Low power WiFi MQTT temperature humidity sensor

[amadeuspzs (Amadeus)]

Hi all,

This custom low power WiFi temperature/humidity sensor has been over a year in the making:

I’m running quite a few at home, with some setup as generic thermostat to control heaters:

In a nutshell, it’s a sensor that will run for 9 months+ on a single 1200mAh Li-Ion battery (which you can then recharge), and submits any temperature differences as measured every 10 minutes (which you can change in the software).

Integration with HA is a sinch:

- platform: mqtt
  state_topic: "livingroom/temperature"
  name: "Living Room Temperature"
  unit_of_measurement: "°C"
- platform: mqtt
  state_topic: "livingroom/humidity"
  name: "Living Room Humidity"
  unit_of_measurement: "%"
- platform: mqtt
  state_topic: "livingroom/vbatt"
  name: "Living Room Battery"
  unit_of_measurement: "V"

• “Product” page
• Github

I’ve got the total build cost to £8 per device which isn’t super cheap, but 40% of that cost is the sensor - so if you just want temperature I would probably make one based on the DS18B20 or similar for a lot less.

I spent several months on breadboard getting the power consumption down, then several months designing and testing PCBs, then a month or two designing and testing the 3D printed case.

The latest case design (case2) only impacts temperature measurement by ~0.05C which is less than the repeatability of the sensor (0.08C):

Overall a big learning project for the esp8266, PCB design and 3D printing which has solved a need and saved my 60% of my electricity bills so far!

Let me know if anyone wants any more details - I’ve tried to write up as I went on Github. I have a few spare bare PCBs I can ship out to UK/Europe for feedback - DM me.

Amadeus

[legolass73]

Hello,

how do you recharge the Li-Ion battery?

I also have a battery type US18650VTC6, do you think it’s possible to adapt them?
And I can’t see where to connect the Si7021 from your plants and instructions.

I would be really interested to have 2 sensors like these, I am unfortunately not as agile with my hands to make this sensor.

[amadeuspzs (Amadeus)]

I use a MCP73831/2 charger which works well (see pictures).

Yes it would work with your battery as it’s a single cell Li-Ion. I designed the board to fit the battery I use so ideally a redesigned PCB and case could fit nicely around your battery.

The Si2071 is surface mount and if I’m honest quite tricky. I extended the pads on the PCB to enable you to squeeze some solder in underneath. I have both hand soldered and “baked” PCBs and neither method is foolproof.

In hindsight I think this sensor is over spec’d (and over priced) for what I need. After a few years I don’t really use humidity at all so could probably get away with a DS18B20 at a fraction of the price (and easier to solder).

I still have some batteries from a batch order I made, but the 18650 format is ubiquitous and probably a better choice. You would get double the lifetime before recharge as well. I imagine you’d have to buy a holder for it which might add some cost.