phloks:
Just starting out with this IoT stuff, and it's fun.
When it works.
Any hints on where to find documentation on how to use the uno as a slave to the esp ?Didn't know that was possible, but for now I don't see how I can achieve my plant watering system without the help of an arduino to control my relais....
Have a look at the link I posted, it explains everything you need to know to program the ESP like an Arduino. Then it's just a matter of getting it to communicate with the Arduino: this can be simply done over serial, for example. Take a look at Robin2's Serial Input Basics.
You can then just send something like "<R1|1>" to turn on relay 1, for example. All the UNO has to do, is read the commands from the ESP, and turn on/off the appropriate pins.
But you could probably do it without the UNO: an ESP-01 has 4 IO pins (you can use TX and RX as well).
If you do need more pins (up to 11), you can get a Wemos D1 mini or similar for around $3. It's an ESP8266 that breaks out all available pins.
It's dirt cheap, and much easier than having to communicate between two microcontrollers.