Measuring battery voltage with ESP8266

If you run IoT devices on batteries then it would be nice to know how much capacity is left on the battery, so you can change it before the device dies. Easiest way to do this is measuring battery voltage. Esp8266 has adc, but it's only from zero to one volt. The correct way would probably be to use Non-Inverting Op-Amp Level Shifter, but that would require many components and negative voltage. Easier approach is to just use voltage divider and just get the maximum voltage to acceptable range.

Using a calculator we get resistor values of 120kOhm and 39kOhm for getting 4.1 volts to 1 volt. For a low voltage of 3 volts these resistors would give 0.736 volts. So we are using 0.264 of esp8266 adc range of 0 - 1 volts. Adc is 10 bits so we have 0.264*1024=270 different values, even with some noise that should be enough for our purpose.

I made a small lua script that reads adc every minute and then posts the result with mqtt. Then used mosquitto_sub from Mosquitto to subscribe to that topic and log the messages.
mosquitto_sub -h roope.local -t /esp/battery | while read line; do echo -n $(date -Ins); echo -e "\t$line"; done | tee esp_battery.log Where roope.local is the mqtt broker machine.

Measurements and calculations can be found here https://docs.google.com/spreadsheets/d/1t18V1jrTRbj8nI5lrpctb8Fs9KzmpQ9O05dR5AeF37k/edit?usp=sharing. Esp_battery tab has raw data and formula has cleaned up data and formula.

I used google sheets linest function to calculate a function to get voltage from adc values, voltage=0.003539154715*adc+0.3756228338.

adc	voltage
1000	3.914777549
900	3.560862077
800	3.206946606
700	2.853031134
600	2.499115663

One problem for voltage divider is that it uses some power all the time. I should probably check how much that is, and if it considerably affects how long the battery will last. But for my next battery powered project, it will most likely report battery status also.

Code can be found here https://github.com/mika-koivusaari/check_battery.

You have mail 2

After having done the hardware for checking mail I had to make some kind of user interface to show that data. I had previously used an old IPhone to display temperature and power usage, so it was an easy choice to continue using it.

Previously I had just used a simple php script to fetch and display data, now I wanted to try python and Flask. I used a tutorial found here to get started.

Code is pretty simple, temperature and electricity usage is fetched every time from database, but this page is usually only open on the extra phone that displays this data so there shouldn't be any reason to cache that data. That data is also updated to database every minute so it should be refreshed every time. Mail status is received over mqtt and it's sent only when it changes so it has to be cached.

I also made a couple of changes to the hardware/firmware section, for the firmware I set the retain flag in the mqtt message so that when clients connect they get the last know value of mail. For the hardware I added a bigger antenna because I have some trouble with the wifi reception. Reception is still not as good as I would like, I have to think about it. Maybe second wifi ap, I'm also thinking if I should build a separate network for IoT-devices, currently there is only one wifi device, but that could easily change. I also changed the psu to one that outputs more current and has a lower quiescent current as I thought that the wifi problem might be because the ESP8266 is not getting enough current to transmit, but that didn't solve the problem. Before I had a cheap chinese buck/boost converter, and now I have https://www.pololu.com/product/2830 which can ouput more current, but is only a buck converter and needs at least 4 volts to output 3.3 volts. So I had to replace the three aaa batteries with four aa batteries to get enough voltage.

Also there is a problem with the mailbox itself:) It let's sunlight through somewhere and that's enough so we can't see if the led is on or not. I'll have to think how to put some seals there.

Future improvements

Hardware

• Monitor battery state.
• Don't turn radio on on wake up, only when we need to send. NodeMCU bug might prevent this currently.

Software

• Move temperature and electricity to mqtt also.
• Add today's forecast.

Software can be found here https://github.com/mika-koivusaari/data_display_phone