I need to trigger the relay on/off (via wireless) that is connected to the power of the pump via an arduino nano.

Very cool! I will dig into things and see if I can figure it out.

I am using Home Assistant as a controller, with a serial gateway. My garden irrigation project required I place my garden water pump directly off of my rain barrel, a full 100' away from the garden and water manifold that is controlled by my arduino nano and 12v solenoids. Is it possible to connect the pump to a nano, relay & NRF24L01 and trigger it (via wireless) to power up when a solenoid is activated on the mainifold and power off again when the solenoid turns off?

Yes, that worked... I downloaded master.zip and replaced the current Liquid Crystal folder with the new one. Restarted IDE and was able to compile and upload the sketch. Thank you!

My apologies... I should have mentioned that I am using your sketch from the Irrigation controller project. Nothing has been changed (just copy and paste) (https://www.mysensors.org/build/irrigation). Renamed it for my garden sprinkler project. I am not near my Desktop PC till Monday, so I am not sure where I found the library. Is there a specific library version I should use? Where will I find it? From reviewing other questions on this subject, it seemed you felt everything needed would be found in the mysensors library. Maybe I misunderstood.

I have researched this error on the forum but I am not clear how to resolve. From other answers Positive is supposed to be declared. I have the mysensors library installed (2.3.1) and using Arduino IDE 1.8.5

"/home/raymond/Downloads/arduino-1.8.5/hardware/tools/avr/bin/avr-g++" -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10805 -DARDUINO_AVR_PRO -DARDUINO_ARCH_AVR   "-I/home/raymond/Downloads/arduino-1.8.5/hardware/arduino/avr/cores/arduino" "-I/home/raymond/Downloads/arduino-1.8.5/hardware/arduino/avr/variants/eightanaloginputs" "-I/home/raymond/Downloads/arduino-1.8.5/hardware/arduino/avr/libraries/Wire/src" "-I/home/raymond/Arduino/libraries/Time-master" "-I/home/raymond/Downloads/arduino-1.8.5/hardware/arduino/avr/libraries/SPI/src" "-I/home/raymond/Arduino/libraries/MySensors" "-I/home/raymond/Arduino/libraries/LiquidCrystal/src" "-I/home/raymond/Arduino/libraries/LiquidCrystal_I2C-1.1.2" "/tmp/arduino_build_167561/sketch/MySensors_Sprinkler.ino.cpp" -o "/tmp/arduino_build_167561/sketch/MySensors_Sprinkler.ino.cpp.o"
MySensors_Sprinkler:174: error: 'POSITIVE' was not declared in this scope
 LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address to 0x27
                                                     ^
Using library Wire at version 1.0 in folder: /home/raymond/Downloads/arduino-1.8.5/hardware/arduino/avr/libraries/Wire 
Using library Time-master at version 1.5 in folder: /home/raymond/Arduino/libraries/Time-master 
Using library SPI at version 1.0 in folder: /home/raymond/Downloads/arduino-1.8.5/hardware/arduino/avr/libraries/SPI 
Using library MySensors at version 2.3.1 in folder: /home/raymond/Arduino/libraries/MySensors 
Using library LiquidCrystal at version 1.0.7 in folder: /home/raymond/Arduino/libraries/LiquidCrystal 
Using library LiquidCrystal_I2C-1.1.2 at version 1.1.2 in folder: /home/raymond/Arduino/libraries/LiquidCrystal_I2C-1.1.2 
exit status 1
'POSITIVE' was not declared in this scope```

Thank you for your reply. I did resolve the problem and wanted to share what I did, in hopes of helping other newbies.
Firstly, I double checked the wiring, compared it to the working gateway and it seemed to be fine but still did not work.

I ran across this post and it turned out to be ideal for my situation:
https://forum.arduino.cc/index.php?topic=421081.0

Got the two radios communicating in a simplistic configuration and once working, uploaded mysensors sketches and the radios continued to work correctly. Maybe dismantling and reconnecting was the fix. Not sure, but things are working now. Thank you for a great resource of information and projects!

I am attempting to set up my first sensor (motion) on Homeassistant but could not see that the gateway was picking up on it, so I enabled debug on Arduino IDE.

(Both radios are NRF24L01)

Serial monitor displays the following:

10040 TSM:FAIL:RE-INIT
10042 TSM:INIT
10048 !TSM:INIT:TSP FAIL
10050 TSM:FAIL:CNT=2
10052 TSM:FAIL:DIS
10054 TSF:TDI:TSL

Serial Monitor on the gateway shows the following:
0;255;3;0;9;0 MCO:BGN:INIT GW,CP=RNNGA---,REL=255,VER=2.3.1
0;255;3;0;9;5 TSM:INIT
0;255;3;0;9;7 TSF:WUR:MS=0
0;255;3;0;9;14 TSM:INIT:TSP OK
0;255;3;0;9;17 TSM:INIT:GW MODE
0;255;3;0;9;20 TSM:READY:ID=0,PAR=0,DIS=0
0;255;3;0;9;24 MCO:REG:NOT NEEDED
0;255;3;0;14;Gateway startup complete.
0;255;0;0;18;2.3.1
0;255;3;0;9;28 MCO:BGN:STP
0;255;3;0;9;34 MCO:BGN:INIT OK,TSP=1
0;255;3;0;9;37 TSM:READY:NWD REQ

Gateway sketch:

/**
* The MySensors Arduino library handles the wireless radio link and protocol
* between your home built sensors/actuators and HA controller of choice.
* The sensors forms a self healing radio network with optional repeaters. Each
* repeater and gateway builds a routing tables in EEPROM which keeps track of the
* network topology allowing messages to be routed to nodes.
*
* Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
* Copyright (C) 2013-2018 Sensnology AB
* Full contributor list: https://github.com/mysensors/MySensors/graphs/contributors
*
* Documentation: http://www.mysensors.org
* Support Forum: http://forum.mysensors.org
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* version 2 as published by the Free Software Foundation.
*
*******************************
*
* DESCRIPTION
* The ArduinoGateway prints data received from sensors on the serial link.
* The gateway accepts input on serial which will be sent out on radio network.
*
* The GW code is designed for Arduino Nano 328p / 16MHz
*
* Wire connections (OPTIONAL):
* - Inclusion button should be connected between digital pin 3 and GND
* - RX/TX/ERR leds need to be connected between +5V (anode) and digital pin 6/5/4 with resistor 270-330R in a series
*
* LEDs (OPTIONAL):
* - To use the feature, uncomment any of the MY_DEFAULT_xx_LED_PINs
* - RX (green) - blink fast on radio message received. In inclusion mode will blink fast only on presentation received
* - TX (yellow) - blink fast on radio message transmitted. In inclusion mode will blink slowly
* - ERR (red) - fast blink on error during transmission error or receive crc error
*
*/

// Enable debug prints to serial monitor
#define MY_DEBUG


// Enable and select radio type attached
#define MY_RADIO_RF24
//#define MY_RADIO_NRF5_ESB
//#define MY_RADIO_RFM69
//#define MY_RADIO_RFM95

// Set LOW transmit power level as default, if you have an amplified NRF-module and
// power your radio separately with a good regulator you can turn up PA level.
#define MY_RF24_PA_LEVEL RF24_PA_LOW

// Enable serial gateway
#define MY_GATEWAY_SERIAL

// Define a lower baud rate for Arduinos running on 8 MHz (Arduino Pro Mini 3.3V & SenseBender)
#if F_CPU == 8000000L
#define MY_BAUD_RATE 38400
#endif

// Enable inclusion mode
#define MY_INCLUSION_MODE_FEATURE
// Enable Inclusion mode button on gateway
//#define MY_INCLUSION_BUTTON_FEATURE

// Inverses behavior of inclusion button (if using external pullup)
//#define MY_INCLUSION_BUTTON_EXTERNAL_PULLUP

// Set inclusion mode duration (in seconds)
#define MY_INCLUSION_MODE_DURATION 60
// Digital pin used for inclusion mode button
//#define MY_INCLUSION_MODE_BUTTON_PIN  3

// Set blinking period
#define MY_DEFAULT_LED_BLINK_PERIOD 300

// Inverses the behavior of leds
//#define MY_WITH_LEDS_BLINKING_INVERSE

// Flash leds on rx/tx/err
// Uncomment to override default HW configurations
//#define MY_DEFAULT_ERR_LED_PIN 4  // Error led pin
//#define MY_DEFAULT_RX_LED_PIN  6  // Receive led pin
//#define MY_DEFAULT_TX_LED_PIN  5  // the PCB, on board LED

#include <MySensors.h>

void setup()
{
	// Setup locally attached sensors
}

void presentation()
{
	// Present locally attached sensors
}

void loop()
{
	// Send locally attached sensor data here
}

Motion Sensor sketch:

/*
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2018 Sensnology AB
 * Full contributor list: https://github.com/mysensors/MySensors/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0 - Henrik Ekblad
 *
 * DESCRIPTION
 * Motion Sensor example using HC-SR501
 * http://www.mysensors.org/build/motion
 *
 */

// Enable debug prints
#define MY_DEBUG

// Enable and select radio type attached
#define MY_RADIO_RF24
//#define MY_RADIO_NRF5_ESB
//#define MY_RADIO_RFM69
//#define MY_RADIO_RFM95

#include <MySensors.h>

uint32_t SLEEP_TIME = 120000; // Sleep time between reports (in milliseconds)
#define DIGITAL_INPUT_SENSOR 3   // The digital input you attached your motion sensor.  (Only 2 and 3 generates interrupt!)
#define CHILD_ID 1   // Id of the sensor child

// Initialize motion message
MyMessage msg(CHILD_ID, V_TRIPPED);

void setup()
{
  pinMode(DIGITAL_INPUT_SENSOR, INPUT);      // sets the motion sensor digital pin as input
}

void presentation()
{
  // Send the sketch version information to the gateway and Controller
  sendSketchInfo("Motion Sensor", "1.0");

  // Register all sensors to gw (they will be created as child devices)
  present(CHILD_ID, S_MOTION);
}

void loop()
{
  // Read digital motion value
  bool tripped = digitalRead(DIGITAL_INPUT_SENSOR) == HIGH;

  Serial.println(tripped);
  send(msg.set(tripped?"1":"0"));  // Send tripped value to gw

  // Sleep until interrupt comes in on motion sensor. Send update every two minute.
  sleep(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR), CHANGE, SLEEP_TIME);
}