Firmware that
finishes the device.

// Cold-chain telemetry — publish + sleep
#include "esp_sleep.h"
#include "mqtt_client.h"
#include "sht4x.h"

void app_main(void) {
    sht4x_init(I2C_NUM_0);
    mqtt_client_t *m = mqtt_connect("mqtts://hub.iot-works.com");

    while (1) {
        float t, h;
        sht4x_read(&t, &h);

        // Signed payload, OTA-rolled keys
        mqtt_publish_signed(m, "v1/telemetry",
            "{\"t\":%.2f,\"h\":%.2f}", t, h);

        esp_deep_sleep(15 * 60 * 1000000ULL); // 15 min
    }
}