Two weeks ago I started on a new project at home to build a telephone that people can use to call their government reps and leave them a message. It’s inspired by 5calls.org which materialized shortly after the dark lord Nyarlathotep took power. 5calls smooths most of the bumps in the process of contacting the government to give them a piece of your mind. The idea popped into my head while biking home from work: A phone I put outside my house that passersby could use. They’ll be so delighted to pick up the handset from the cradle, mash the keypad, and listen to the sweet DTMF tones before getting down to business. I wish I could bottle the optimism that comes with the spark of a greenfield project idea.
Given the richness of the hobby electronics ecosystem there was no worry as to whether I could find the parts I needed or write the code. The worry always comes the next day, usually in the form of “Is this idea garbage?” and it’s easy to stop there, but I think we owe it to ourselves to test that question with a little bit of work.
To get started I knew at the very least I needed a cellular transceiver — turns out there are some nice packages available and their capabilities really surprised me. The SIM800 series of chips can provide cellular, GPS, Bluetooth, FM tuner and more all in one package. I started with AdaFruit’s breakout because they make cool things and deserve support. All of the SIM800’s capabilities are accessed via a massive AT Command set which I love because it brings me back to the simpler times of the modem. The other bit I needed was a SIM card and Adafruit’s docs led me to Ting. Ting provides pay-as-you-go cell service over T-mobile’s network and they’re perfect for a project like this. Setting up the SIM card was extremely smooth and they have a very clear website to show your usage.
Wiring the FONA board up to my Mac was straightforward using a USB-TTL cable and a breadboard. I spent a few hours going through the AT Command set and trying out some of the features like the built-in HTTP client! My only complaint about the FONA board is that it requires a battery to be plugged in to operate, which is fine for experimentation but I planned to couple the board with a Raspberry Pi so it added some complexity in terms of power supply. The next board I purchased was this GSM Hat from Waveshare and it’s got a really slick feature that allows you to change how the USB port and Rx/Tx pins are routed. Since the hat takes over the Pi’s GPIO header it’s inconvenient to access the Pi’s console over serial, but the board’s designer solved that problem with a set of jumpers:
- A: control the SIM868 through USB TO UART
- B: control the SIM868 through Raspberry Pi
- C: access Raspberry Pi through USB TO UART
OK so the communication part of the project is figured out. In the next installment I’ll talk about old-timey telephones and TFT screens.
- Ting Referral Link — help subsidize the phone project!