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Hi all, I'm making a project that needs to use as little power as possible. I've read that most of the power consumption of a project is the sensors etc (not the arduino itself), but I can't really change that very much. I've read the following concerning power consumption:
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agg. tab plus enter means bad news. oh well.

I've read this:
-the voltage regulator uses up unnecessary power
-the chip can be put into a 'sleep' mode, and brought back by a reset or an interrupt (shaky on the meaning of interrupt)
-you can use an external device to turn the arduino on for a bit and then off for a bit
-something about pins and pullup resistors

my project requires that the arduino check a sensor every five seconds or so and possible store a time to eeprom. It can be off the rest of the time, assuming eeprom is stable without power (yes?). Would a 555 timer be able to do this, or an RTC? I haven't used either, so I dunno.

Any information regarding reducing power consumption would be excellent. Thanks!
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the voltage regulator uses up unnecessary power

You need a voltage regulator that is low in quiescent current else it might eat up ore current than the MCU itself. Some LDO are very low in quiescent current and you can also opt for switching based regulator. But it also depends how long you want your system to run without permanent power.

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Would a 555 timer be able to do this, or an RTC?
You can use an RTC because they consume very low current. You can look at jcl's solution, he is using an RTC and some very low quiescent LDO. The RTC time stamp can be used as your sensor data time stamp too. Besides RTC, you can also use external low power crystal provided you have the necessary MCU pins to drive it.  

You can also shut down the sensor when not using them. Some sensor has sleep mode, you can do that provided the sleep current is low enough for your application. Else you can just cut the supply to the sensor.

Cheers.
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- When talking about reduction of power consumption there is always a "battery" in the background. You will need a regulator, when the voltage is far away from 4 Volts. In many cases a good solution is to use the unregulated battery voltage, as long  as it is between 3 an 6 volts.

- Nearly all switching regulators can be "switched off" (and than to become switched on by a time pulse from the RTC).
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I didn't know you could use an arduino without providing exactly 5 volts. If I use rechargeables AAs, 1.2 * 4 is 4.8 volts. Would that damage the arduino at all? Also, can you program an RTC to give a pulse every X seconds?

As for a switching supply, I assume that that is a power supply that turns on or off when it detects a pulse? In that case, the RTC would give it a pulse at t = 0, the arduino turns on, then maybe the RTC (or arduino, I guess) sends another pulse at t = .1 seconds or so to turn the arduino off. The cycle then repeats?
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I didn't know you could use an arduino without providing exactly 5 volts. If I use rechargeables AAs, 1.2 * 4 is 4.8 volts. Would that damage the arduino at all?
No, this is just fine for the AVR and the FTDI chip (3.3 to 5.3 is a safe margin). Note that fresh 1.5 batteries have an open circuit voltage of around 1.65 volts.
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Also, can you program an RTC to give a pulse every X seconds?
One second will be fine as well, just go to sleep (x-1) times again; my alarm clock calls that "snooze"

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As for a switching supply, I assume that that is a power supply that turns on or off when it detects a pulse? In that case, the RTC would give it a pulse at t = 0, the arduino turns on, then maybe the RTC (or arduino, I guess) sends another pulse at t = .1 seconds or so to turn the arduino off. The cycle then repeats?
Quite similar. You will need an external chip, a "retrigerable one-shot". A switching regulator (as the popular LM2574 or LM2575) needs a  permanent "On"-signal. They generally works between (Voutput+1) and 40 volts. So it is fine to use 12 or 24 car batteries for suply. There are also so called "boost" regulators which convert from 3 volts (=2x AA cells) to 5 volts
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okay, that's good to know. how exactly do you put the arduino to sleep? Is there some command, or do you do something special with a pin or something? I'd just as soon turn the arduino off instead of putting it to sleep, if the two are not the same thing. I'm actually planning on running this off of either a 9 volt or AAs.

So a retigerable one shot turns a pulse into a constant signal? kind of like a switch? So the RTC sends out a pulse, the retrigerable one shot receives it and puts out a constant signal to the switching regulator, which powers the arduino. Then, the RTC sends out another pulse, the one shot receives it and kills the signal, the regulator turns off the arduino. Correct?

I'm also unsure of how an RTC works. Is it standard for them to just put out a pulse every second?

Thanks!
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RTCs can be configured to do this. search in the forum oder the playground for RTC - you will get lots of useful information.
A "one-shot" will extend a (very) puls. ASo it can be used as a kind of external watchdog. The arduino, when awake must than re-trigger this device each milisecond or so, so that is upholds its output signal.

If might even be vice versa. IF the second pulse from the RTC is quite long - say 1/2 second - then you have to shorten it...

There are lots of design options about
- whicht part of the system to completely shut down
- whether the procesor to shut off or to send to sleep.

The mentioned types of  swtching regulators will not need noticably more power than the processor in sleep they are supplying.

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