The integration of legacy energy meters into modern smart grids remains a challenge due to the unviable cost and logistical complexity of full-scale replacement. In Brazil, this problem is acute, with millions of meters operating on a sort of Protocol for Advanced Measurement Infrastructure (AMI) legacy called Protocolo para Infraestrutura de Medição Avançada (PIMA), provoking incompatibility with modern internet of things (IoT) networks and impeding the rollout of AMIs. This letter presents a novel low-power sensor interface platform designed to resolve physical-layer data acquisition barriers in legacy infrastructure. The system architecture addresses the specific sensing constraints of open-collector serial outputs, decoding proprietary PIMA frames with high precision and transmitting key measurements over long range wide area networks (LoRaWAN). An experimental validation confirmed the sensing accuracy, with a perfect match between transmitted data and the meter’s physical display. This retrofit solution provides a verified and viable pathway for the rapid, scalable, and economical integration of legacy assets into modern AMI, bypassing the need for costly meter replacement.