Weather Station Midea

Overview

The AC unit is treated as a black box: the firmware never talks to it electrically. Instead it captured the unit's own IR remote codes ahead of time — using a 555-timer monostable to clean up the receiver's noisy output and a logic analyzer to record the resulting pulse train — and now simply replays those exact waveforms through an IR LED whenever the climate logic decides the room needs cooling, heating, or the unit should turn off.

Every second, an HDC1080 sensor reports temperature and humidity. Humidity is adjusted by a fixed calibration offset, then a heat index is computed and checked against a hysteresis band around a configurable setpoint. Two independent state machines exist for this decision — one for cooling (ac_processing), one for heating (heater_processing) — but only one is ever active, chosen by a physical jumper (HEATER_MODE) read once at boot. A third state machine (humidifier_processing) runs independently of both, driving a relay purely off room humidity.

A bank of four DIP switches lets you nudge the target setpoint by up to ±3.5°C in 0.5° steps without reflashing — read as a binary-weighted offset and applied identically to whichever mode is active.

Signal architecture

One sensor feeds three independent decision loops; only the IR LED talks to the AC unit itself.

Pin reference

DIP and mode pins use INPUT_PULLUP and are read inverted (!digitalRead) — switch closed = logic true.

State machines

All three machines share the same debounce shape: a steady state, a "bouncing" transitional state that must hold for several ticks before committing, and a fault/disabled branch.

Function reference

IR capture & replay

Rather than implementing Midea's IR bit protocol from a spec, the codes were captured directly from the unit's own remote and are replayed byte-for-byte.

const PROGMEM uint16_t cool_25_on_a[] = {
  0, 109, 100, 0, 168, 166, 21, 61, …
};
sendProntoFromProgmem((uint16_t*)(cool_25_on_a));
sendProntoFromProgmem((uint16_t*)(cool_25_on_b));
// most commands are captured as an "a" and "b" half

Six codes are baked in: cool_25_off, cool_25_on_a/b, heat_22_off, heat_22_on_a/b. There is no general-purpose "set to N°C" command — the firmware always commands the same two fixed setpoints, using its own sensor loop (not the AC's internal thermostat) to decide when those commands fire.

Tuning constants

All thresholds are plain globals at the top of the sketch.

const float HC_MAX_HIDEX = 27.2;
const float HC_MIN_HIDEX = 26.8;
const float ABOVE_ZERO_LEVEL_HUMD = 33;
const float ZERO_LEVEL_HUMD = 30;

min_temp = 19.8 + dip_offset;
max_temp = 20.2 + dip_offset;

const float MIN_HUMD = 39;  // was 55
const float MAX_HUMD = 43;  // was 69

const int MAX_BOUNCE_COUNT = 6;      // ~6s hysteresis debounce
const int BUTTON_BOUNCE_COUNT = 1;   // ~1s button debounce
const long REPEAT_ACTION_INTERVAL = 600000; // 10 min IR keep-alive

if ((room_temp > 10 && room_temp < 60) && (room_humd > 20 && room_humd < 95)) {
  room_hidx = dht.computeHeatIndex(room_temp, room_humd, false);
  reading_error = !(room_hidx >= 1 && room_hidx <= 95);
} else { reading_error = true; }
// room_humd is read with a fixed -15 calibration offset applied first

Notes & known quirks