Using a Redboard qwiic, I want to change the logic levels to 3.3 volts. I cut the 5 volt jumper and soldered the 3.3 volt jumper. It appears that the i2c interface no longer operates. Is there another step?
If you’ve switched to 3.3 already and are using i2c I believe you’ll need to disable the 2 jumpers on the rear as well RedBoard Qwiic Hookup Guide - SparkFun Learn
It might help if you share a photo of the soldered area too
I looked at the hook up guide and it does not go into detail about how to use I2C and logic level conversion. I did look at the schematic for the Redboard and it implies that cutting the 2 jumpers disconnects the qwiic connector. Could you please check for sure about what to do? I double checked my jumper soldering connection with a voltmeter and then put it back to original setting to make sure that everything still worked. It would be so much better if there was a header on the board with a movable jumper. The format of the cut and solder is tiny, and difficult to go back and forth between settings.
I did 
All of these are easily reversible; post the photo
Also…the guide covers the jumpers in-depth…go to the hardware section, click ‘jumpers’
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If you measure Vcc with the jumper set to 3.3v, does it read 3.3v?
How are you checking the I2C operation; with o-scope; using A4/A5 pins; using Qwiic?
I don’t have a Redboard but have a few Blackboard-Cs (the pre-cursor of that Redboard) and they are all working with 3.3V (except one which I kept at 5V); all needed was to adjust the 3.3v-5v jumper.
With the 5 volt jumper cut, I measured the voltage from the center I/O pad to ground and measured 0 volts verifying that I cut the jumper properly. I then soldered the 3.3 volt pad to the center pad. I checked with a volt meter being careful to touch only the center pad, not the solder blob to ground, which was 3.3 volts. This verified the solder connection. At this point my Redboard output changed from correct output to no output. I am using a qwiic sensor that lights up LED 13 on the Redboard when something magnetic passes by the sensor. (3d hall sensor).
I actually just took the time to do microsurgery and solder a header to the IO port, so that I can easily toggle between both voltage levels. I got the same result. There must be something else to it.
I don’t think cutting these jumpers are the answer. Please help. Where the schematic says Qwiic 3.3v conversion, this is ambiguous with what it says right below about disconnecting the Qwiic port. Trust me in that I am not a novice. I can definitely understand schematics, solder, use a VOM, an Oscilloscope, and follow instructions. This system is advertised as being simple.
That is some pretty nifty “surgery”. I do agree that cutting the A4/A5 jumpers should not be needed. For reference you can find the Blackboard C that is nearly identical to this Redboard here and which I use without these issues (just as a point of reference). Since you have experience with electronics, I would suggest to actually monitor the communications from A4 and A5 (and from the Qwiic output) with +5 and +3.3 switch to note if you see any differences. One should expect none (except the level from A4/A5 when using 5V should be at 5V). Only other thing I can think off is that you are using the Vcc from the Redboard for something else that does not like the 3.3V.
I would try soldering it as prescribed instead and testing…all 3 of those look shorted to me when I zoomed in (or at least test with a multimeter to see)
I know that Russell is stuck on the idea that it is soldered incorrectly, but it was tested to be correct, in two different ways. There is definitely another issue, and it is more complicated than read the instructions better or learn to solder. I will make my own logic level shifter, even though I should not have to. I like the idea of the qwiic concept, but this is the second time it has let me down. And, if you play with the IO pads too much the traces will come off the board. A header or a rocker switch would be nice.
