How does an ESP8266 "advertise" it can handle Alexa "relative volume" commands

Question

As I understand more I WILL edit this question. For now, I am guessing at what I need. To make it easier for people to help, I'll tell you the over all purpose:

I have programmed an ESP8266 to advertise it is the TV and that it can turn the TV on / off. The ESP8266 actually transmits the absolute on / off codes to the TV using IR signals. I believe I have added a second "advertisement" for yet another on / off feature to the same ESP8266 device.

However, what I really want to add is a "relative volume" device. I believe I need to do this by using XML. That is, I believe I need to modify the XML transmitted to Alexa to not only advertise the on / off device but to also advertise a relative volume device.

Where can I find examples where a relative volume device is advertised to Alexa?

To clarify my objective, let me add an example:

If I say

"Alexa, turn on the TV"

the TV will turn on. But, if I say

"Alexa, turn up the volume on the TV"

Alexa will respond

"TV does not support that"

I started by using the code here in this github.com project and added additional code to handle transmitting the IR signals to the TV. This project appears to transmit this XML in response to an Alex asking for what the ESP8266 is capable of doing:

HTTP.on("/eventservice.xml", HTTP_GET, [](){
    Serial.println(" ########## Responding to eventservice.xml ... ########\n");
      
    String eventservice_xml = "<scpd xmlns=\"urn:Belkin:service-1-0\">"
        "<actionList>"
          "<action>"
            "<name>SetBinaryState</name>"
            "<argumentList>"
              "<argument>"
                "<retval/>"
                "<name>BinaryState</name>"
                "<relatedStateVariable>BinaryState</relatedStateVariable>"
                "<direction>in</direction>"
                "</argument>"
            "</argumentList>"
          "</action>"
          "<action>"
            "<name>GetBinaryState</name>"
            "<argumentList>"
              "<argument>"
                "<retval/>"
                "<name>BinaryState</name>"
                "<relatedStateVariable>BinaryState</relatedStateVariable>"
                "<direction>out</direction>"
                "</argument>"
            "</argumentList>"
          "</action>"
      "</actionList>"
        "<serviceStateTable>"
          "<stateVariable sendEvents=\"yes\">"
            "<name>BinaryState</name>"
            "<dataType>Boolean</dataType>"
            "<defaultValue>0</defaultValue>"
           "</stateVariable>"
           "<stateVariable sendEvents=\"yes\">"
              "<name>level</name>"
              "<dataType>string</dataType>"
              "<defaultValue>0</defaultValue>"
           "</stateVariable>"
        "</serviceStateTable>"
        "</scpd>\r\n"
        "\r\n";
            
    HTTP.send(200, "text/plain", eventservice_xml.c_str());
});

I assume, in order to support (offer up to Alexa) relative volume control, all that needs to be done is add a description of the volume control feature to the above XML. However, I have not been able to find out how to do that.

Answer 1

I don't believe you can do this with a local only device. The WeMo support is a legacy approach that has very limited capabilities.

You need to build an Alexa Smart Home Skill which requires a cloud component (With a lambda to trigger it).

The skill reports the capabilities of the device. (https://developer.amazon.com/docs/smarthome/smart-home-skill-api-message-reference.html#discovery-messages)

Answer 2

This question generated considerable interest (7 as of this writing). So I am posting a followup answer with a local only solution.

I have accepted @hardillb answer as I have yet to find a method allowing Alexa to control relative Volume using a local only device.

However, there is a way to control relative TV sound levels using a local only device. By using a device name like "TV sound" and phrases like "Alexa, turn up the TV sound", Alexa can be coaxed into thinking it is turning up and down the brightness of a device called "TV sound". In accepting this approach we are forced to use Alexa's absolute brightness levels while trying to control a relative sound level TV. The first thing we notice is that we can only turn down TV sound a few times before we exhaust Alexa's brightness range (Alexa jumps about 25% for each dimming command). But we can also tell Alexa the brightness our device is set to at the end of each command. If we tell Alexa the brightness is always 50% then Alexa will always respond with more than 50% when we tell Alexa to "turn up TV sound" and less than 50% when we tell Alexa to "turn down TV sound".