top of page

Fixing the Main-Driver Subassembly


I had taken over a set of FlowIO components from a previous student and was trying to get it to run. However, when supplying power to the main driver subassembly and pushing the on/off button, nothing turned on. I therefore suspected that the main board had some sort of faulty on/off circuit or on/off button. Hereafter is what had been done to troubleshoot and to resolve the problem.

Troubleshooting Steps

0. The components that I had available are shown in this image (+ the tools I used for troubleshooting):

1. The problem basically occured when following the 'Main Driver Subassembly' page; after connecting a 3.7V power supply in step 9 (instead of the LiPo battery), when pressing the power button in step 10 nothing happened:

No current seemed to be drawn, and the microcontroller didn't seem to be powered as there was no LEDs lighting up on it. 2. Also, when trying to read the voltage directly on the main board's 3.3V and GND pin (I had tried it both from the header pins as well the solders at the bottom of the board) I read 0.0V, whereas I would have expected to read 3.3V there and this was presumably the reason why the microcontroller didn't get powered:

3. To exclude the possibility that the Feather board is faulty, I tried hooking it up directly with the power supply's 3.7V as described in the instructions, and everything seemed fine (LEDs are lighting up):

4. Similarly, when connecting to the Feather board via Arduino on my laptop everything worked as expected and I was for example able to upload the 'Hardware Test' Script. So the Feather board itself seemed to be fine:

Troubleshooting conclusion: This made me conclude that the fault had to most likely lie with the main board, with either the on/off button or the on/off circuit not working properly.

Solution Steps

0. After posting an inital version of this article, Ali kindly took the time to analyze the problem and suggested that I replace the small Mosfet below the on/off controller (green circle in the image below), and if that didn't work to also replace the on/off controller itself (blue circle in the image below).

1. As both these components are quite cheap (you will most likely pay more for shipping than for the components themselves...), I directly bought both of them on Mouser rather than waiting for one to arrive only to then have to yet order another one and wait some days again. I also suggest to order several of each, as they are really inexpensive and for reasons I explain below.

Once the components arrived I then directly tried replacing both at once. Here is the approach I chose as a person not very experienced with electrical assembly (so you might find a better way of doing it):

2. First I taped off everything but the two components I wanted to replace. Then I used a heat gun to melt the solder on the two components and removed them with a pair of tweezers.

3. Next, I took each of the components and placed it in the correct position, and used a very thin soldering iron tip that I tinned beforehand to solder all the legs to the board (using only the solder that was still on the pads respectively the bit of solder that was tinned to the soldering iron).


- Steps 2 and 3 can be quite tricky as the parts are small. For each component I lost one of them in the process as they dropped when trying to solder them on and as they are so tiny it can be near impossible to find them on the floor... This is why I suggest to directly order maybe a few of each, to have some buffer for error.

- Also, I unfortunately more or less melted away the battery connector with the heat gun, so try to be smart about how you apply the heat.

Result: With the two components replaced, now going back to step 1 of the 'Troubleshooting Steps' section, when pushing the on/off button, the LED lights up and everything works as expected. I can use the FlowIO and all its channels as outlined in the building instructions.

Hope this helps other people too.



119 views3 comments


Ali Shtarbanov
Ali Shtarbanov
Mar 14, 2023

Looks like the problem in this case may be with the small Mosfet below the on/off controller. I would try replacing it first. If that still doesn't fix things, you can also replace the on/off controller.

Replying to

Hi Ali,

It looks like this did the trick by the way. Thank you!

bottom of page