Hacked an old phone to control Sonos system with Alexa and keypad.
I've had an antique-looking phone for a while, it looks like an old rotary phone, but it has a touch-tone dial pad. Since I haven't had a standard (POTS) line for some time, I thought I'd turn it into a project.
Step one was to gut the phone and put in a Raspberry Pi Zero, add a power source, wire it up so the handset's microphone and speaker work with the Pi, and make the phone hook trigger an event on the Pi so we know when someone has lifted or set down the receiver.
I loaded the Pi with AlexaPi and made one feature change to the code. I didn't want to have the microphone always-on or use a "wake" word, so I modified AlexaPi to only listen when I take the phone off-hook. (Mar 2023 this has been removed)
The phone is a Crossley CR55-BK. When taking it apart, the first thing you notice is that it is loaded with about 1.5 lbs of lead. This gives the phone a lot of heft and makes it feel like a classic, indestructible Bell telephone. I removed the lead from the phone body but kept it in the handset. I gutted everything else from the inside of the phone, including the ringer, and then cut out as many mounting pegs as I could. Removing the mouth and earpieces from the phone was troublesome. Lots of glue, but they did come off eventually.
It was unlikely that the existing microphone and speaker would work (and they didn't), so I replaced them. I kept the wiring intact, but removed the microphone and speaker, soldering in an 8 ohm, 2W speaker and added an amplifier to the Pi.
I had a difficult time finding a microphone that would work, so I tried an experiment with something really cheap in case it failed. I bought a super inexpensive USB microphone and basically took the whole thing apart and used its internal components. The USB mic was dismantled to remove the mic and then soldered to the two leads that come from the phone to the USB component. This basically puts a long wire on the mic. The mic was then placed in the handset and soldered.
Internally, the phone hook is just a switch with two wires, so this was easy to wire to the Pi and use GPIO events to know if it was triggered or not.
So this gave me a cool, retro phone to control Sonos.
I am by no means a hardware engineer, so circuit design takes a lot of web searches in the hope that I can find someone who has done something similar. Figuring out how the rotary-style keypad worked seemed like a daunting task as I hadn't come across any other keypad like it. It has eight wires coming out of it. As it turns out, even though it is in a circular layout, the keypad works like many other keypads. The wires correspond to rows and columns. For any button press, two of the wires become "active". I brought out an ohmmeter and started testing. 8 wires, many combinations, but eventually found the pattern.
| Digit | Color 1 (row) | Color 2 (col) |
|---|---|---|
| 1 | yellow | gray |
| 2 | yellow | purple |
| 3 | yellow | white |
| 4 | orange | gray |
| 5 | orange | purple |
| 6 | orange | white |
| 7 | red | gray |
| 8 | red | purple |
| 9 | red | white |
| * | brown | gray |
| 0 | brown | purple |
| # | brown | white |
Next, I wired each of these to an open GPIO port on the Pi and wrote some Python code to listen for these key presses. I used a keypad library as the guts and wrote my button UI to interpret the keypresses.
I wanted the UI to be simple, so numbers 1-99 correspond to Sonos favorites and playlists. By editing each favorite/playlist name to have the pattern [XX] in their title, these can be referenced through the phone. (Ex: "Jazz Standards [1]" is triggered by just pressing the "1" key. No coding or configuration file to edit. Just edit the Favorite in the Sonos app itself. Favorites can be anything, a song, an album, a playlist, or even a link to a radio station in one of the Sonos services.
The other key commands I have wired up, including '**' for volume up, '*#' for volume down, and '*7' for shuffle.
The zero (0) key toggles play/pause.
When a command is received, we then need to tell Sonos what to do. For that I used the very nice library from @jishi called node-sonos-http-api. This library is a node web service that makes it super easy to control your Sonos. So, when the '1' key is pressed, I send a GET to this web service like http://localhost:5005/Office/favorite/Fav1.
Installed a camera inside the phone case and cut a thin slot in the side of the phone. Small, credit-card sized albums are now inserted into the slot where they are read by the camera. Once the album is recognized, the phone will play the album or playlist.