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(You can also add a series resistor to limit the buzzer current,it will decrease beep sound.)
Overhere,we use D2 as the output pin to control the buzzer.
Connect the NodeMCU to computer via USB cable,open this sketch by using Arduino IDE(Version1.6.4+):
Edit the code to fit your own WiFi and MQTT settings as following operations:
const char* ssid = "your_hotspot_ssid";
const char* password = "your_hotspot_password";
Find above code line,put your own ssid and password on there.
2)MQTT Server Address Setting
const char* mqtt_server = "broker.mqtt-dashboard.com";
You can use your own MQTT broker URL or IP address to set above mqtt_server value. You can also use some famous free MQTT server to test the project such as "broker.mqtt-dashboard.com", "iot.eclipse.org" etc.
3)MQTT Client Settings
If your MQTT broker require clientID,username and password authentication,you need to change
if (client.connect(clientId,userName,passWord)) //put your clientId/userName/passWord here
If not,just keep them as default.
After do that,choose the coresponding board type and port type as below,then upload the sketch to the NodeMCU.
About how to config the MQTT client,check this link.
Onece the upload done,if wifi hotspot ssid and password setting is ok and MQTT broker is connected, open the Serial Monitor,you will see following result: the NodeMCU serial port will show the IP address and the connection satatus.
Then you will see the NodeMCU serial output. When you publish "1", the NodeMCU will subscribe this message and print "Command from MQTT broker is : [OsoyooCommand Turn On Buzzer" on the serial monitor,the buzzer will beep; When you publish "0", the NodeMCU will subscribe this message and print "Command from MQTT broker is : [OsoyooCommand Turn Off Buzzer",the buzzer will be turned off.