monitor-modem detects when an Internet connection has gone out and reboots the modem. Its behavior is configurable via signals. See the Signals section for details.
It is designed to run on a Raspberry Pi 3 model B+, but may work on other Pi models.
WARNING: High voltage is dangerous. Do not attempt to work with high voltage if you lack sufficient training in electrical safety or feel even slightly uncomfortable about the prospect of working with high voltage. If you make a mistake, you could be injured or killed.
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Get a Raspberry Pi 3 model B+. If you are a Raspberry Pi expert, you may wish to use a different model.
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Download and install the raspbian desktop image from https://www.raspberrypi.org/downloads/raspberry-pi-desktop/
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Build or buy a relay capable of controlling the power supply to your modem.
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Connect the relay to the Raspberry Pi GPIO header according to the relay instructions. Typically this will involve connecting one terminal on the relay to a ground pin on the GPIO header and another terminal on the relay to a regular 3.3V GPIO pin. The relay should be wired so that when the GPIO pin is at 0V relative to ground, power should be applied to the modem, but when the GPIO pin is at +3.3V relative to ground, power to the modem should be interrupted.
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If you have the most common kind of modem, one that contains an integrated ethernet switch and is configured to provide a private IP address to connected devices via DHCP, connect an ethernet cable between the modem and the Pi. If you have a different kind of modem, you may need to connect or configure the Pi differently so that it has Internet access. This is left as an exercise to the reader.
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Clone this project to your Pi.
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Copy monitor-modem.cfg.example to monitor-modem.cfg, and change the RELAYPIN variable to the GPIO pin you chose in step 4. Use the BCM pin number from this graphic: https://pinout.xyz/ For example, if the relay is wired to the pin in the lower right corner of the image, use RELAYPIN="21", not RELAYPIN="40".
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(Optional) Customize the HOSTS variable to suit your tastes. The default will be fine for most people.
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From the project directory, run sudo make install.
That's it!
Sometimes it is desirable to power down the modem for administrative reasons. This can be done by sending signals to the monitor-modem.sh process.
SIGUSR1 puts monitor-modem into keep_modem_off mode. In this mode, the modem is powered off and kept off until monitor-modem receives a signal putting it into a different mode. Take care when putting monitor-modem into this mode remotely as the modem will stay off until monitor-modem is sent SIGUSR2, the Pi is power cycled itself, or someone physically restores power to the modem on site.
SIGUSR2 puts monitor-modem into keep_link_up mode. This is the default mode, so SIGUSR2 is intended to reverse the effect of sending SIGUSR1.
monitor-modem comes with a control script for sending these signals called monitor-modem-control.sh. Here is a sample crontab that turns off the modem between 2am and 5am every day, and leaves it on the rest of the time (unless the modem dies and needs to be power cycled).
0 2 * * * /usr/local/sbin/monitor-modem-control.sh keep_modem_off
0 5 * * * /usr/local/sbin/monitor-modem-control.sh keep_link_up
Q: What happens if the Pi loses power?
A: The modem will stay on, provided it has a good, independent power source. If the Pi loses power, the GPIO pin controlling the relay will stay at 0V. In this condition, the C and NC terminals on the relay will be connected, and power will flow to the modem.
Cutting power to a Pi without first shutting down gracefully does sometimes cause SD card corruption. You can mitigate this by booting from a USB flash drive (https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md) or using a UPS. If the Pi does suffer SD card corruption and fails to boot properly after power is restored, the GPIO pin controlling the relay will stay at 0V and power to the modem will stay on.
This project has no dependencies beyond what comes preinstalled with the Raspberry Pi desktop.
If you wish to contribute, please send a pull request.
This project is licensed under the GPLv3 or, at your option, any later version.