TTN and Cayenne: how to do calculations or how to return data

Community Help With My Project
1

I just got the Dragino LDDS75 LoRaWAN Distance Detection Sensor and connected it successully to TTN, configured the payload decoder and can receive the sensors readings in Cayenne.

What I want to achieve with the distance sensor is to monitor the amount of fluid level in a tank. What I get is a distance reading to the surface of the fluid. So I need to do some calculations:

($TANK_HEIGHT - $SENSOR_READING) x CONSTANT

to get the amount of fluid in the tank.

Where is the typical place to do such a thing? Should I modify the Payload Decoder or is there any other place to do this kind of stuff?

I thought, i could do some calculations in the payload decoder and return some aditional data from the payload decoder like this

function Decoder(bytes, port) {
  // Decode an uplink message from a buffer
  // (array) of bytes to an object of fields.
  var value=(bytes[0]<<8 | bytes[1]) & 0x3FFF;
  var batV=value/1000;//Battery,units:V
   
  value=bytes[2]<<8 | bytes[3];
  var distance=(value);//distance,units:mm
  //var distance=((1000-value)*7)+" mm";//distance,units:mm
  if(value===0)
    distance = "No Sensor";
  else if(value<280)
    distance = "Invalid Reading";
    
  return {
       Bat:batV +" V",
       Temp:20 +" c", 
       prox:distance +" cm",
       Distance:distance +" mm", 
       tl: 50 +" null", 
       liquid: 700 +" l"
  };
}

but the additional (bogus) data did not appear in Cayenne. Any ideas what I could do instead? Any hints you can give me?

Thank you very much!

2

you cannot do something like this as codec for Dragino LDDS75 LoRaWAN Distance Detection Sensor is meant to accept only certain payload to show the particular widgets.

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3

Thank your for your reply!

Does cayenne receive the “raw” data direct from the device without any need to install a packet decoder?

Anyway: being able to do (simple) calculations like this would be on my wish list.

Your help is appreciated very much!

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4
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5

Be very nice to be able to do something with the data - the ldds75 is a great very affordable sensor from what I can see but without being able to “process” the data in cayenne to show a tank level instead of a distance its use is limited somewhat. There needs to be the ability to “post process” in TTN or Cayenne to get the data to the required format

6

@tim2 we have cayenneLPP format for the same. You can do all the needed processing on the device itself and then send it to cayenne.