I'm installing an air quality monitor outside my apartment. I have no power outlet outside, so I have to plug in the power supply inside and run the wire through my wall. The collected data will be shared via wifi. The wall is concrete, and the power supply is pictured below.

power supply


I'll need to drill to route the cable, and I'd like to drill as small a hole as possible. This presents a couple of challenges. In order to fit the head of the USB cable through the hole it'll need to be around 11 or 12 mm, and even larger to fit the ferrite bead. I would like to avoid drilling such a large hole.


In the interest of drilling as small a hole as possible, can I cut the USB cable and solder it back together once it's been threaded through the hole? This would mean that the hole would only need to be a few millimeters wide. Assuming I go this route, must I reconnect all interior parts of the USB cable (the bare copper wire, the foil shield, the red 5V cable, the black ground, and the other two data cables)? Or can I get away with just reconnecting the 5V and ground cables?

Obviously this would mean I would have to cut the cable again if/when I decided to move it. Is there any way to avoid this? Are there other connecters I could use for this sort of purpose? Is there something else I haven't thought of?

  • 1
    \$\begingroup\$ Since it is an apartment, you don't own the walls and drilling holes to get cable outside is most likely forbidden at least without permission and licenced professional doing the installation propely to prevent e.g. rainwater seeping in via cable hole . Buy a flat flex USB extension cable to fit through the seams of a window or something. \$\endgroup\$
    – Justme
    Commented Mar 1, 2021 at 9:08
  • \$\begingroup\$ depending on the required current, it may be possible to use a wireless charger circuit to pass power through a glass window \$\endgroup\$
    – jsotola
    Commented Mar 1, 2021 at 15:51
  • \$\begingroup\$ @Justme Thanks for your thoughts, but this is not my situation. I'll look into the flat flex cables. \$\endgroup\$ Commented Mar 2, 2021 at 2:03
  • \$\begingroup\$ @jsotola Hadn't thought of that. Thanks. \$\endgroup\$ Commented Mar 2, 2021 at 2:03
  • \$\begingroup\$ Just a note from experience, if your hole is very close to the size of your cable, it's much harder to seal, especially if the layer you need to seal is inside another(eg stucco or siding over blueskin). Even a very small hole should be sealed, so there is an advantage to drilling the larger hole. All of that said, you can buy a replacement male microusb cable end instead of attempting to cut and solder the cable (Which is possible, but tricky if the cable is to be used for data at all). If the cable is only used for power, cutting and rewiring is quite easy. \$\endgroup\$
    – K H
    Commented Mar 2, 2021 at 2:53

2 Answers 2


You haven't given any details about the air quality monitor. Judging from the picture, it's a simply DC power supply. So the data wires most likely aren't used.

Therefore you can go ahead as proposed:

  • Cut the wire
  • Thread it through the hole
  • Reconnect the 5V and ground wires

There is no need to reconnect the data wires or the shield (which shields the data wires).

You can also use a different wire with two conductors, e.g. to extend it. It should be sufficiently thick to carry the current, in particular if it is extended.

  • \$\begingroup\$ The monitor is from Purple Air. More details here: www2.purpleair.com/collections/air-quality-sensors/products/…. Since the cable simply connects the monitor to a power outlet I don't think the data cables are used. That said, I am curious why there's a ferrite bead. \$\endgroup\$ Commented Mar 1, 2021 at 14:26
  • \$\begingroup\$ @AlexJohnson Bead is there to suppress RF emissions from the device that would otherwise use the power cable as an antenna. They probably needed it for EMI compliance, or just put it on to be safe. \$\endgroup\$ Commented Mar 1, 2021 at 14:44
  • \$\begingroup\$ @user1850479 Makes sense. Thank you. \$\endgroup\$ Commented Mar 2, 2021 at 2:04

it appears to be a fixed voltage power supply so it be failry easy cut and reconnect the DC cable.

to reconnect semi-permanently you could use wire nuts, or lever, or screw operated splices. enter image description here

You will probably find that that cable has only two conductors, but if it has more you may need to reconnect all of them.


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