I have built a sprinkler controller using my existing 24VAC valves. I have a Shelly Pro 3 controlling the valves. I found a Node Red flow that was shared on the forum. I am using the flow created by enosrevlah. Here is the flow as he posted it:
I am having trouble controlling the relay outputs using MQTT. I’ve enabled MQTT in the Shelly app and configured the MQTT server in Node Red, but the Shelly does not trigger the relay. I know its a syntax issue with how to set up the MQTT out node but I can not get it to work. Here is how my flow is set up. Note: I changed the zone 1 MQTT node and the start node, but the others are original code.
This is the error I get from the MQTT node:
4/20/2026, 3:45:48 PMnode: Lawn
msg : error
error
name: "Error"
message: "Invalid topic specified"
stack: string
Error: Invalid topic specified
at MQTTBrokerNode.node.publish (/opt/node_modules/@node-red/nodes/core/network/10-mqtt.js:1214:35)
at doPublish (/opt/node_modules/@node-red/nodes/core/network/10-mqtt.js:365:33)
at MQTTOutNode._inputCallback (/opt/node_modules/@node-red/nodes/core/network/10-mqtt.js:1489:21)
at /opt/node_modules/@node-red/runtime/lib/nodes/Node.js:214:26
at Object.trigger (/opt/node_modules/@node-red/util/lib/hooks.js:166:13)
at MQTTOutNode.Node._emitInput (/opt/node_modules/@node-red/runtime/lib/nodes/Node.js:206:11)
at MQTTOutNode.Node.emit (/opt/node_modules/@node-red/runtime/lib/nodes/Node.js:190:25)
at MQTTOutNode.Node.receive (/opt/node_modules/@node-red/runtime/lib/nodes/Node.js:506:10)
at deliverMessageToDestination (/opt/node_modules/@node-red/runtime/lib/flows/Flow.js:824:40)
at Immediate._onImmediate (/opt/node_modules/@node-red/runtime/lib/flows/Flow.js:840:21)
at processImmediate (node:internal/timers:504:21)
Can anyone tell me how to format the start node and MQTT node to control my Shelly?