brainwagon

Last night, the weather was pretty rainy. I woke up a couple of times during the night (once, when my little cat friend Bailey decided to check my breathing and started head butting me around 4 AM) and heard a lot of rain falling on the roof. When I woke up, it was fairly wet outside. I made myself a cup of tea and settled into check messages and email…

And the power went out.

Sigh. I could have used my phone and the cell service, but instead, I just spent a few minutes thinking.

Like about my battery backed up temperature/pressure sensor I had running in the garage, which is battery backed up. And since it sends its data to the MQTT server at io.adafruit.com, it should wo…

Oh, except of course my network is down.

It got me thinking that perhaps logging data to a local SD card isn’t actually the worst idea. Maybe once a day or so, it could wake up and send all the backlogged data to the server, but continue to log data locally.

And while I could use a microcontroller like the ESP8266, those things take a lot of current. Maybe it was time to look into something else that could be efficiently powered down and then only power up the remote peripherals (like a radio) when needed. And instead of using any kind of fancy charging circuit, maybe I could make something that is just powered from either Alkaline batteries with a run time of maybe a year.

As it happens, last night I found a couple of Arduino Minis and a USB/serial connector, so it seemed to me that using one of those would be pretty reasonable. But I was pondering a couple of issues: just what is the standby current of an Arduino Mini when in deep sleep mode? How much current does the ubiquitous power LED consume?

So, during a break I dug around and found information about power consumption by the Arduino Mini.

The breakdown seems to be that if I use a 3.3V 8Mhz Arduino Mini which is unmodified, I could expect about a 4.74mA current running flat out, and about 0.9mA while sleeping. Most of the sleep current is actually the Power LED, which draws about 0.84mA. It isn’t had to get rid of that (just cut a trace and/or destroy the LED with some snips) and then you are down to just 0.054mA during sleep. Very nice.

You can further reduce the sleep current by getting rid of the voltage regular, which may or may not be feasible depending on the use case. I was curious about what voltage I could use as input if I got rid of the regulator, and it appears that the ATmega328P is good from about 2.7V to 5.5V, which is a very broad range. You can simply feed that into the VCC. Sleep current is down to around 4.5?A in power down mode. Really, really good.

But it might be worthwhile use a better regulator. The Microchip MCP1700 has a quiescent current down in the 1.6uA range, which might be worth looking into. There is a lot of information about the efficiency of these regulators on the page linked above. I’ll have to look into it some more.

I should note that some of my thinking about this today was probably nudged by the following video @JeremySCook

New video showing how to make a very compact @arduino nano setup: https://t.co/OVsCIkhWDJ w/ DIP switches as pullup inputs, CR2032 batteries, and of course, shrink wrap! pic.twitter.com/hVy0auctDP

— Jeremy Cook ? (@JeremySCook) March 19, 2019

I thought there were a couple of cute ideas in here. He used the Arduino Nano instead of the Micro but the form factors are pretty similar. I liked his way of soldering down DIP switches to provide some inputs, and using some shrink wrap to make a nice tight little package. He also pointed out these cute little battery holders with built in power switches that I hadn’t seen before. But I was a bit concerned about the use of the Nano. The USB interface chip draws about 15mA, and the Nano is nominally a 5V chip, so I thought that supplying just 6v to such a system would result in very short life, as the CR2032 batteries are pretty low capacity already, and 6v is not really enough headroom above what the voltage regular needs to provide 6v. Still, definitely some good ideas.

I also think it’s probably a good idea to read some of the “low power” tagged items on JeeLabs, as well as some information like this on the Arduino forums.

Anyway, more to think about.