Arduino minimal power usage

Question

I know there is a lot on this subject on the net but i cant seem to be able to join the dots together. I am trying to make sensor system than can run for as long as possible if not indefinitely using an Arduino, and also to do so very cheaply. I have a £1 Arduino Nano clone that is running with sensor and alarm fine, but currently its runs off USB power from PC.

I have followed this guide to minimize power usage code side: https://www.openhomeautomation.net/arduino-battery/

And I have read that the Vin pin needs at least 7 volts to work properly, so 2 volts would be wasted in heat making a 5V level.

I have a 3.7V lithium cell 4300mAh so I thought I would need a step up boost module.

But it looks like the Nano can run using 3V. But the 3.3V rail is apparently made for external sensors, assuming I want a mixture of cheap/simple and high efficiency. Can i just hook a 3.7 lipo into the 5V rail of the Nano and a 5V solar panel (with diode) up to same point? Will this kill my Nano? If not, is it worth stepping the 3.7V down to the minimum the Nano can run with?

Underclocking is not an issue, it essentially wakes up every hour reads 5 sensors and if all is well (99% of the time) goes back to sleep for an hour. Linear voltage regulators seem to burn the excess voltage as heat. And then what will happen when the battery dies off and is no longer 3.7 volts? And if my solar panel is around 5V in good light, if its partly shaded and only has 3.5 volts for example it would not charge my lipo to 100% capacity. Should I step up DC/DC boost the panel voltage to charge my cell.

Guidance please.

Answer 1

I have to agree with @Olin Lathrop about the fact that Arduinos are not really tuned for your goals.

I am doing microcontroller projects for like 20 years, and I found the nanowatt PIC microcontrollers offering the best price/performance/tricks ratio if you need to have something running on batteries for a seriously long time (and has not too much to calculate). There we're talking about nanoamperes in sleep, and there are a lot more tricks you can consider.

You have chosen Arduino for a valid set of good reasons. But which Arduino please? They come with different CPUs and hardware!

If you go with Uno:

• power your board directly by batteries, e.g. 3xAA batteries at the 5V pin to bypass power loss on the 5 V regulator (you can also give a try to feed power via the USB socket if you feel that more convenient)
• remove the 3.3 V regulator (assume you don't need 3.3 V)
• sleep a lot and sleep well, listen to @Nick Gammon and check his page about power saving

Then comes the minor tricks:

• do not let any pin float. Before going to sleep, put all inputs to output. I.e. if you have a button input, change port direction to output and set to 0. (assuming that you don't need the button in sleep)
• turn off not only all internal peripherals, but also external ones. For example, things like I2C temperature sensors may have remarkable 0.5 mA sleep current. The easiest thing is to power those from a port pin, and set the port to 0 before going to sleep.
• turn off all pullups (and all inputs to outputs), don't forget the I2C as well. Although the bus will be idle, there is a serious leakage current (serious means microamperes, and you wanna go to the nanoampere range, right?)

Then if you want to go really to the hard parts:

• throw away all hardware you don't need. The power checking comparators (I am referring to this schematics http://brittonkerin.com/annotateduino/arduino_uno_rev3_schematic.png), the USB interface chip altogether
• throw away u1 (the 5 V regulator) - also has some leakage current

And the final steps:

• I would check the electrolytic capacitors on the power rails, e.g. C2. These also have a leakage.
• Although Arduino does not have suppressor diodes to protect anything from any voltage spikes, I have to emphasize that single high capacity unipolar and bipolar suppressor diodes may have a leakage current of 1 uA.

Note about underclocking: this is not necessarily a wise thing to do. It is true that on slower speed the CPU consumes less. However at slower speed the CPU has to stay awake much longer to perform the same tasks.

• I found that if you need to perform some complex calculations (not just reading some sensors) it may make sense to switch to a higher CPU speed, and do the calculation in less time and go back to sleep more efficient.
• On the other hand, if your CPU is awake just to wait for some sensor readings and just burning cycles while waiting, a slower CPU speed is more beneficial.

Answer 2

I have a project along those lines. I am reading temperature, humidity, and light level and logging it, with a timestamp, to an SD card.

Schematic (click for larger image):

Photo:

It was designed with low power in mind. It runs from 3 x AA batteries which I think I have only changed once since I made it (in August 2013).

Also see my page about power saving.

I don't see any problem with running it directly from 3.7V - you would need to slow the processor down to 8 MHz (which I did anyway in that project).

The code (available here) compiles under the Arduino IDE.

Answer 3

Arduinos are for rapidly getting something going without having to know much about the underlying electronics and microcontroller. As a result, some design tradeoffs were made for you, generally optimized for development simplicity.

You now have a different primary concern, which is low power consumption. If that is truly important, then you have to design the system with that goal up front. This means starting with the bare microcontroller, designing the electronics around it carefully to minimize power, and designing the firmware architecture to minimize high-power cycles and maximize sleep time.

You are asking where to strap the big engine on a Honda Civic to make a high performance racing car. You can make a Civic higher performance, but to get really high performance you have to start with that goal from the beginning.