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I have been playing a little with my new Arduino UNO and built a little "timer" that blinks an LED 3 times after a selectable time delay of from 1minute to 4 hours.  the ONLY hardware I have attached to my UNO is 16pin DIP switch and a single LED/resister.  The switch grounds the "input" pins, program does a little math, delay(SelectedDelay) happens and then blinks the LED on the "output" pin .... Everything works exactly as I expected it to when loaded..

Then I decided to see if I can use a 9v battery to power my UNO and run the timer without being hooked to my USB cable.  It works fine except that it doesn't last very long at all .. if set for 20min delay I get about 3 hours before the green power LED on my UNO goes out.  Is this normal??  Why does the UNO suck the life out of a 9v battery so fast?

Thanks,
dmac257
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Probably the onboard voltage regulator is probably disapating half the power dropping the voltage from 9v to 5v.
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And if you're running delay, you've got the processor running full time too, got the power LED on the whole time, got the at8mu2 running full time, if your unused pins are left floating as inputs you've got potentlal switching losses there (best to set unused pins as inputs with internal pullups enabled).

You'd be way better off running of 3 AAs direct to the 5V header pin, bypassing the regulator.
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Then I decided to see if I can use a 9v battery to power my UNO
That was a mistake, these batteries have very little current capacity they are not really practical of using with an arduino.
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And they are expensive... you would be better of with multiples of AA batteries.
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A rechargeable 9V battery reduces the cost over alkaline---but they are normally 150mAh or so, whereas AA's are 2000mAh (13 times the life).  And if using rechargeable AA's then use 4, not 3, as they are about 1.25 to 1.3V each rather than 1.5V for alkaline.
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Thanks for the quick answers .. unfortunately now raises several new questions:

Crossroads said:
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And if you're running delay, you've got the processor running full time too, got the power LED on the whole time, got the at8mu2 running full time, if your unused pins are left floating as inputs you've got potentlal switching losses there (best to set unused pins as inputs with internal pullups enabled).

You'd be way better off running of 3 AAs direct to the 5V header pin, bypassing the regulator.

I am still learning but is there a way to shut down the at8mu2, power LED, and processor for a specified length of time.  I can set the unused pins as you suggest but have not done it in this case as all the examples leave all the unused pins alone in the sketches and didn't know this would be a problem.

Last part of your reply was to apply 3 AAs to the 5V header .. 3X1.5(or as pointed out for rechargables1.25)=4.5V(3.75v) .. I was under the impression that you needed to have a 5v supply to the 328p but after looking it up Vcc can be 1.8-5.5v .. does lower voltages effect the ocillator timers and such or the reliability of the chip to operate?


MarkT said:
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A rechargeable 9V battery reduces the cost over alkaline---but they are normally 150mAh or so, whereas AA's are 2000mAh (13 times the life).  And if using rechargeable AA's then use 4, not 3, as they are about 1.25 to 1.3V each rather than 1.5V for alkaline.

Untill you told me I had no idea that there was that much of a difference in mAh between the two types of batteries.  For now I am working on a computer upstairs and the breadboard in the basement and thought being able to run the UNO from a "portable 9V supply" would be a good idea.  I will just rip a PC supply out of a junk PC and use for 5V supply on the workbench (come to think of it I will have 12V and 3.3V also)  Eventually I will have a PC downstairs running Linux with Arduino IDE running on it so will have the USB port downstairs too.

Thanks again guys,
dmac257
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You can put the atmega into power down sleep mode. I don't think you can do anything about the at8mu2. The power LED is on all the time, current strictly based  on the voltage.  There have been numerous discussions on what state to put unused pins for power reduction.
You'd have to read the datasheet to ascertain oscillator timer effects, not sure you will see anything noticeable as the voltage drops.
I have run my duemilanove on 3 AAs, ran great as remote sending RF data from a keypad on 3 AAs. Then changed to a 3.7V LiPo battery and a 8 MHz prominin, still runs great.
If you  have  a DC supply, then running on batteries is kind of a moot point.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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