# Controlling Relay using mqtt
**URL:** https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459
**Category:** General
**Created:** [10 May 2022 14:43 UTC](https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459 "2022-05-10T14:43:50Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@rolandoartha](https://discourse.nodered.org/u/rolandoartha)
#### Post date: [10 May 2022 14:43 UTC](https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459/1 "2022-05-10T14:43:50Z")
hello guys,
i want to control my relay by mqtt broker, it's already subcribe the data from Soil Moisture Sensor, but the relay is always off
this is the logic i use
this is the callback function
```auto
void mqttCallback(char* topic, byte* message, unsigned int length) {
Serial.print("Message arrived on topic: ");
Serial.print(topic);
Serial.print(". Message: ");
String messageTemp;
for (int i = 0; i < length; i++) {
Serial.print((char)message[i]);
messageTemp += (char)message[i];
}
Serial.println();
if (String(topic) == DeviceConfig.MqttSub) { // DeviceConfig.MqttSub = /command
Serial.print("Changing output to ");
if(messageTemp == "ON"){
digitalWrite(AIO_SENSOR_PIN_3, HIGH);
Serial.println("ON");
}
else if(messageTemp == "OFF"){
digitalWrite(AIO_SENSOR_PIN_3, LOW); //AIO_SENSOR_PIN_3 expands to pin32 esp
Serial.println("OFF");
}
}
}
```
and the value already sended
but the relay state is not affected at all
can you guys help me please?
---
### Author:  [@dynamicdave](https://discourse.nodered.org/u/dynamicdave)
#### Post date: [10 May 2022 16:33 UTC](https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459/2 "2022-05-10T16:33:38Z")
At first glance, the piece of code you have posted looks OK.
I assume `digitalWrite(AIO_SENSOR_PIN_3, HIGH);` maps to an actual pin?
If I was doing it, I would have used the \ library in your sketch as you could send json from your flow which would give you much more flexibility in decoding a number of different commands.
So, for example, you could send commands like...
`msg.payload = {"relay":"main","action":"on"};`
or
`msg.payload = {"relay":"secondary","action":"off"};`
The reason I say this is... you are bound to add extra relays or LEDs to your system. Plan ahead.
PS: It's not normal to start a topic-name with a` /`
Maybe you can change it to just... `command` (in your flow and in your sketch).
**PPS: If you write a simple test-sketch (like the 'blink' sketch), can you turn the relay On and Off ?**
---
### Author:  [@craigcurtin](https://discourse.nodered.org/u/craigcurtin)
#### Post date: [10 May 2022 22:27 UTC](https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459/3 "2022-05-10T22:27:33Z")
What are you running the sketch on ? i.e. arduino uno, mini, ESP8266 etc etc Do you have a Fritzing schematic so we can see how that side looks ?
Craig
---
### Author:  [@dynamicdave](https://discourse.nodered.org/u/dynamicdave)
#### Post date: [11 May 2022 16:37 UTC](https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459/4 "2022-05-11T16:37:18Z")
One of my IoT students was asking about nearly the same question that you posted on the forum.
He was using a Wemos D1 Mini - not sure what you are using. But I'm sure it could be adapted.
I've posted what we did on the forum as I think it might go some way to solving your problem.
> [@Controlling LEDs on a Wemos D1 Mini (Arduino code) via MQTT](https://discourse.nodered.org/t/controlling-leds-on-a-wemos-d1-mini-arduino-code-via-mqtt/62523):
>
> One of my IoT students was asking how to control a set of LEDs on a Wemos D1 Mini, that had been programmed using Arduino code, via MQTT commands from Node-RED. This is what the breadboard lash-up looks like... The red LED is connected to D8 (GPIO-15). The green LED is connected to D7 (GPIO-13) The yellow LED is connected to D6 (GPIO-12) This is the very simple test-flow we created in Node-RED. [Screen Shot 05-11-22 at 06.35 PM] [{"id":"bb2bbcc236726f09","type":"tab","label…
---
### Author:  [@system](https://discourse.nodered.org/u/system)
#### Post date: [10 July 2022 16:37 UTC](https://discourse.nodered.org/t/controlling-relay-using-mqtt/62459/5 "2022-07-10T16:37:57Z")
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