Run arduino on small battery

I'm building (yet another) stopwatch as my first Arduino project. I do a lot of public speaking and have always wanted to build my own countdown timer that has different colored LEDs as "warning" indicators (10% of time left, 5% of time left, etc).

The software won't be hard - I'm a programmer by trade and have already prototyped the software on my Diecimila board. I'll need to interface it with a four digit 7-segment display, but I think that will also be fairly straight-forward. I'm planning on prototyping it with the Diecimila board and breadboard, then taking the 168 out and installing it on a pre-printed PCB to put in my Altoids tin.

The big question I have is: how do I power the thing? The tin won't allow enough room for three AA batteries (and the display, board, etc). A watch battery would be perfect, but I don't know if there is one that will supply enough power, etc. I can install a cutoff switch so that it powers down when the tin is closed to save battery. But I'd like it to be able to use it for at least 20-30 lectures each varying 15-45 minutes without replacing the battery.

I've seen several posts here that suggest LiPo batteries - but they also all have warning about blowing up my house if I do something wrong. I'm too new to take that risk :-X

I've used these for battery power...

http://www.ladyada.net/make/mintyboost/ http://www.sparkfun.com/commerce/product_info.php?products_id=8248

The MintyBoost seems to be more efficient but requires assembly. The SparkFun device gets erratic when the battery gets low. In my experience, they both work well.

I've also powered a no-bootloader 328 and 168 from two and three AA batteries. I believe the Arduino fuse settings enable the Brown Out Detector for just over 4 volts. You may not be able to use just two batteries unless you can change the fuses.

In my experience, one or two LEDs consume about the same amount of power as the processor.

Hi another good source of portable power are the Lithium Ion battery packs for power tools. The packs are based on A123 cells and deliver good a current performance that is similar to Lithium Polymer cells but without the limitations and fire risk. Have a look on Ebay , you can often pick up De-Walt or Makita packs significantly cheaper than that charged by your local hardware outlet.

HTH :)

why notbuild a joule thief: http://www.emanator.demon.co.uk/bigclive/joule.htm

The very simplest is a Bodilabs VPack (that's the sparkfun thing referenced above).

The next simplest is a 9V battery and a regulator.

The next simplest is the minty-boost (you could argue if the 9V + regulator is easier than minty, I think it is)

The next simplest is a Nicad or Lipoly battery and an external charger.

The next simplest is a nicad or Lipoly battery with an internal charger.

Once you have a USB charger you want to reduce the power consumption of the Arduino as much as possible.

→ get rid of the regulator if the USB charger already has a suitable regulator.

→ “sleep” the Arduino as much and as long as possible
http://www.arduino.cc/playground/Learning/ArduinoSleepCode

→ while not sleeping keep the frequency as low as possible. Take notice that this may interfere with all other timings (including serial communication)
http://www.arduino.cc/playground/Code/Prescaler

cheers, Udo

You probably need to revisit your design as well and calculate how much power you actually need. Unless you know this there really is no way of specifying battery, charge and expected runtime.

I'll need to interface it with a four digit 7-segment display.

The 7-segment display by itself will have 4x7 leds plus punctuation (e.g. a colon) for a total of say 30 leds. Each LED will happily consume 20mAH at full brightness. A small Arduino (such as Pro Mini) will need 10-15 mAH. The items Udo listed above will allow you to save power on the 10-15mAH for the Arduino itself, but this really becomes insignificant in comparison to the 4x7 segment display.

If you reduce brightness of the 4 segment display to 25% and factor in that on average only half of the segments will be lit you will still need (15*5mAH) 75mAH. In addition you have your green/yellow/red LED's that will need another 5-20 mAH.

All in all this may be hard to squeeze into a mint-box with a prospect of operating without power for 15 hours+.

That's the reason why most low power designs would use a reflective LCD instead of a LED display. However if he would only light the LEDs after pushing a button and switch them off 10s later it would work with LEDs as well.

However if the battery is close to dead then the current hunger of the LEDs would clearly finish it off -> better to go for LCD immediately.

Cheers, Udo