Is it possible to use Wi-Fi as a wireless connection to my Arduino/Windows PC LAN without using the internet
I have a hydroponic setup working. Actually I'm working in a second version. My solution is this:



I'm using a nodeMCU, because it have WiFi and lots of memory. In the breadboard I also have a real time clock (RTC), a LDR to measure real light level, and a temp/humidity sensor (DH-11), plus a small OLED display. The green led represent the HID light for testing purposes. I still need to add more elements, but it's good enough to show you my setup.

The nodeMCU send information to a MQTT broker . I can watch that information using an MQTT client in my Android smartphone. With the same client I can send orders to the nodeMCU (like changing light schedule, etc.)

MQTT is a protocol designed for Internet of Thing. It small and simple. You need a broker (server) to receive and send messages. I'm using Mosquitto running on Linux, on a old laptop that is on all time.

This is the big picture (no Internet involved):



Mosquitto is free, and it's available for PC, Mac and Linux here.

You have free clients for Android, iPhone, Windows, Mac, and Chrome. I'm using MQTT Dash.

MQTT is lightweight, easy, multiplatform, multiclient, and the best part of it: minimal programming.