Thermostat fed by Home Assistant

A wall thermostat on the Olimex ESP32-EVB with its 2.8" glass — with one twist: the room temperature does not come from a wire, it comes from Home Assistant, read off a Zigbee sensor somewhere in the house. The board's own relay switches the heating; two small HA automations feed it the temperature and a heartbeat; a ladder watchdog decides how much to trust them.

The project is installed with the editor in tutorials\ — open it while reading. The general route of connecting a board to HA (broker, discovery, what a variable becomes) is on Home Assistant; this page builds a complete working system on top of it.

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The design decision worth copying

The temperature arrives over WiFi, ethernet, a broker and a radio — any link in that chain can die silently. So the board never trusts it blindly: remote data gets a watchdog. Home Assistant toggles a ha_beat variable once a minute; the ladder counts the silence, and three minutes without a beat means the heating goes OFF, a banner appears on the glass and an alarm lands in the table. Manual mode keeps working without HA on purpose — the person standing at the equipment outranks the network.

The same shape protects drive positioning against stale encoder data — it is a doctrine, not a trick for this page.

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What you create, step by step

1. Project and board

New project, board Olimex ESP32-EVB, resolution 240×320 portrait. Ethernet needs no configuration — the board's catalog starts the LAN8710 with DHCP by itself.

2. WEBview off — deliberately

On the WEBview page, untick WEBview on. This classic board has no PSRAM, and the HTTP/WS server and the MQTT client do not fit its internal RAM together — the editor's memory tiers would skip MQTT and say so in the boot log. Here the choice is easy: the operator has the glass, and the remote view is Home Assistant. The server's memory pays for the MQTT client.

webview:
  enabled: false

3. The variables

4. MQTT

Broker = the machine running Mosquitto (the HA add-on), one topic per variable, discovery on:

mqtt:
  broker: 192.168.0.53
  username: ctrl32
  password: ctrl32mqtt
  client_id: ctrl32-evbtherm
  topic: ctrl32/evbtherm
  publish_mode: per_tag
  discovery: true
  interval_ms: 2000
  tags:
    - { name: ha_temp,   write: true }
    - { name: ha_beat,   write: true }
    - { name: temp_sp,   write: true }
    - { name: auto_mode, write: true }
    - rel1
    - ha_ok

write is the permission: only these four may be changed from the broker side. rel1 and ha_ok are read-only sensors in HA — the outside world watches the relay, it never drives it.

5. The ladder

Five things, one rung each (open the project to see them drawn):

1. Heartbeat — a 1 Hz BLINK for the lamp in the corner.
2. The watchdog — R_TRIG + F_TRIG on ha_beat make one pulse per edge; the pulse feeds a TOF with PT = 180 000 ms. Its Q is ha_ok: any beat restarts the three minutes.
3. Hysteresis comparator — HYST on ha_temp around temp_sp (0.5 °C band). A bare comparison would chatter the relay.
4. The relay rung — (auto AND cold AND ha_ok) OR (manual AND command) into the rel1 coil. Note where ha_ok sits: it gates only the automatic branch.
5. The setpoint spinner — R_TRIG per button press, SEL chain, LIMIT 15–30 °C clamped last.

6. The screens

Three portrait screens: the dial (two nested arcs — blue room temperature, orange setpoint), Manual, and a two-curve Trend (ha_temp + temp_sp). The watchdog banner is one label with visible_if: n_haok — drawn only while HA is silent, exactly like the states and motion page describes.

7. Download

F5 over USB. The boot log prints the MQTT connection, and within seconds Home Assistant shows a new device ctrl32-evbtherm with six entities.

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The Home Assistant half

Two automations — create them in HA (Settings → Automations, or paste into automations.yaml). First the temperature: every change of the Zigbee sensor (and every 5 minutes as a refresh) is written into the board's ha_temp number entity:

alias: ctrl32 EVB thermostat - temperature from Zigbee
trigger:
  - platform: state
    entity_id: sensor.tuya_teplota      # your sensor's entity id
  - platform: time_pattern
    minutes: "/5"
condition:
  - condition: numeric_state
    entity_id: sensor.tuya_teplota
    above: -40
    below: 80
action:
  - service: number.set_value
    target:
      entity_id: number.ctrl32_evbtherm_ha_temp
    data:
      value: "{{ states('sensor.tuya_teplota') | float }}"
mode: single

The condition matters: a battery Zigbee sensor reports unavailable while it sleeps, and writing that into a number would fail — the guard lets only a sane reading through.

Then the heartbeat — the whole automation is one toggle:

alias: ctrl32 EVB thermostat - heartbeat
trigger:
  - platform: time_pattern
    minutes: "/1"
action:
  - service: switch.toggle
    target:
      entity_id: switch.ctrl32_evbtherm_ha_beat
mode: single

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Proving it

• Watch the dial follow the sensor; hold the sensor in your hand and the number climbs.
• Kill the beat (disable the heartbeat automation): after three minutes the banner appears, the alarm fires, the relay drops — and Manual still heats.
• Change the setpoint from HA (the temp_sp number): the dial's orange arc moves on the glass. The setpoint is retentive — pull the plug, it comes back.

When it does not work