Powering Arduino/ATMega328 & 3.3V components from AA batteries

I have hacked the remote of a remote-controlled power outlet, and hooked it up to a 7-segment multiplexed LED display that I scavenged from a digital clock. It's a working remote-controlled countdown timer outlet.

I've been prototyping with Arduino Uno, but I eventually want to flash an ATMega328 chip with the sketch and use it in place of the Arduino for the finalized project. So I won't have the power management stuff from the Arduino Uno unless I put it there myself.

The remote control runs on a 3.3V coin battery. I don't understand its internals so I just hacked it by replacing the remote's buttons with NPN transistors. The Arduino supplies a HIGH signal (through a 220 ohm resistor) to the transistor's base pin to "press" a button, shorting the 2 button terminals. So the 3.3V system is sort-of isolated from the 5V Arduino but they share a ground.

The LED display is driven by the Arduino and runs somewhat dimly at 5V. (By "driven by" I mean: the display has 2 common cathodes, so I have the Arduino sink current through a single digital pin to the cathode I want, and source current to one of the 11 anodes. It illuminates only one segment at a time. There's a 220 Ohm resistor between each cathode and each "sink" pin.)

Everything is working nicely now when powered by USB and the coin battery.

What I want to do is run both the Arduino, the Arduino-driven multiplexed LED display, and the 3.3V remote control off of batteries, probably AA batteries. I want to get rid of the 3.3V coin battery.

  1. Could I run the ATMega328 at 3.3V, and use smaller current-limiting resistors on the LED display to compensate for the lower voltage? Is this a good idea?

  2. Can I get 3 AA batteries, run the 3.3V remote off of just two batteries (1.5V+1.5V), and the Arduino+display off of all three (4.5V)?

2b) Since NiMh AA batteries have a lower voltage, would they be much harder to use than normal AA batteries for this application? I'd prefer to use rechargable batteries.

All my previous projects have just run while plugged in, so I'm confused by batteries especially when 3.3 devices are also involved! Thanks for your help. I hope I explained this alright.

2 AA alkaline batteries (nominal 1.5V)= 3.1V when new, and go down from there.

2 NiMh have a nominal 1.2V, so 3 of them to get above 3V…but, that is 3.6V. Possibly higher when freshly charged.

Running an ATMega at 3.3V requires a peek at the data sheet. Operating at a lower voltage means a decrease in clock frequency. That will create errors with the arduino libraries if you do not properly adjust the chip parameters.

“It’s a working remote-controlled countdown timer outlet”, an AC outlet? If so, why using batteries, and why eliminate the coin cell in the remote?

How accurate does the timing have to be? Photo quality? Any preset times?

-fab

NiMH are 1.3V, its NiCd that are 1.2V.

4 NiMH are about 5.2V (upto about 5.6V freshly charged I think) which is handy for 5V circuitry.

Many lithium types are 3.7V nominal, 4.2V freshly charged.
LiFePO4 is 3.2V nominal and a good match to 3.3V circuitry (and safer!)

The standard runtime supports 8MHz and 16MHz only, using
anything else may require modifications to the runtime (and maybe recompiling
the bootloader?)

Thanks for your responses.

fabelizer:
The microcontroller is inside the portable transmitter, a remote control for an AC outlet.
Similar to this: http://www.amazon.com/Woods-32555-Outdoor-Control-Converter/dp/B001Q9EFUK/ref=sr_1_2?ie=UTF8&qid=1424028946&sr=8-2&keywords=remote+control+outlet

It doesn’t have to be really accurate, +3/-3 secs for every hour would be ok.

Would I have any problems powering an ATMega328 directly from 4 NiMH AA batteries in series?
Could I put a wire between the 3rd and 4th batteries and use that to get 3.9 V which is probably close enough to 3.3 V to run the transmitter remote circuit?

Would this simple setup work or would I fry something?

Is it safe to connect the Vcc pin of an Arduino / ATMega directly to the positive terminal of the 4th AA NiMh battery as in my ugly schematic?

Would this be a decent way to get both approx 3.3V and 5V from the same set of batteries or am I missing something?

Should I get some kind of voltage regulator instead or is it not necessary?

Thank you!

Not good to have batteries in series and draw from any point in the chain except at the end.
Your batteries won't last. The batteries voltages will be mismatched and trying to correct themselves in series will put strain on them.
You could use a 3.3V pro Mini , 2 x AAs and a booster like this one.

Thank you elac. That 3.3V pro mini would be nice for this project. I was worried that drawing from the middle of the battery chain would strain the batteries, glad you confirmed it.

For the sake of learning, what would be a good way to get both 5V and 3.3V out of 2 AAs without straining anything or being very inefficient? Could I take the positive terminal of the 2-AA-battery chain and connect it to two separate booster regulators to get both 5V and 3.3V rails out of the same 2-battery chain?

Would there be any undesirable interactions between the two boost converters?

For the sake of practicality, is there any inexpensive 2-output boost regulator component I can buy which takes variable ~2.5V input and outputs regulated 3.3V and 5V separately? I tried searching TI and other places but couldn't find anything. I turned up some references to them so they seem to exist, but no part numbers.

Thanks!