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" !
Best Answer
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: