5v vs 3.3v arduinos

If I try to drop in replace a 5v arduino with a 3.3v one, what would you expect to need to change, and what would the side effects be?

Obviously we need to change the power supply. It looks like we will be running at half the clock speed? Would you expect attached LEDs to get notably dimmer? Anything else?

Thanks!

The regular Atmega328 16 Mhz like in an UNO is on the edge of the specification if run at 3.3V.. That said, I've done it LOTS of times and have not found a sample that fails yet. The latest version of our YourDuino RoboRED has a 5.0V to 3.3V switch that runs the processor and feeds all the 3-pin connectors the 3.3V . I have not found a case where it fails, but "some chip from some batch some time, some temperature" could fail. See that example HERE:

What do you need to do with 3.3V on the processor or peripherals?

Oh, specifically I am looking to swap the pro mini 5v in some of my projects with the pro mini 3.3v, for ease of operation with some 3.3v devices as well as lasting longer on battery. It says 8mhz instead of 16mhz. Any idea what else will change if I simply drop it in?

Any idea what else will change if I simply drop it in?

Hardest: Some timing-dependent libraries won't work..
Easy: Hardly see the LED difference..

Easy and inexpensive to try, right??

blah44:
Oh, specifically I am looking to swap the pro mini 5v in some of my projects with the pro mini 3.3v, for ease of operation with some 3.3v devices as well as lasting longer on battery. It says 8mhz instead of 16mhz. Any idea what else will change if I simply drop it in?

Based on the information given? No idea at all.

What does your Arduino do? What's connected to it...?

Biggest problem is likely to be instability of serial - has been reported in forum a few times that serial did not 100% of the time at 3.3V and 16 MHz.

If you are running 3.3V & 8 MHz, you should be fine as long as the correct bootloader is installed - if it has the 16 MHz bootloader by accident, timed events will seem to take twice as long.
Serial will be off too - will need to set sketch to 9600, but serial monitor to 4800 to match for instance.

One of the nice side effects is lower power/current consumption - the 8MHz clock reduces
power by a factor or 2, the drop to 3.3V reduces it further by a factor of 2.3, so about
5 times lower power draw...

You can also power from a single LiPo / lithium ion battery conveniently, or a LiFePO4
cell direct to Vcc (this particular battery chemistry is 3.2V conveniently).

(For CMOS chips the power consumption is proportional to f x V^2, f being
clock frequency, V = supply voltage. This is why more and more chips are
becoming lower voltage such as 3.3V, 1.8V, 1.1V - CPU chips are already down
at 0.6V or something like that, at 10's of GHz internally - as the voltage goes
down the speed can go up).

fungus:

blah44:
Oh, specifically I am looking to swap the pro mini 5v in some of my projects with the pro mini 3.3v, for ease of operation with some 3.3v devices as well as lasting longer on battery. It says 8mhz instead of 16mhz. Any idea what else will change if I simply drop it in?

Based on the information given? No idea at all.

What does your Arduino do? What's connected to it...?

Hah sorry to be vague, I wanted to get General/hypothetical issues.

For this first project all it does is read serial and light up LEDs via shift register, so configuring do serial and the brightness of LEDs is all that really matters. Well and stability of course but that part is always true. I am wondering if changing the arduino will change LEDs enough that I should be changing out the resistors too

If the LEDs are running off the same power supply as the Arduino, you'll probably have to change the ballast resistors, yeah.

Note that if you're driving a bunch of LEDs, your power usage concern should be the LEDs, not the Arduino controlling them.

If you have a LED that pulls 10mA, with 1.8v across it (this is the case with some red ones; shorter wavelengths = higher operating voltage), and you've got it running off 5v through a resistor (330 ohm or thereabouts), you're pulling 10mA from the 5v supply - 50mW, with 18mW in the LED and 32 in the resistor. Running off 3.3v, you'd need a smaller resistor - 150 ohm or so. Still pulling 10mA but at 3.3v, so 33mW total, so you're only wasting ~15mW in the resistor.

I don't always run the electronics off the same power supply as the LEDs, particularly if the LEDs use a lot of power relative to the controls. I'll run LEDs at the minimum voltage (and with low value resistors) that I can get the LEDs working at (using a buck converter to lower the voltage reasonably efficiently)