user18990: I've successfully migrated and ATMEL328P-UP microcontroller from the Arduino Uno Board to a breaboard, following these instructions: https://www.arduino.cc/en/Tutorial/ArduinoToBreadboard. Everything is OK, the bootloader worked and code is already uploaded an running.
The ATMEL is connected to a pair of sensors, and all the system works using 3.3V. And here comes the problem: I'm using a 5V USB power bank to power up everything. When I make this connection:
USB Power Bank @ 5V -> Arduino Uno board (without the microcontroller) -> 3.3 V Output from the board -> ATMEL/Sensors on the breadboard
everything works just fine! Now I decided to replace the Arduino uno board with an LM317 voltage regulator which gives me the 3.3V needed. The connection is this:
USB Power Bank @ 5V -> LM317 with 3.3 V output -> ATMEL/Sensors on the breadboard
and nothing happens! Is as if there wasn't enough current to feed the ATMEL nor the sensors. I've checked the Vcc/GND pins of the sensors and the ATMEL and they're all getting the required 3.3 V, but for some reason the system seems to be OFF.
What could be happening? I'm sure the USB power bank is at full charge, so it should be able to supply enough current. Any suggestions?
Thanks guys!

user31038: According to the datasheet, the LM317 needs an input voltage 3v higher than the output. Since 6.3<5, there's not enough margin for the LM317.
You can use an AMS117 LDO @3.3, which only needs an input of 3.9v to operate. I find these modules very handy. If the MCU has some tolerances, you can likely get away with using a diode or two in series with the 5v rail to drop it down; the actual forward voltage fluctuates slightly on load, but it will likely stay in the acceptable range for your MCU.
you can also use a switch-mode regulator, commonly sold as "buck converters" on ebay, to more efficiently reduce the voltage. This is the way to go over linear regulator like the 317 if you power your project from batteries or have concerns about heat. I go through a lot of these style converters, and they seem to work very well.

18990: <p>I've successfully migrated and ATMEL328P-UP microcontroller from the Arduino Uno Board to a breaboard, following these instructions: <a href="https://www.arduino.cc/en/Tutorial/ArduinoToBreadboard" rel="nofollow noreferrer">https://www.arduino.cc/en/Tutorial/ArduinoToBreadboard</a>. Everything is OK, the bootloader worked and code is already uploaded an running.</p>

<p>The ATMEL is connected to a pair of sensors, and all the system works using 3.3V. And here comes the problem: I'm using a 5V USB power bank to power up everything. When I make this connection:</p>

<p><em>USB Power Bank @ 5V -> Arduino Uno board (without the microcontroller) -> 3.3 V Output from the board -> ATMEL/Sensors on the breadboard</em></p>

<p>everything works just fine! Now I decided to replace the Arduino uno board with an LM317 voltage regulator which gives me the 3.3V needed. The connection is this:</p>

<p><em>USB Power Bank @ 5V -> LM317 with 3.3 V output -> ATMEL/Sensors on the breadboard</em></p>

<p>and nothing happens! Is as if there wasn't enough current to feed the ATMEL nor the sensors. I've checked the Vcc/GND pins of the sensors and the ATMEL and they're all getting the required 3.3 V, but for some reason the system seems to be OFF.</p>

<p>What could be happening? I'm sure the USB power bank is at full charge, so it should be able to supply enough current. Any suggestions?</p>

<p>Thanks guys!</p>
 31038: <p>According to <a href="http://www.ti.com/lit/ds/symlink/lm317.pdf" rel="nofollow noreferrer">the datasheet</a>, the LM317 needs an input voltage 3v higher than the output. Since 6.3&lt;5, there's not enough margin for the LM317.</p>

<p>You can use an AMS117 LDO @3.3, which only needs an input of 3.9v to operate. I find <a href="http://www.ebay.com/itm/181847233524" rel="nofollow noreferrer">these modules</a> very handy. If the MCU has some tolerances, you can likely get away with using a diode or two in series with the 5v rail to drop it down; the actual forward voltage fluctuates slightly on load, but it will likely stay in the acceptable range for your MCU.</p>

<p>you can also use a switch-mode regulator, commonly sold as "buck converters" on ebay, to more efficiently reduce the voltage. This is the way to go over linear regulator like the 317 if you power your project from batteries or have concerns about heat. I go through a lot of <a href="http://www.ebay.com/itm/142147326064" rel="nofollow noreferrer">these style converters</a>, and they seem to work very well.</p>
