Sending a JSON encoded UDP message to a remote server

Projects Programming
1

Evening all !

I've constructed a box of electronics and I'm using a NodeMCU ESP8266 connected to a temperature sensor in order to control a pair of PWM fans. The idea is that the temperature in the box will be measured and the fans will speed up or slow down as required to maintain the temperature.

This all works fine.

As the box of electronics is on a remote hilltop I'd like to send a JSON message containing the current temperature and fan speed via UDP to a remote server for logging purposes.

I just cannot get this to work. Here is my code:

// Libraries for sending UDP over Wifi
#include <ESP8266WiFi.h>
#include <WiFiClient.h>
#include <WiFiUdp.h>
#include <SPI.h>

// Library for JSON
#include <ArduinoJson.h>

// Create a UDP object
WiFiUDP Udp;

// Define server that we'll send temperature and fan data to
IPAddress serverIP (A, B, C, D);
unsigned short serverPort = 50000;

// Create the JSON object
StaticJsonDocument<128> doc;

// Build the object values
doc["source"] = "ESP8266";
doc["temp"] = sensorTemp; // Value from elsewhere
doc["fan"] = fanSpeedPercent; // Value from elsewhere

Serial.println("Pretty JSON message: "); // This works
serializeJsonPretty(doc, Serial);

// Create and send UDP packet
Udp.beginPacket(serverIP, serverPort);

char payload[] = "";

// Serialize the JSON directly to the udp port and send a newline character at the end
serializeJson(doc, payload) + "\n";
Udp.write(payload);
Udp.endPacket();

Unfortunately absolutely nothing is sent to the server. The code on the server works as it receives JSON sent via UDP from some of the electronics in the box.

What am I doing wrong ? I'm sure that there's just one obvious thing that I've missed but after several hours of going around in circles I still can't see it.

2

a lot of code appears to be missing, e.g. setup() and loop() functions

post the complete code?

have a look at udp-examples.html

3

Thanks, I deliberately didn't post the setup() and loop() because they aren't really relevant. All I've posted are the bits that construct the JSON and (in theory) construct the UDP packet as those are the parts that aren't working.

I've already several hours going through lots of conflicting examples including the ones that you linked to. The only difference I can see is that I'm defining the payload with:

'char payload[] = "";

and then actually assigning it with:

'serializeJson(doc, payload) + "\n";

In the example that you post, the UDP message string is already defined and is fixed in length:

'char replyPacket[] = "Hi there! Got the message :-)";

Could that be the issue ?

4

try defining the char array size, e.g.

char payload[50] = "";
5

I do not think the following is legal.

serializeJson(doc, payload) + "\n";

I agree with horace. Give the payload array a size.
payload is a C NUL terminated string so use strcat to append the newline.

char payload[50];	// This must be big enough to handle the worst case/longest JSON.
serializeJson(doc, payload);
strcat(payload, "\n");
Udp.write(payload);
6

Thanks Horace, I'm not a C-programmer so I assumed that payload[] = "" would define an array that would grow as assigned my string to it. I was wrong.

Thanks customcontroller, now changed and I was able to send my UDP packet !