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Driving LCD backlight and contrast ping by Arduino
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VEGETA:
I've seen this: http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/

They claim that Arduino pro mini at 3.3v will consume 23uA only! This is due to no power LED and disconnecting the on-board regulator as well as sleep mode, although I don't know how to let it wake up when I need to update measurement and display. I guess now I can power the board with soldering 3.3v wire directly at the 5v pin? or RAW pin? I guess it is Vcc (5v pin).

So assuming 0.1mA maximum for the board, it is perfect. Now the LCD is a different story but overall I have just doubled or even tripled the running hours by doing what is in that link. I would test stuff to see if it is possible without problems.
BrianHG:

--- Quote from: VEGETA on April 25, 2018, 07:46:44 am ---I've seen this: http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/

They claim that Arduino pro mini at 3.3v will consume 23uA only! This is due to no power LED and disconnecting the on-board regulator as well as sleep mode, although I don't know how to let it wake up when I need to update measurement and display. I guess now I can power the board with soldering 3.3v wire directly at the 5v pin? or RAW pin? I guess it is Vcc (5v pin).

So assuming 0.1mA maximum for the board, it is perfect. Now the LCD is a different story but overall I have just doubled or even tripled the running hours by doing what is in that link. I would test stuff to see if it is possible without problems.

--- End quote ---
Now you are heading in the right direction.
I don't know about Aduino, but, with a PIC, when putting it to sleep, I can set a wakeup timer/timeout on interupt, or use the watchdog timer to force a wakeup.

Basically removing that 5v regulator means that if you use the switching regulator, you send 3.3v direct to the Arduino removing that LDO's wasted quiescent current.

The LCD should also draw around 1-2ma, especially if you are lucky enough that it will also take the 3.3v supply.  It's just how often and how bright you power that backlight.

I don't know about the rest of your connected hardware, but if you are driving thermocouples, they should only be powered on once every sample, like once a second for 1000th of a second.  Same with battery monitoring.

If you get your average power consumption down below 1ma, your 9v battery will last in the neighborhood of 3500hours(145 days).

Note that without any LDO, using a really small DC-DC switcher powering direct from a 2x 3v 1000mAh lithium coin cell you will only get around 2500 hours.

Get your average current down to 0.2ma average, that 9v battery will last 17000 hours, almost 2 years.  IE, if you are sampling, just sample once a second, keep the Arduino in sleep for 99.9% of the time and only turn on both the LCD and the backlight when needed.
VEGETA:
Parts needed for the project:

- Arduino pro mini.
- 9v battery.
- DHT11 sensor.
- Altoids project tin.
- 16x2 LCD.

As for requirements, DHT11 outputs once a second so if I make Arduino sleep for 3 seconds then wake up and take 1 measurement + update display... that would be perfect. temperature and humidity won't change in a matter of 1 second anyway.

LCD must be always on, how much current will it take then? what if back light is on too? I would put one tactile push button to activate 2n2222 to connect 3.3v to back light. this button is connected to arduino so how will it do the job with all this power down thing?

BrianHG:
Which 16x2 LCD module?
(This is the lowest power one at digikey)
Example: https://www.digikey.com/product-detail/en/newhaven-display-intl/NHD-C0216CU-FSW-GBW-3V3/NHD-C0216CU-FSW-GBW-3V3-ND/2165870
Data sheet: http://www.newhavendisplay.com/specs/NHD-C0216CU-FSW-GBW-3V3.pdf

This is a 3.3v to 5v compatible unit.
The character module takes around 0.3ma at 3.3v.
The LED takes 20ma for full brightness.

DHT11 datasheet:  https://akizukidenshi.com/download/ds/aosong/DHT11.pdf  (Requires 3.5v)

So, leaving the character module on, powering and measuring with your sensor once every 3 seconds and updating the display, while keeping the backlight mostly off, you can attempt a life of 700-1200 hours.  You will need to run everything at 3.5v to be compatible with the DHT11.

Now, it will take a little quality programming and clock settings to get that 700-1400 hours.  Tweak everything out really well and you might almost double that.  It now comes down to the quality of your 9v to 3.5v regulator.

Using 4 AAA batteries would give you around 1500-3000 hours.  (Around the same size as an 9v if all 4 are grouped together in a square)


Also remember that Alkaline weakens in cold temperatures.  Energizer has a 9v lithium battery which will last longer and be more immune to sub-freezing temperatures.
VEGETA:

--- Quote ---Which 16x2 LCD module?
--- End quote ---

typical one from aliexpress, bought them years ago.


--- Quote ---DHT11
--- End quote ---

I have a breakout board version of it.

Anyway, here it says 3-5 volts: https://www.adafruit.com/product/386  so 3.3v is enough.


--- Quote ---Using 4 AAA batteries would give you around 1500-3000 hours.
--- End quote ---

1.2v * 4 = 4.8v so you mean connecting dc-dc here to get 3.3v? will work fine. How is it that these give better life?

Plus, how to wire them? will be messy


--- Quote ---Now, it will take a little quality programming and clock settings to get that 700-1400 hours.  Tweak everything out really well and you might almost double that.

--- End quote ---

How? you mean the power down line I wrong above?




--- Quote ---  It now comes down to the quality of your 9v to 3.5v regulator.
--- End quote ---

Here it is: https://www.banggood.com/Small-Mini-360-Adjustable-DC-Power-Supply-Module-Mini-Step-Down-Module-p-917568.html?rmmds=myorder
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