An ESP32 ESPHome device in the furnace room reads a differential-pressure sensor across the filter and watches the furnace control board’s status LED. Home Assistant turns the raw pressure into blower speed, filter loading, and a replace-filter alert. The threshold comes from a real clean-filter-vs-old-filter experiment, not a guessed limit.

Python tool for validating cellular SIM activation on a Digi WR21 router. Auto-detects carrier, cycles through known APNs, and reports results via a 2-line I²C LCD with RGB rotary encoder interface.