We have fan coil valves in 140 hotel rooms doing this, so here is our setup.
Hermetry sends commands to the device on its own downlink topic. The fan board subscribes to v1/rpc/req/+, a command arrives as v1/rpc/req/<id> with a method and params, and the board answers on v1/rpc/res/<id>. The id in the topic is how the answer is matched.
On the fan profile, open the Commands tab and add a command, for example method fan.set, one parameter on of type Yes/no, tier Basic. Basic matters because a dashboard Switch only sends Basic tier commands. Then the supervisor’s board gets a Switch card, its action is Run a command fan.set with the control’s value, and the Current state is bound to a telemetry key the board reports after it switched, we use fan.on.
Rough Arduino side with PubSubClient:
void onMsg(char* topic, byte* payload, unsigned int len) {
String t(topic);
if (!t.startsWith("v1/rpc/req/")) return;
String id = t.substring(11);
StaticJsonDocument<256> doc;
deserializeJson(doc, payload, len);
if (doc["method"] == "fan.set") {
bool on = doc["params"]["on"];
digitalWrite(RELAY, on ? HIGH : LOW);
client.publish(("v1/rpc/res/" + id).c_str(), "{"ok":true}");
client.publish("v1/telemetry", on ? "{"fan.on":1}" : "{"fan.on":0}");
}
}
// after every connect:
client.subscribe("v1/rpc/req/+", 1);The docs say a command can arrive twice, so keep the last few ids and skip repeats. Page: Sending commands to devices. For setpoints that should survive a reboot (heater target temperature) we use shared attributes on v1/attributes/set instead, the board asks for them right after connecting.