user15413: Scenario: I would like to monitor my closet's door as I highly suspect a family member is opening and using my items without permission. To respect privacy and also because my closet is nowhere near a power mains, I don't need video nor audio recording. I also do not need indicator lights etc because I intend to turn it on when I go to work and check the logs on the SD card when I return home. A typical day would require the device to run for 12 hours minimally.
Aim: Is it possible for Arduino to achieve the following,

Run on batteries for at least 1 month
Detect motion
Logging the detection motion onto a text file in a SD card

Or do I have to look elsewhere for this functionality?
Current understanding: I am aware Arduino can run on batteries though I've never been able to get it to last more than 3 hours. Probably has to do with me not using the low power mode or something and that my batteries were being stepped up. I also know that Arduino has some PIR motion sensors though I don't quite remember how power hungry these sensors are. I have NEVER tried logging onto an SD card using Arduino. This sums up my experience with Arduino.
Any advice would be much appreciated. Thank you.

user13432: It is possible to modify an Arduino Pro Mini (clone or original) to less than 1 mA and give at least a month on standard batteries (1000 mAh). Basically you need to remove the Power indicator LED and use low power mode in the sketch. 
You could start simple by just lighting a LED when motion is detected and add the SD log later. 
There are several techniques to detect motion. Advanced accelerometers such as ADXL345 but also simple wire sensing. The standby power requirements for these have a large variation. 

user11037: For low-power, just some tips (link references)
https://github.com/petervojtek/diy/wiki/Arduino-with-Very-Low-Power-Consumption
claim the below results,
LED off, without the JeeLib library: 6.7 mA
LED on, without the JeeLib library: 8.8 mA
LED off, with the JeeLib library: 43 uA (!)
LED on, with the JeeLib library: 2.2mA

https://github.com/petervojtek/diy/wiki/Arduino-with-Very-Low-Power-Consumption
Detailed step by step tutorial.
For sensing PIR will depend on exact components; using SMD low-leakage components will do the trick but cost would increase. There are very-low-power RTC on the market and maybe SD logging will be more difficult as can consume up to 200 mA per reading/writing operations.

user612: You would be better off with a bare ATmega chip than an Arduino - you want to eliminate extraneous circuitry like a USB converter, regulators with quiescent current draw, and any resistors pulling signals against their usual state.  Fortunately the ATmega chip has a fairly wide voltage range, especially at 8 MHz or below, so you can use something like 3 cells, alkaline or rechargeable, over most of their range.
An SDcard is neither necessary, nor a good idea.  If your circuit won't be discovered you can probably hold the data in static ram while the ATmega sleeps; if not, the quantity of data would probably not exceed the on-chip EEprom and would certainly not exceed that of a 4 megabyte SPI flash. Either of these will be far more robust (and low power) in a battery powered situation than an SD card.  They will also be simpler to support in software.  Either an SPI flash or an SDcard will probably have a narrower supply range the the ATmega and require a regulator - you may be able to enable that only when you need to access the memory.
Your main power consumption win however will be sensors.  If you can use a contact sensor - either a direct switch, or a magnetic reed one, that will result in the lowest power consumption.  Your next option might well be an accelerometer chip, specifically if you choose one which can run in a micropower mode and fire a wakeup interrupt to the MCU when it detects that a door it is mounted on is being opened.  Anything with light is likely to have much higher power consumption and could well be impractical, unless you figure out how to operate it in short bursts with a lengthy inactive time in between. 

15413: <p>Scenario: I would like to monitor my closet's door as I highly suspect a family member is opening and using my items without permission. To respect privacy and also because my closet is nowhere near a power mains, I don't need video nor audio recording. I also do not need indicator lights etc because I intend to turn it on when I go to work and check the logs on the SD card when I return home. A typical day would require the device to run for 12 hours minimally.</p>

<p>Aim: Is it possible for Arduino to achieve the following,</p>

<ol>
<li><p>Run on batteries for at least 1 month</p></li>
<li><p>Detect motion</p></li>
<li><p>Logging the detection motion onto a text file in a SD card</p></li>
</ol>

<p>Or do I have to look elsewhere for this functionality?</p>

<p>Current understanding: I am aware Arduino can run on batteries though I've never been able to get it to last more than 3 hours. Probably has to do with me not using the low power mode or something and that my batteries were being stepped up. I also know that Arduino has some PIR motion sensors though I don't quite remember how power hungry these sensors are. I have NEVER tried logging onto an SD card using Arduino. This sums up my experience with Arduino.</p>

<p>Any advice would be much appreciated. Thank you.</p>
 13432: <p>It is possible to modify an Arduino Pro Mini (clone or original) to less than 1 mA and give at least a month on standard batteries (1000 mAh). Basically you need to remove the Power indicator LED and use low power mode in the sketch. </p>

<p>You could start simple by just lighting a LED when motion is detected and add the SD log later. </p>

<p>There are several techniques to detect motion. Advanced accelerometers such as <a href="http://www.analog.com/en/products/mems/mems-accelerometers/adxl345.html" rel="nofollow">ADXL345</a> but also simple <a href="http://www.interlogix.com/intrusion/category/wired-sensing" rel="nofollow">wire sensing</a>. The standby power requirements for these have a large variation. </p>
 11037: <p>For low-power, just some tips (link references)</p>

<p><a href="https://github.com/petervojtek/diy/wiki/Arduino-with-Very-Low-Power-Consumption" rel="nofollow">https://github.com/petervojtek/diy/wiki/Arduino-with-Very-Low-Power-Consumption</a></p>

<p>claim the below results,</p>

<pre><code>LED off, without the JeeLib library: 6.7 mA
LED on, without the JeeLib library: 8.8 mA
LED off, with the JeeLib library: 43 uA (!)
LED on, with the JeeLib library: 2.2mA
</code></pre>

<p><a href="https://github.com/petervojtek/diy/wiki/Arduino-with-Very-Low-Power-Consumption" rel="nofollow">https://github.com/petervojtek/diy/wiki/Arduino-with-Very-Low-Power-Consumption</a></p>

<p>Detailed step by step tutorial.</p>

<p>For sensing PIR will depend on exact components; using SMD low-leakage components will do the trick but cost would increase. There are very-low-power RTC on the market and maybe SD logging will be more difficult as can consume up to 200 mA per reading/writing operations.</p>
 612: <p>You would be better off with a bare ATmega chip than an Arduino - you want to eliminate extraneous circuitry like a USB converter, regulators with quiescent current draw, and any resistors pulling signals against their usual state.  Fortunately the ATmega chip has a fairly wide voltage range, especially at 8 MHz or below, so you can use something like 3 cells, alkaline or rechargeable, over most of their range.</p>

<p>An SDcard is neither necessary, nor a good idea.  If your circuit won't be discovered you can probably hold the data in static ram while the ATmega sleeps; if not, the quantity of data would probably not exceed the on-chip EEprom and would certainly not exceed that of a 4 megabyte SPI flash. Either of these will be far more robust (and low power) in a battery powered situation than an SD card.  They will also be simpler to support in software.  Either an SPI flash or an SDcard will probably have a narrower supply range the the ATmega and require a regulator - you may be able to enable that only when you need to access the memory.</p>

<p>Your main power consumption win however will be sensors.  If you can use a contact sensor - either a direct switch, or a magnetic reed one, that will result in the lowest power consumption.  Your next option might well be an accelerometer chip, specifically if you choose one which can run in a micropower mode and fire a wakeup interrupt to the MCU when it detects that a door it is mounted on is being opened.  Anything with light is likely to have much higher power consumption and could well be impractical, unless you figure out how to operate it in short bursts with a lengthy inactive time in between. </p>
