One of the first things that I found out when I started reading up on the ESP8266 was that it used AT commands, something I have never used before. I also (luckily enough) found out before I tried to hook it up that it works at 3.3V with a maximum current of 250ma.
The UNO have a 5V and a 3.3V pin. The problem is that the 3.3V pin on the UNO only allows 50ma, so the UNO can not handle the ESP8266 at full blast. The ESP8266 is also said to have a maximum voltage of 3.6V so using the 5V pin on the Arduino may just end up frying the ESP8266 chip.
When I bought these things there were NOTHING that gave me warnings about this. No manuals, no connection schemes, nothing from the retailer.
To make things even more interresting I have found out that the PIR sensor need 5V input, but have a 3.3V output… way to make things complicated to hook up.
This acutally made me kind of dispair and drop the project for quite some time before I decided to take it up again. Problems are meant to be conquerd and overcome… and besides, whats the fun if everything goes your way right from the start?