MQTT NodeMCU servo can react only on decimal array payload - mqtt

Question

I'm creating a fish feeder with SG90 servo and NodeMCU
I used this sketch:
#include <ESP8266WiFi.h>
#include <PubSubClient.h>
#include <Servo.h>
// Update these with values suitable for your network.
const char* ssid = "your_wifi_hotspot";
const char* password = "your_wifi_password";
const char* mqtt_server = "broker.mqttdashboard.com";
//const char* mqtt_server = "iot.eclipse.org";
Servo myservo; // create servo object to control a servo
WiFiClient espClient;
PubSubClient client(espClient);
void setup_wifi() {
delay(100);
// We start by connecting to a WiFi network
Serial.print("Connecting to ");
Serial.println(ssid);
WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED)
{
delay(500);
Serial.print(".");
}
randomSeed(micros());
Serial.println("");
Serial.println("WiFi connected");
Serial.println("IP address: ");
Serial.println(WiFi.localIP());
}
void callback(char* topic, byte* payload, unsigned int length)
{
Serial.print("Command from MQTT broker is : [");
Serial.print(topic);
for(int i=0;i<length;i++)
{
if((int)payload[i]>194||(int)payload[i]<0)
break;
myservo.write((int)payload[i]); // tell servo to go to position in variable '(int)payload[i]'
}
}//end callback
void reconnect() {
// Loop until we're reconnected
while (!client.connected())
{
Serial.print("Attempting MQTT connection...");
// Create a random client ID
String clientId = "ESP8266Client-";
clientId += String(random(0xffff), HEX);
// Attempt to connect
//if you MQTT broker has clientID,username and password
//please change following line to if (client.connect(clientId,userName,passWord))
if (client.connect(clientId.c_str()))
{
Serial.println("connected");
//once connected to MQTT broker, subscribe command if any
client.subscribe("OsoyooCommand");
} else {
Serial.print("failed, rc=");
Serial.print(client.state());
Serial.println(" try again in 5 seconds");
// Wait 6 seconds before retrying
delay(6000);
}
}
} //end reconnect()
void setup() {
Serial.begin(115200);
setup_wifi();
client.setServer(mqtt_server, 1883);
client.setCallback(callback);
myservo.attach(D1); // attaches the servo on pin D1 to the servo object
}
void loop() {
if (!client.connected()) {
reconnect();
}
client.loop();
}
The servo is working when I use MQTTBox in order to send a payload as "Decimal Array", it is however is giving me a hard time when I send payload as JSON string.
If I send "Decimal Array" 1 it does turn Servo to position 1, however if I simply send 1 as a payload as a string it moves Servo to position 49.
If I send payload as 2 it moves to position 50.
If I send payload as 10 then position is 4948
looks like position of 1 and position of 0 at the same time.
My ultimate goal is to send those payloads via HomeAssistant which are sent as string or JSON, however I don't find a correct solution at the moment.
I would highly appreciate any help or solution.

Answer 1

MQTT payloads are UTF-8 encoded so the Arduino PubSubClient library treats the payload as an array of uint8_t.
If you want to send and receive JSON then you can use the ArduinoJson library to parse a JSON payload. So assuming a JSON payload like:
{
"position": 123
}
Then you can implement a callback such as:
#include <ArduinoJson.h>
// Assuming a fixed sized JSON buffer
StaticJsonBuffer<200> jsonBuffer;
void callback(char* topic, byte* payload, unsigned int length)
{
JsonObject& root = jsonBuffer.parseObject(payload);
if (root.success() && root.is<JsonObject>())
{
int position = root.as<JsonObject>().get<int>("position");
myservo.write(position);
}
}

Answer 2

Looks like the motor is taking the position of the ASCII equivalent of the number when you send it as a string.
i.e.
ASCII equivalent of the character '1' in 49 in decimal
ASCII equivalent of the character '2' in 50 in decimal
Try sending the character 'a', the motor will go to 97.
If you want to send a string, you will have to change the following code:
for(int i=0;i<length;i++)
{
if((int)payload[i]>194||(int)payload[i]<0)
break;
myservo.write((int)payload[i]); // tell servo to go to position in variable '(int)payload[i]'
}
to:
int location=String((char*)payload).toInt()
if((location>194)||(location<0))
return;
myservo.write(location);