resonators, its corresponding resonance disappears from the allocated band. The proposed CRFID keypad sensor tag is built upon a Rogers substrate of 85\times 95\times0.8 mm volume. The backside of the proposed tag is fully copper filled for an easy interface with a variety of backgrounds, including metallic walls. The design exactly mimics the shape of ordinary keypad. The finger model used in the simulations realizes the practical scenario. This CRFID keypad is deemed an intriguing solution for future automated cities and will help in the conceptualization of Internet of Things (IoT) systems.