I already have some switches controlled using MQTT, but I am searching for a cheap wireless sensor. I will need around 6.
I have had a look at xiaomi but I don’t know if this is the cheapest/easiest way to go. The gateway can be found for around 25€ and sensors below 10€.
Any other brands to check?
How handy are your soldering skills?
A Wemos D1 Mini with a DHT22 Sensor attached, is pretty cheap (< $10) - though you also need a 5v power supply. Plus some Homie code. These have the advantage of being able to do more than one thing (I have one that’s a reed switch plus a temp sensor with a dht shield - but I prefer having the wired sensor - as the dht shields get a bit of heat).
Might be some pointers here: https://www.geekzone.co.nz/davidcole/8960
Have you looked at https://www.mysensors.org/build/temp ? I have a couple that have been up and running for a while now, one of them battery powered and been running probably 6 months so far on AA batteries.
A tidy solution is using https://www.mysensors.org/hardware/newbie-pcb and https://www.mysensors.org/build/battery
My soldering skills are low (being optimist…). I would feel more comfortable with a solution using http://www.rflink.nl and any sensor compatible with that.