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I decided to create another post in order to get feedback for my ideas. I have created a network of wireless sensor nodes which connect to a central arduino with serial connection to a RPi.

I initially followed maniacbug's awesome post: https://maniacbug.wordpress.com/2011/10/19/sensor-node/. What do I want my sensors do at a first phase?

  • Connect to the base with an NRF24L01+ each
  • Temperature, humidity (DHT11 or 22)
  • Soil moisture
  • Soil temperature
  • Light
  • Battery voltage
  • Atmospheric/barometric Pressure

Right now I have connected the NRF24 and the voltage divider with the internal reference enabled (at 1.1V). I have also bootloaded the Mega328P with a 8Mhz bootloader for lower consumption. The circuit runs directly from two batteries (AA - 2.8V/2700mAh max) and the node sends data every 1 minute to the base.

When I tried to connect the DHT11 previously connected on an Uno board, I realised that it needs 3-5.5Volts which I don't possess and 3-3.3V is also necessary for other types of sensors. The ATMega328 works fine @8Mhz considering the manual from 2.4V and up. From a quick calculation and some tests, the battery voltage decreased 0.02V in a week. I haven't yet measured the current but I am willing to do so.

My choices are:

  • A voltage booster to 3.3V like this one https://www.pololu.com/product/2561. I don't know what the consumption will be but I suppose much higher than running the battery directly to the board. It's also extra hardware, cost and circuit size.
  • A bigger battery pack with 3(theoretically 4.5V) or 4 batteries (6V) AA(2700mAh) or AAA(800-1000mAh): bigger size, more weight (not a problem), the batteries run directly to the board, but the circuit will need to shut down the sensor readings when the voltage falls below 3V.
  • A 9V battery (200-280mAh) which is small and light in weight, but I will definitely need a regulator which will consume also power.
  • Use a LiIon 3.7V battery. Higher cost, lighter weight and the need of a charger?
  • Find other sensors which work in the 2.4-3V range. Extensive research is necessary and orders that will usually take more than a month to arrive here. Yes, I am in the middle of creating something, I don't want to wait for ages ! :)

Please tell me your opinion. I don't possess deep electronics knowledge (developer) so don't just propose "create a booster, it's easy" !

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    \$\begingroup\$ A boost converter will be 80~90% efficient. But if you can go with 3x AA then that would be much easier. And the batteries will be dead before they ever reach < 3V. \$\endgroup\$ – Passerby Aug 21 '15 at 7:47
  • \$\begingroup\$ Am I missing the question? What is your goal, are you trying to achieve a certain battery life time? Or is it just: what is the best way to battery power my system which needs 3.3V? \$\endgroup\$ – Arsenal Aug 21 '15 at 11:24
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The most efficient method (in term of power) is connecting primary batteries (not rechargeable) directly to the devices without any converter (buck/boost). You can use 3x1.5v primary battery (AA/AAA/whatever).

Li-ion, lead-acid, or other rechargeable batteries will self-discharge itself over time. Primary batteries, which aren't designed for recharging between manufacturing and use, have much lower self-discharge rates. You can see their discharge rates in this table:

enter image description here

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  • \$\begingroup\$ This is not possible as the NRF24 needs 1.9-3.3V and DHT11 operates in 3.0-5.5V (which works better for 3.3-5.5V). So I will definitely need to work at 3.3V and the voltage must be stable. \$\endgroup\$ – Radolino Aug 21 '15 at 9:53
  • \$\begingroup\$ So you may use 3x1.5 battery with 3.3v efficient regulator for the devices that need 3.3 volt only, electronics.stackexchange.com/questions/38278/… \$\endgroup\$ – Oka Aug 21 '15 at 10:02

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