Heatwave Monitor: A Low-Cost ESP32-Based Heat Index Early-Warning Device

Abstract Heatwave warnings are issued for whole districts, while the heat stress a person experiences depends on the temperature and humidity of the room or site they are in. This paper presents a standalone heat-risk indicator built from only three modules, an ESP32 development board, a DHT22 temperature and relative-humidity sensor and a 0.96-inch SSD1306 OLED display, on a single-sided printed circuit board (PCB) drawn with nail-polish resist and etched in ferric chloride. Every 2.5 s the ESP32 reads the sensor, computes the Heat Index with the NOAA (Rothfusz) regression, classifies it as SAFE, CAUTION, WARNING or DANGER and shows the result on the display, which flashes at DANGER; each reading is also sent over USB to an offline browser dashboard. The firmware scans the I2C bus at start-up and runs a command-by-command display self-test. This self-test located a fault in the hand-made board: the display controller acknowledged every command until the panel was switched on and then stopped responding, which was traced to poor solder joints on the display tracks; after the tracks were re-soldered the complete system ran on the PCB. Heat Index values computed on the device agreed with an independent implementation to within 0.1 °C in simulation and on hardware. Mounted beside the ESP32, the sensor read up to about 5–9 °C high because of self-heating; a temperature offset with a Magnus-formula humidity correction is applied in firmware, and a full calibration against a reference instrument remains future work. The core hardware costs roughly Rs. 1,000–1,300.