Running pro mini on CR2032 - strange behavoir

Hi,

I've got a project with an Arduino 3V pro mini which runs ok on the FTDI breakout powered by the PC's USB.

But, once I disconnect and run it off a CR2032 battery, the RX (or TX ?) LED flashes as if it's loading something or trying to run, and the project doesn't do what it's meant to?
(I have other traffic light LEDS attached which dont light up when they're meant to)

Can anyone shed any light of what is happening when the RX LED flashes fast?

The obvious question is - Do have a multimeter to check the battery voltage while it's trying to power the Arduino?

What else is connected to the Pro Mini?

There's a BMP280 barometric sensor and a 3 LED traffic light cluster.

And how are you connecting the CR2032 to the Pro Mini? Via the RAW pin or the VCC pin?

CR2032 can only provide a tiny current, a couple of milliamps, before starting to droop in voltage.
The normal brown-out voltage setting on a Pro Mini or similar Arduino is 2.7V, you'll need to
change that to 1.8V by altering the fuse settings in boards.txt if you want reliable running from
3V or less (the one you have is setup for 3.3V).

But I suspect you'll need a battery capable of more current anyway, like two LR44's in series or
a single LiFePO4 cell, try that first.

Its connected to the VCC pin.

My battery holder accomodates two CR2032's.

Is it a good idea to use two CR2032's then?

skyboyflyboy:
Its connected to the VCC pin.
Is it a good idea to use two CR2032's then?

No.

How much voltage would that put on the VCC of the ATmega on the Prop Mini ?

(assuming the battery holder puts the CR2032s are in series)

It would be a good idea to use a much bigger battery

what's your motivation for using a coincell? If you're going for something super-compact, a Li-Po will give you far greater capacity and current-output.

Like mentioned above, a coincell's chemistry can't keep up with more than a few mA's at a time, and will experience non-negligible voltage droop pretty much right away. They may have a capacity of ~230 mAh, but that doesn't mean they can put out 230 mA for an hour continuously. The harder you drive them, the less of their capacity is actually usable.

A Li-Po, however, will maintain its rated voltage for most of its capacity, and can be recharged.

Thanks guys,
The reason I wanted a cr2032 was simply to save space.

But it sounds like a lipo is the way to go.

However, I have seen devices similar to the one I'm developing using a single cr2032. Maybe they're optimized in some way?

Maybe they're optimized in some way?

They use less current.

skyboyflyboy:
Thanks guys,
The reason I wanted a cr2032 was simply to save space.

But it sounds like a lipo is the way to go.

However, I have seen devices similar to the one I'm developing using a single cr2032. Maybe they're optimized in some way?

Who knows ?

Have you actually measured, with a multimeter what is happening to the VCC on the Pro Mini when it being powered from a CR2032 ?

This was suggeted in the first response to your original post, but you chose not to reply.

As forum members we can speculate what might happen, but we cannot see what is actually happening with your circuit.

That LED on your Arduino can take 3-5 mA by itself. Start by removing that.

The LEDs of your traffic light system also require power - how much depends on your actual circuit, which you didn't show. Make sure your current limiting resistors are large enough, keeping the current down. 1-2 mA is probably enough to make them shine, albeit dimly. That way you should be able to last a week on that coin cell.

For even more power savings: get an ATtiny.

Another thing: make sure you have a 8MHz, 3.3V version. Otherwise you won't be able to run at 3V in the first place. At 8 MHz you can go down to about 2.7V.

Things that run off coincells typically have extensive energy-saving implementations such as utilizing sleep-code with interrupts and possibly dramatically-reduced clock speed.

For example, if you run an Attiny85 always-on at 16 MHz, it pulls in the neighborhood of 5-6 mA. But if you reduce the clock to run off the internal WDT at 128 kHz, that same chip now runs at around 0.4 mA. If you enable deep-sleep with either external-event or wdt interrupts, then the chip will pull a small number of microamps while asleep, and still only pull the 400 uA during the fractions of a second its awake. Be sure to note that there are different types of Attiny85 that are designed for different clock-speeds at different voltages. Per the datasheet (pg 206), the Attiny85-10 versions are rated for 1.8 - 5.5V with a nominal clock speed of 10 MHz (which can changed by re-burning fuses); while the Attiny85-20 versions are rated for 2.7 - 5.5V with a 20 MHz nominal clock speed. So if you're using a coincell, you can expect voltage droop. And with the coincell only starting at 3.0V, you don't have much overhead even for using an Attiny85-20x, even with a slower clock speed, because it will brown-out much sooner.

A pro-mini, with all its ancillary hardware (regulator, on-board LED, etc) will work against you for power-consumption as stated above.

Also, if you're able to choose which LEDs are used in your traffic lights, I would strongly suggest looking into the Heli2 line of LEDs by Kingbright. These units are awesome!!! Most LEDs this size and brightness typically pull between 10 - 20 mA. These LEDs have the same brightness, but only pull 2 mA!!! I was blown away when I came across them.