My office has a piece-of-junk Comcast cable modem that often needs to be rebooted. I got tired of walking back to the network closet, so I bought a USB relay for $30.

Yeah, I could have simply bought a better modem, but where’s the fun in that?

Here’s a photo of the relay board:

After connecting it to the modem’s power, I wrapped it in heat shrink:

And its final resting place where it will happily power cycle my cable modem for its entire useful life:

I put the relay between the modem’s power adapter and the modem itself, and connected the relay to my Linux server’s USB port for control. Once connected, the Linux kernel happily registered it as /dev/ttyUSB0 (it uses an FTDI chip for its USB-to-Serial translation, which is supported by all modern Linux kernels).

When connecting the power, I used the “C” and “NC” ports (“C” = “Common” and “NC” = “Normally Closed”). I didn’t use the “NO” port (“NO = “Normally Open”) because I don’t want my modem powering off just because the relay loses power (e.g., when the Linux box reboots).

This simple shell script power cycles the modem, which I run nightly using cron:


#!/bin/bash
device=/dev/ttyUSB0
printf '\xFF\x01\x01' >$device # Power off
sleep 60
printf '\xFF\x01\x00' >$device # Power on


I noticed that I needed to let the modem sleep for longer than 10 seconds, or there are problems on the network. In particular, our Outlook clients seem to lose their brains to our (off-site) Exchange server unless I sleep for longer. I used 60 seconds just to be safe.

The next step is to setup another cron job on my Linux box that will power cycle the modem if it ever discovers that it can’t reach the internet, or that its internet bandwidth is degraded.

I also wrote a small C++ program for Windows (about 20 lines) just to test the relay. It worked fine, but Linux was a lot easier to setup (no compiler necessary). In the time it took me to write that stupid C++ program, I was able to create both a working Python script and working shell script to do the same job (and Linux didn’t even require me to know the baud rate).

By the way, I would have avoided this whole exercise if my modem, like many others, had a software controllable reboot (some can be rebooted with wget, but not mine).

Happy (hardware) hacking!