Total beginner question probably. We are piloting cold storage monitoring for our logistics warehouse. Six ESP32 boards publish temperatures to v1/telemetry and that works fine, chart and alarm are there.
Now I want actuators: door frame heaters and two extra fans, each on a relay board with an ESP32. In my head pub/sub works both ways, so my plan was: the fan board subscribes to warehouse/zone3/fan/set, and something (a dashboard button? a script?) publishes on or off on that topic.
So I tested it from my laptop with paho-mqtt, logged in as one of the sensor devices, and published to v1/zone3/fan/set. Nothing arrived on the fan board. After three tries my laptop connection got closed. The Log tab of that sensor device shows the publishes were rejected as unknown_reserved_topic.
Is pub/sub just the wrong model for actuators here? Or am I missing a setting somewhere that lets devices talk to each other?
Isn’t that what Custom topics on the profile are for? I remember a setting that accepts any topic outside v1/.
Custom topics are inbound only, they become telemetry of the device that published. And devices never get each other’s messages in Hermetry, so a sensor publishing for a fan will never work.
Also from 14 years of plant work: you want the actuator to answer. Fire and forget to a relay is how you end up with a fan that shows on in the control room and is off in the room.
ok that makes sense. So how does something get down to the fan board then? I still need a button somewhere for the warehouse supervisor.
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.
One thing from the field: make the command set a state, never toggle. A command that times out only means no answer came back, it may have run anyway. If the supervisor presses again, fan.set on twice is harmless, fan.toggle twice turns it off.
Working. Switch card on the supervisor board, fan clicks, state comes back as fan.on.
Two things I had wrong. I first made it fan.toggle, changed it after Tyler’s post. And I only subscribed once in setup(), so commands worked until the first WiFi drop and then never again. Moved the subscribe into the reconnect function and it survives now.
- You must be logged in to reply to this topic.
Join the conversation
Sign in to ask and answer, like helpful posts and mark the reply that solved your problem. Every contribution earns points and levels.