Hi, thank you very much for your reply.
I already put one ardunio powering leds... my weak is the electronics. I am a former C develper with good grades on 0's and 1's. eheh
This project is to build step by step to achive the final... i dont know how long it will take, but it must be finished by the summer... 9 months from now 
The ideia of using the a nano as master, is to learn how to talk to wifi module on HW serial, and RS485 on SW serial ports.
The current project, to my GF present at school, includes moisture and temperature sensors, an LCD 16x2, and 6 LEDS. As i dont use the TX and RX pins, i used a 74LD139 to decode the leds...
The fases i intend to follow:
- put two ardunios talk with RS485 and then set the BUS for the three. I want this, to use the same cabe (CAT5 shielded) to transport power for the slave ardunios. This will avoid, i hope, one DC adapter on each arduino.
- activate the water valve with relays or not. it will depend on the valve. i think will need to use some power driver with transistors to activate the relays.
- study the EEPROM to store and load the configurations or some other persistence model that someone advise.
- set the LCD and keyboard. Monitor information, write setup, manual open the valves, etc..
- set the WIFI on the master to connect with blynk.
I need ideias on wich will be a good and durable moisture sensor, if powered with 12V better.
I found this ones, https://www.dfrobot.com/wiki/images/thumb/b/b4/SEN0193.jpg/300px-SEN0193.jpg
