Batteries

I am thinking about making a project (haven't made it yet, just an idea). It will consist of an ATtiny and a servo. The ATtiny can run from 2.7V-5.5V, the servo from 4.8V-6.0V. I want to power my project off batteries. Of course, i could use a 7805 with a 9V battery, but I want to keep the part count as low as possible. 3xAA = 4.5V, to low for servo, 4xAA=6V, to high for ATtiny. And the rechargable li-ion batteries are only 3.4V! any suggestions? Is there a way to get a steady 5V with as few parts as possible?

Thanks! baum

4 * NiMh AAs = 4.8V, everyone is happy.

But 4.8 is the minimum for the servo! So once the battery voltage begins to drop off slightly, servo stops working!

Can't say I've ever really noticed that happening.

Is there a way to charge those batteries without taking them out of the project, like you can do with li ions?

baum: I am thinking about making a project (haven't made it yet, just an idea). It will consist of an ATtiny and a servo. The ATtiny can run from 2.7V-5.5V, the servo from 4.8V-6.0V. I want to power my project off batteries. Of course, i could use a 7805 with a 9V battery, but I want to keep the part count as low as possible. 3xAA = 4.5V, to low for servo, 4xAA=6V, to high for ATtiny. And the rechargable li-ion batteries are only 3.4V! any suggestions? Is there a way to get a steady 5V with as few parts as possible?

Thanks! baum

Keep in mind, motors typically have an ideal voltage and temperature. Running at the minimum voltage will likely result in slightly higher current demands on your battery. That means reduced battery life. If you're concerned about battery longevity, you should probably explore targeting the sweet spot in voltage. At a minimum, see if you can find a graph depicting amp load at voltage for your servo, and determine if its even worth consideration.

No datasheet. http://www.sparkfun.com/products/9065

Maybe, to be safe, I will use a 3.4V li ion with a step up connected to an lm317 set to about 5.2 volts.

nevermind!

this should work:

http://www.sparkfun.com/products/8249

•Typical current: 200mA or less

For servo(s) ?

how much do you think the above servo would draw?

I noticed in the link for the servo, a comment said the servos did not have ball bearings. A ball bearing servo typically has less internal resistance and accordingly, should require less load on the servo motor. Should translate into less overall load on the battery. Again, its up to you how much you really want to chase the low current load.

What if I connect 2 of the AA power supplies to get 400mA? will the servo draw more than that?

How about this one Baum? http://www.pololu.com/catalog/product/791 I use this to step up 5V to 12V for driving LED strings. (Having a seperate 5V & 12V wallwarts seemed silly, altho cheaper! Having a 12V supply switching down to 5V was costlier due to needing ?600mA of 5V current)

"The Pololu adjustable boost regulator is a very flexible switching regulator (also called a switched-mode power supply, SMPS, or DC-to-DC converter) that can generate voltages higher than its input voltage. We offer two adjustable ranges: approximately 2.5 V to 9.5 V and 4 V to 25 V. The output voltage can be set using the trimmer potentiometer in the upper-right corner of the board. The input voltage range is 1.5 V to 16 V (the input voltage should be kept below the output voltage). The integrated 2 A switch allows for output currents high enough to drive small motors,"

A ball bearing servo typically has less internal resistance

I assume you’re now talking mechanical resistance?

Having a seperate 5V & 12V wallwarts seemed silly, altho cheaper! Having a 12V supply switching down to 5V was costlier due to needing ?600mA of 5V current)

What?

What is the advantage of that one?

AWOL:

A ball bearing servo typically has less internal resistance

I assume you're now talking mechanical resistance?

Yes. Sorry for the confusion.

Youcan get higher voltage for your servo's with more current capability:

2 A switch (and input) limit

So what should I do, exactly?

Here's what I'd do: Get yourself 4 C or D-cells to run the servo with (or AAs if you want it smaller but less long lived), and either pick off the 3rd battery for 4.5V, or use a stepdown converter to run the uC with. http://www.bgmicro.com/batteryholdersandsnaps.aspx http://www.pololu.com/catalog/product/2101

Get yourself 4 C or D-cells

I would have done that, but the batteries have a nominal voltage of 1.5V-- meaning that ON AVERAGE the 4 cells will be at 6V, but off the shelf, it may add up to around 6.5V, blowing the servo. (its rated for 6V max)

baum