3G to WiFi Raspberry Pi gateway

[Edit]: Thanks Hackaday for the highlight! Part two of the hack – having a 77 year old learning how to use an Android phone – is ongoing. Proof of success: her instagram account.

After building my DIY 16.5 dBi (probably with pornographic levels of VSWR – hopefully i can get an antenna analyzer on that by the next few weeks) and proofing the concept of a feasible 3G gateway, i had to put everything inside a nice IP65 box and set it up outside. Initially i was aiming at using a modded WR703N with Rooter firmware and a Huawei E3131. After burning both of them, i had to resort to a Raspberry Pi Zero.

Principle of operation: Raspberry Pi handles the PPP session from the USB modem, creates an WiFi network via hostapd and uses iptables for the rest.

Notes on weatherproofing something

Basics first:

  • water + electronics = bad. Solution?
    • Sealed box.
  • heat + electronics = bad. Solution?
    • Sealed box with ventilation.
  • Sealed box with ventilation + electronics = moisture during low temperature. Solution?
    • Heat it when it’s cold.
  • 230VAC outside = bad idea. Solution?
    • Industrial grade 230VAC to 5VDC PSU – i used this one from Mean Well – RS-35-5 (link to manufacturer) placing it within the attic and routing 5m of 2×0.75mm2 cable outside – 6m total, minimum voltage drop.

So, this is not a trivial problem to solve. My solution? IP65 grade box (IP rating table right here) with a breathing hole, DHT22 sensor inside, DS18B20 outside, python measuring things. More details below…

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