Recently, I have been building my own phone for fun!

Why?

I have been programming the operating system from scratch (well, not really. I cheated and I’m using the pico SDK now).

The goal was to build a RISC-V phone with all my own components for fun, since normal operating systems have become over bloated with unnecessary security protections (DEP, a kernel, etc). I wanted something that booted instantly, no separating of kernelmode and usermode, and ran in Machine Mode in RISC-V at all times.

I also believe that a phone should only be able to text, call, and play music over bluetooth. That is all it needs to do. So that is all my phone will do. Without many unnecessary features, I should see battery life of around 2 months per charge on a 3,500 mah 18650 battery.

Since then, I have replaced every single component at least 3 times, but I have been using this as a daily driver for the past month. Currently, this beautiful hunk of junk can text and play music to my bluetooth earbuds/speaker.

I used an eink display, since they do not require any power once an image is on. I used an SSD1306 OLED, since they are simple to work with and easy.

I used a 5v BMS and buck booster combo, but next time I will just use a BMS that does not have a booster. There is no need to hit 5v, as the sim card module can take in VBAT from a Lithium ion battery (3.3v - 4.2v). I then used a Pi Pico 2W in RISC-V mode, a typical SD card reader, and a CardKB keyboard. All of these parts are available online for about 80$ total.

So, No calls. No What?

The current LTE module (Sim 7670g) does not have any working breakout boards that expose audio. Luckily, I downloaded some footprints form JLCPCB and messaged SIMCOM, and they gave me a datasheet. Testing with an osciliscope shows audio is actually functional without a DAC.

(This was so annoying I designed a whole DAC in KiCAD just to find out I no longer need it).

My voice during phone call:

Setup to make this possible:

So, I spent the last two months learning circuits enough to develop my own board in KiCad that exposes these two pins.

It is now getting printed! I really hope it works :)!

What next?

The next update will occur when I actually have a case, the board works, and it’s pretty much ready to go as a normal phone. That should be achievable in ~4 months?

I also cannot receive any MMS messages, which I will add later. Battery life is only around 3 days as of now, as I have not added any of my mosfet’s to toggle off systems.

Open source?

Probably. If anyone actually wants to use this other than me lol. The OS was all written by scratch from me, without AI usage (except for in the BT driver a bit, it’s hard ok).

If anyone actually wants this or asks for it, I can give way more info, such as the datasheet, pin location, and more. This breakout board I am building will actually be extremely useful. It will cost about 35$ per board with components and be able to make phone calls over the current network!