[solved] How to send MQTT topic w/o payload

General
1

Hello all,
I am searching for a solution to send only the topic without the payload.
Background: I have a Heos 1 Wifi speaker. it is connected to my homeautomation system FHEM. FHEM is able to publish ans subscibe MQTT messages.
This speaker starts to play on /cmnd/entertain/wz/heos1/play and stops on /cmnd/entertain/wz/heos1/stop
Both commands sendet by hand on the MQTT broker UI do very well.
How do i tell node red to send only the topic without any payload.

Best regards O.J.
(please excuse my weak english)

2

It is easy to remove the payload property from the msg object. A change node configured to delete the payload property would do the trick, but..

Doing a quick testing shows that the mqqt out node will send an empty string whenever msg.payload equals null or undefined

Therefore the question to be asked is : is it a problem for you to send an empty string as the payload ?

3

Yes just set msg.payload = “”; ie empty string and mqtt will do the right thing

4

Does it work if you send play or stop messages on the topic /cmnd/entertain/wz/heos ?

5

sorry, Im an absolute noob. how do i convert the msg.payload to “”

Skizze%20node()
Skizze%20node()886×130 16.3 KB
Skizze%20node()
Skizze%20node()886×130 16.3 KB

6

This forum has changed the normal double-quotes into smart quotes, making it in invalid.

Try this:

msg.payload="";
return msg;

You could also use a Change node to set msg.payload to an empty string.

7

found a better way:
I changed the payload type to "buffer" then in to the 3 points to change the content, but didnt change anything.
Injecting this gives an empty buffer (msg.payload : buffer[0])

Thank you all for your help