A battery fed MQTT weatherstation

(Note: if you consider building this… I am working on a similar station with a bare ESP12 and a BME280 for even better energy efficiency)

Weatherstations are a popular build for DIY-ers and with the ESP8266 WiFi capabilities that has become very easy.
Here I will present a simple weatherstation (DHT sensor BMP sensor for temperature, humidity and atmospheric pressure) that can be battery fed or mains fed.
It will send the info by MQTT and in order to save batteries, go to sleep most of the time.
Components needed are:

  • A wemos mini D1
  • A DHT11 or DHT22
  • A BMP280
  • A 100k resistor for the battery
  • A small switch or jumper
  • A LithiumIon charger module

Just a word for the wise… the Wemos D1 is not the best option to battery feed, as it has some peripheral components (such as the CH340 chip) that draw current. A better choice would be the bare ESP8266, but it is not for everybody to solder that.

The application has OTA. Using OTA together with deep sleep is always a bit of a tricky situation as you cannot do OTA when the ESP8266 is in deep sleep.
basically there are 3 solutions for that:

  1. Include a pause in which you can do OTA. Problem with that is that you would need some indication of when that pause starts (e.g. once the MQTT messages have come in). Another problem is that that pause in which the processor is not in deep sleep, it wil consume more energy.
  2. Make the deepsleep dependent on the state of a pin. With a jumper or Switch you could make one pin high or low and thus allow deepsleep or not.
  3. By having the software check on a seperate website whether or not there is an update for the software and then download that software. Obviously that requires some more organization and if the Wemos will do that check whenever it awakes, it will do that say every 15 minutes for something that may never happen.

Considering the above, Option 2 seemed the best, so I added a small switch to tell the software whether or not to go into deepsleep.

Software

The program once loaded (get it here) has several tabs in the IDE.
The main program “esp8266_weatherstation” is quite straightforward, it sets up all the usual parameters, variables and the sensors.
It reads the sensors and sends these via MQTT to a broker.
Once that is done, it goes into deepsleep, depending on two parameters:

  • the “FORCE_DEEPSLEEP” variable: if present there will be a 15 min deepsleep. If not set, the deepsleep will be determined by the battery voltage.
  • the state of the ‘SLEEPPIN (D6):  if LOW there will be deepsleep, if HIGH, there will be no deepsleep

If there is no deepsleep, a pause of 30 secs is added to avoid the MQTT messages being send in a frenzy.

You may be puzzled by the following code section:

float h = dht.readHumidity();
float t = dht.readTemperature();
delay(2000);
float h = dht.readHumidity();
float t = dht.readTemperature();

The reason for this is the response time of the DHT22. If you  start a reading, the results you get are from a reading that was started 2 secs before. Usually that is not an issue, but if you put the processor to sleep for 15 minutes, that first reading will give you 15 min old values. If that is not a problem, by all means remove the delay and second reading. It will not make a difference on your battery life as your  DHT22 is not put to sleep.
technically ofcourse it is possible to feed the DHT22 from one of the I/O pins and switch it off when not needed, but the DHT22 also needs a warm-up time of about 2 secs, so you would need to take that into account. No doubt that warm up time could be catered for with the delay(2000) command mentioned above.

The “OTA” section sets up the OTA function. As explained before, OTA and DEEPSLEEP is a bit of an uneasy combination. In order to use OTA, tou would need to put the switch on D6 in the right position (HIGH) and either reset  the WEMOS or wait till the next wake up. Mind you that when DEEPSLEEPing the OTA port  can disappear, but it should come back once the processor is out of DEEPSLEEP.

the “config.h” file defines some variables

the “webconnect” file connects to WiFi. If for some reason that connection cannot be made after 10 times trying, the processor is going back to sleep for 30 more seconds and then tries again.

Libraries

The libaries used are quite standard, with the exception of the PubSubClient library. This is not the well known PubSubclient library from Nick O’Leary, but a fork made by Ian Tester. The reason for using that one is that it is easier to send variables in the MQTT payload.
The DHT library is the DHT sensor Library from Adafruit. If you prefer another DHT library (there are many) you will probably only need to make minor changes.
The BMP280 library is also from Adafruit.

The sensors

DHT22

The DHT22 is the more accurate sibling of the DHT11, but both have a tendency of failure. generally the DHT22 needs a pull up resistor, but one can also use the internal pullup of the I/O pin for that.

There are two specification where the DHT11 is better than the DHT22. That’s the sampling rate which for the DHT11 is 1Hz or one reading every second, while the DHT22 sampling rate is 0,5Hz or one reading every two seconds and also the DHT11 has smaller body size.

If you are using this sensor outside, you can protect it against dirt and  creepy crawlies by covering it with a single layer of teflon tape. Supposedly this is fully permeable for humidity.

The DHT sensors do not have eternal life. they give up working alltogether (and usually permanently) when the humidity is 100%

Properties DHT11 DHT22
Voltage 3-5 Volt 3-5 Volt
Operating current 2.5mA 2.5mA
Temperature range 0 t/m 50 ºC -40 t/m 80 ºC
Temperature accuracy ±2 ºC ±0,5 ºC
Humidity range 20-80% RH 20-95% RH
Humidity accuracy ±5% RH ±2-5% RH
Sampling rate 1Hz 0.5Hz
BMP280

The BMP280 is the successor of the BMP085, BMP 180 and BMP183

Ofcourse it is possible to use one of the above predecessors, but these older sensors are generally more expensive than the BMP280.
The BMP280 can measure barometric pressure with ±1 hPa absolute accuraccy, and temperature with ±1.0°C accuracy. It can be controlled both by SPI and I2C.

Do not mix it up with the BME280. That sensor can also read humidity, but it is slightly more expensive.
The BME280 ofcourse would be a perfect chip to go to if you want to avoid the DHT22 sensor. It can read temperature, humidity and barometric pressure. I just didnt have it lying around, thus couldnt try.

If you are no fan of the DHT11/22 consider the I2C HTU21D, or theSi7021 (replaced by the HTU21D).
If you are only interested in say temperature and not in humidity, the BPM280 is the best choice and you can omit the DHT22. If you only want temperature, the DS18B20 is probably the best choice.

The battery

Lipo Charge module

It is hard to give recommendations for what battery to use. I am using the battery shield to which I connect a LiPo battery. That makes it all very simple but it is not the most efficient way of using the energy from a battery as the battery shield first boosts it up to 5 Volt after which the Wemos makes 3V3 Volt from it.
Another easy way is to use a Lipo battery with a lowdrop 3V3 regulator and a cheap charging module (pictured top). You could combine the charging module with a small solar cell (with a 5-6 Volt max output). The 100k resistor in the  circuit sketch should go directly to the V+batt, to monitor the battery voltage. The choice for the 100k resistor is explained here.  If for power efficiency you decide to use a bare ESP8266 instead, one has the opportunity to increase the resistors of the voltage divider to come to a smaller current through these resistors

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6 thoughts on “A battery fed MQTT weatherstation”

  1. Since I am a sucker for everything minimalistic, I am considering replicating this with a BME (ditch the DHT). I’d love to use an ESP-01. But not having GPIO16 for the wakeup AND missing GPIO4 and 5 for I2C makes this a bit too much of an effort. So I guess it will be an ESP-12 or gutting a Wemos D1.

    For power I am contemplating either
    1) the Red Neck approach: Use an old 16850 in a holder, calibrate the internal Vcc monitor and use delay instead of deep-sleep while Vcc > 3.9 (calibrated) volts.
    2) simply use 2 good AA cells.
    Either should give it enough oomph to survive winter I think (did you make a mAh per day calculation?). Swapping out twice per year or so I can handle. It could send me an alarm message once Vcc drops under 2.5 volt (still enough for both the ESP and the BME).

    1. Thanks Jeroen, I am with you on the minimalistic approach. I feel uneasy to do ‘overkill’ with a Wemos that can do so much more than just 2 sensors and in fact have used an ESP8266-01 with quite a few sensors and even an OLED in the past. Even though it is possible to solder a wire onto the chip to get a reset from GPIO16, that is just too delicate a task for my eyes. However, as I have this working, I will put it in a bare ESP12.

      Didnt make a mAh/day yet, but will, in my considerations for final set up (BME BMP+DHT, Wemos vs ESP12/ESP01).

      Though I have electric available in the garden, I’d ideally like to run this from a battery/solarcell. The batteries are a bit of a concern as LiPoś and Nicad are usually not performing very well during the winter….. regular AA disposables may do better.

      I am seriously considering dropping the DHT’s, have had too many failing on me, but it is what most people have.. The BME280 is indeed a much better choice, also in terms of power consumption. The BME uses 3.6uA vs the DHT 2.5mA that is a big difference, also, I understand that in between readings the BME goes to slep, only using 0.1uA. That the Wemos itself AND the battery shield are not the best choices for battery life is very clear: boosting the battery up to 5V simply to then feed it to a 3V3 LDO is foolish. Also the CH340 chip is not going to deepsleep while its current consumption I think is some 50mA,
      So see it (1) as a proof of concept as I had little experience till now with deepsleep. Currently I leave it running just to see how it is doing: after 21 hours (or better 84 deepsleep cycles) battery is still at 4.20 volt, so that is promising. And (2) as an easy to build for people who like it ‘plug and play’ and hate soldering.

      On an afterthought.. I wouldnt be surprised if the current consumprion on an ESP01 with delays could rival the current consumption of a Wemos/Wemosbattery shield on deepsleep, mainly because of the CH340 chip and the inefficiency of the battery shield.
      The ESP01 ofcourse has no way to read the battery coltage so that would be a guessing game… but it also would save the battery from draining through the voltage divider

      rather than gutting a Wemos as you mentioned, I would go for a bare ESP12. I think I sent you a ESP12 baseplate long time ago and putting an ESP12 on it is I think easier than gutting a Wemos.

  2. What about extending with a small solar cell? The battery then only has to survive (long) nights, since a small solar panel will feed also under cloudy conditions. That will make the setup actually a ‘hands-off’ project and you don’t have to bother much about deep sleeps, polling intervals etc. Unless you regard the minimal energy consumption a challenge on its own, of course… Anyways, nice project!

    1. Thank you, That is definitely a possibility and I have done that in the past.
      Eventhough with a solarcell you need to make sure that your batteries will have enough load to get through the night and depending on the cell and the battery, going to sleep may help.
      Having said that, I am not really a fan of putting things to sleep and in this project I just wanted to see how much I could squeeze out of batteries….. but I would definitely advise to take an added solarcell into consideration, eventhough you then most certainly do need an LDO as well.

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