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I've noticed that on the back of an Arduino PRO mini are two Jumpers next to the A4 and A5 pins, marked with red in the following :

Pro mini Back view with jumpers selection

Looking at the leads one seems to be a bypass for the POWER Led and the other a bypass for the regulator, but not sure.

Further, the schematic reveals just one Jumper SJ1 :

Arduino Pro mini - Power schematic with jumper selection

https://cdn.sparkfun.com/datasheets/Dev/Arduino/Boards/Arduino-Pro-Mini-v14.pdf

So, my questions are :

  1. Why are there two physical Jumpers on the board ?
  2. Are these two jumpers actually just one : SJ1 ?
  3. And if so how to connect them in order to enable Power isolation ?

4 Answers 4

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Both responses are correct but be mindful that this is a SparkFun feature. An Arduino Pro Mini from China won't necessarily have the SJ1 pad.

1) Like Gerben said, the pads on the underside of the board are for I2C pull-up resistors.

2) Like David said, SJ1 is on the top of the board next to the GND and RST pins. (It looks like a small solder blob.)

3) To isolate the regulator from VCC, remove the solder blob on the top of the board with a sucker or solder wick.

The Sparkfun schematic shows all of these elements: https://cdn.sparkfun.com/datasheets/Dev/Arduino/Boards/Arduino-Pro-Mini-v14.pdf

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I think those aren't jumpers, but pads to place two pull-up resistors for I2C.

It seems the eagle files Sparkfun shares aren't the same as the board in the picture.

But you don't need to worry about this. If the RAW pin is not connected the MIC5205 regulator uses only 0.01μA according to the datasheet.

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These pads are not jumper SJ1. On the front of the board just beneath the GND pad there is a blob of solder. This is SJ1 (I believe - I am writing from memory, I do not have a Pro Mini sitting on my desk at the minute) and removing the solder blob will disconnect the LED and output of the regulator from Vcc.

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You can look up the location of SJ1 in the Eagle project files that are provided by Sparkfun). For example, you can import them into KiCad (Free and Open Source), which opens a PCB and a Schematic editor.

To locate SJ1 you can simply click on it in the Schematic editor such that it's highlighted in the PBC editor, as well. For better overview, you may want to hide all the layers besides the foreground one (e.g. 'F.Cu') and click View/Zoom-to-Fit:

-> That means SJ1 is located on the front of the board, right of pin 9 (ground), i.e. it's highlighted in a lighter red. For comparison, I included an image (Sparkfun, CC BY 2.0) of the front, where you can see that SJ1 is marked by a white rectangle (painted on the board) and that it's closed, by default by a solder blob, which can be de-soldered (e.g. using solder wick and a bit of flux).


NB: To enable power isolation (i.e. such that the MCU can be directly powered without also powering the power-on LED and one side of the on-board LDO, e.g. in order to save power), you have to open SJ1.


Thus, the pads on the back of the board, you marked in an image you included in the question, are something different. In KiCad you can look them up as well: that means switch to the back layer (e.g. 'B.Cu') and click on the pads such that the related parts are highlighted in the schematic editor:

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