dht11 CR2032 longevity

[Nca78]

@fhenryco said:

MMM OK, hesitating just because i'm not sure removing this regulator is going to be as easy and safe for board as i would like ...

I never had problem to remove it, try it it's easy !

just to be sure , if i keep the regulator what you say about connecting power directly to Vcc is in case the power is provided by a single CR2032, right ?
Yes, in that case voltage will be a bit over 3V with full battery and go down to 2V (but at 2.4V the battery will be neard dead already.

About your third other points (BOD modification) : this is necessary in case the voltager that the atmega eventually receives (either through RAW without regulator , or through Vcc) is lower than a regulated 3.3 V, do i understand well ?
Yes, this is in case you are using cell only.

about your last point (capacitor and sleep between consecutive transmissions): is this again related to the above harware choices for powering the board ? i mean is it made necessary again by the fact that the atmega is running on a less than 3.3V CR2032 ? just to be sure i understand

No that's necessary because you are using a CR2032. These type of battery has a high internal resistance, that will go higher and higher when the voltage decreases. It means if you draw a lot of current, you will lose power because of this internal resistance. Then it means voltage drops and as it drops internal resistance increases.... etc etc So you need extra capacitor to help the battery by providing a bit of extra current during transmit.
Using 2 cells will not improve that much, you might even not have better battery life life that as the regulator will consume a bit of power, and as your input voltage will be from 6V to 5V it makes a sharp drop in voltage meaning the efficiency of the regulator will be far from it's best.

thanks again

[fhenryco]

@Nca78

Last question : what about 3 AA LR6 in series then :

• also has the problem of a large internal resistance ?
• sharp drop of voltage again because voltage ~ 4.5 V still too much greater than 3.3 ==> bad regulator efficiency , right ?

[mfalkvidd]

@fhenryco 2xAA without regulator is what is recommended in https://www.mysensors.org/build/battery

[fhenryco]

@mfalkvidd

Oh! sorry, i had completely missed this battery section on this site "build" page. All right
There is just something that is not yet completely clear for me for this recommended usage of 2 AA batteries in series.
The boost 3.3 V regulator is presented as a good way to take "all the juice from batteries" for them to last longer, but at the same time, as a regulator it is expected to continuously draw current even during sleeping time ... and in this way reduce lifetime of batteries
so i remain a little bit confused as for the actual usefullness of such step up regulator : is it only usefull in a project where sensors are transmitting very often so that the sleeping current is not the dominant factor reducing battery lifetime ?

[mfalkvidd]

@fhenryco there is no agreement in the community. Some have had success without step-up, some with step-up.

[Nca78]

A lot of people don't change the BOD settings on the prominis and so when using only 2 AA or AAA the sensor will stop running when the batteries are around 1.35V which is a lot of waste. In that situation yes you will get better battery life with the booster.

But if you use a sensor that can go below 2V and set your promini to have 1.8V BOD and bootloader to run at 1MHz (so the pro mini can run below 2V) the batteries can go down a bit below 1V and at that level there's not much "juice" left in them. With that configuration you'll get a much better battery life than with a booster, and you will avoid the main disavantage of the booster: the interferences with the radio that sometimes make the sensors unreliable.

[Nca78]

And no, no problems with AA/AAA regarding internal resistance. You can draw a lot of power from them without problem, so you don't need a capacitor for that. Just keep the capacitor for the radio.

[fhenryco]

Now that i have received pro minis 3.3 i can practice !

following the method in http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/
i just tried to cut the trace to the LED with a cutter, ... but the LED remained shining after several deeper and deeper attempts .... so i just removed the LED, but now my mini pro is not responding anymore: dead!

so i'm wondering if i have destroyed something else trying to cut this trace , and considering how easy it is rather to just remove the led, i'm wondering why the guy advices to cut the trace which is probably a more dangerous method for the pro mini

[fhenryco]

eventually, i was more skilled with a second mini pro : after removing both LED and board regulator, the consumption i measured is about 19mA when active and 80 microA when sleeping , i guess mainly because of the step up regulator (from 2xAA to 3.3V)

[Nca78]

@fhenryco I agree with you, I find it more simple to just remove the led. Cutting trace can lead to mistake (like you did on your first board) and there are a bunch of different promini clones, some might have traces using different paths than on the example.