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I found out that my 3G modem keeps closing the connection every couple hours. I tried to set up a reset script, everything works fine and I know how to do it, with one little problem: The only way I've found how to reset the device is physically unplug it and plug it back.

So my question is, how can I perform the unplug+replug operation software-wise? I've seen both How do I reset a USB device using a script? suggesting echo 0/1 > authorized and Resetting usb device from terminal suggesting usbreset, but usbreset doesn't manage to reset the device properly, and writing to authorized managed to cause two things: 1) freeze RPi, 2) forbidding echo 1>authorized with Error: Broken pipe.

So I look for another method how to truly reset the device without having to touch it. If it's easily possible via GPIO with some trick, I'm open to such solution as well.

Details: Debian GNU/Linux 7.0 (wheezy), the device is Huawei E220 12d1:1003, plugged through a powered USB hub 05e3:0606.

  • I have the same problem with the Huawei E220 USB modem. Did you find a solution to this? or do you know any other USB modem that does not crash every few hours? – Arya Jul 2 '15 at 16:08
  • This solution works well: electronics.stackexchange.com/questions/103678/… – yo' Jul 2 '15 at 18:21
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I have no idea on how to do that in software, but I do have an idea on how to implement such a feature in hardware, using one GPIO from the Pi.

You need a N-channel MOSFET like BS170 (It switches 500mA, maybe you want a bigger one) and a USB extension cord for this. Remove the protective shielding of the cable and find out its power wire, ususlly red. Cut that wire and insert the MOSFET, source pin to the MODEM and drain pin to your Pi, then wire the Gate pin to one of the GPIO on your Pi.

When you are using the modem, write 1 to the GPIO pin, turning the 3G modem on; and when you need to reset it, write 0 to the GPIO and wait for a little while, write 1 again to power cycle the modem.

  • Sounds cool. I'll definitely try this. I think that 500mA is quite tight, but should be fine, I'll see if my electronics shop has something stronger. Thanks a lot! – yo' Mar 20 '14 at 7:31
  • I can't make it work :( I found out a scheme at elinux.org. They suggest a bit different thing: putting the MOSFET on the - wire instead of the + one. As well, the maximum gate voltage is 3V and I gave it 3.3V :-/ Have you got an idea what could I have done wrong? – yo' Mar 20 '14 at 23:49
  • You need another MOSFET, a bigger one. I am no guru on that so maybe you need to find a datasheet collection and look for your ideal model. Alternatively, you can use low-voltage solid-state relays. Those switches 1A or up at 5V. – Maxthon Chan Mar 24 '14 at 5:23
  • I've got a working solution, but it took me a lot of effort and remembering my physics knowledge from the high school. You need an N-FET on the GPIO side and a P-FET on the USB cable side, see my self-answer on EE.SE – yo' Mar 24 '14 at 9:38
  • @tohecz I have implemented one myself too but using both MOSFET and BJT. I used a 2N7000 N-MOSFET to push out a little current then a C8550 PNP-BJT to push out the power. Also the four-terminal adjustable low-drop linear regulator PQ30NV11 but that is very difficult to find and quite expensive – Maxthon Chan Apr 1 '14 at 5:49

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