# Control a relay - Wio Terminal In this part of the lesson, you will add a relay to your Wio Terminal alongside the soil moisture sensor and control it based on the soil moisture level. ## Hardware The Wio Terminal requires a relay. The relay you'll use is a [Grove relay](https://www.seeedstudio.com/Grove-Relay.html), a normally-open relay (meaning the output circuit is open or disconnected when no signal is sent to the relay) that can handle output circuits up to 250V and 10A. This is a digital actuator, so it connects to the digital pins on the Wio Terminal. The combined analog/digital port is already in use with the soil moisture sensor, so this relay plugs into the other port, which is a combined I2C and digital port. ### Connect the relay The Grove relay can be connected to the Wio Terminal's digital port. #### Task Connect the relay.  1. Insert one end of a Grove cable into the socket on the relay. It will only fit one way. 1. With the Wio Terminal disconnected from your computer or other power supply, connect the other end of the Grove cable to the left-hand Grove socket on the Wio Terminal as you look at the screen. Leave the soil moisture sensor connected to the right-hand socket.  1. Insert the soil moisture sensor into the soil if it isn't already from the previous lesson. ## Program the relay The Wio Terminal can now be programmed to use the attached relay. ### Task Program the device. 1. Open the `soil-moisture-sensor` project from the last lesson in VS Code if it's not already open. You will be adding to this project. 2. There isn't a library for this actuator—it's a digital actuator controlled by a high or low signal. To turn it on, you send a high signal to the pin (3.3V), and to turn it off, you send a low signal (0V). You can do this using the built-in Arduino [`digitalWrite`](https://www.arduino.cc/reference/en/language/functions/digital-io/digitalwrite/) function. Start by adding the following to the bottom of the `setup` function to set up the combined I2C/digital port as an output pin to send a voltage to the relay: ```cpp pinMode(PIN_WIRE_SCL, OUTPUT); ``` `PIN_WIRE_SCL` is the port number for the combined I2C/digital port. 1. To test if the relay is working, add the following to the `loop` function, below the final `delay`: ```cpp digitalWrite(PIN_WIRE_SCL, HIGH); delay(500); digitalWrite(PIN_WIRE_SCL, LOW); ``` The code writes a high signal to the pin that the relay is connected to, turning it on, waits 500ms (half a second), then writes a low signal to turn the relay off. 1. Build and upload the code to the Wio Terminal. 1. Once uploaded, the relay will turn on and off every 10 seconds, with a half-second delay between turning on and off. You will hear the relay click on and then click off. An LED on the Grove board will light up when the relay is on and turn off when the relay is off.  ## Control the relay based on soil moisture Now that the relay is working, it can be controlled in response to soil moisture readings. ### Task Control the relay. 1. Delete the 3 lines of code that you added to test the relay. Replace them with the following code: ```cpp if (soil_moisture > 450) { Serial.println("Soil Moisture is too low, turning relay on."); digitalWrite(PIN_WIRE_SCL, HIGH); } else { Serial.println("Soil Moisture is ok, turning relay off."); digitalWrite(PIN_WIRE_SCL, LOW); } ``` This code checks the soil moisture level from the soil moisture sensor. If it is above 450, it turns on the relay, and turns it off when it goes below 450. > 💁 Remember, the capacitive soil moisture sensor reads lower values for higher soil moisture levels and higher values for drier soil. 1. Build and upload the code to the Wio Terminal. 1. Monitor the device via the serial monitor. You will see the relay turn on or off depending on the soil moisture level. Test it in dry soil, then add water. ```output Soil Moisture: 638 Soil Moisture is too low, turning relay on. Soil Moisture: 452 Soil Moisture is too low, turning relay on. Soil Moisture: 347 Soil Moisture is ok, turning relay off. ``` > 💁 You can find this code in the [code-relay/wio-terminal](../../../../../2-farm/lessons/3-automated-plant-watering/code-relay/wio-terminal) folder. 😀 Congratulations! Your soil moisture sensor successfully controls a relay! --- **Disclaimer**: This document has been translated using the AI translation service [Co-op Translator](https://github.com/Azure/co-op-translator). While we aim for accuracy, please note that automated translations may include errors or inaccuracies. The original document in its native language should be regarded as the authoritative source. For critical information, professional human translation is advised. We are not responsible for any misunderstandings or misinterpretations resulting from the use of this translation.