Fm Transmitter
USB FM Transmitter
What am I making?
My plan is to make a small device that plugs into your phone or computer and then can stream audio from that computer over FM over a short distance (1-2 meters) essentially allowing you to create your own mini radio station. Devices like these already exist and are mainly used to stream songs from your own device over to the car radio. My goal with this project is to create something similar and make it as plug and play as possible.
Goals for this project
- [ ] Low Cost PCB
- [ ] Tiny USB Drive form factor
- [ ] 1-2 Meters of Range
- [ ] USB C (if not too expensive)
- [ ] Programmable (set frequency,gain etc)
Time Spent Overall: 19 Hours
May 20th - Initial Research
Well this project seems a lot simpler than I expect it to be, all I really have to do is stream audio from a DAC over to the FM TX Chip and that's basically it. In order to tune the TX chip's frequency and other parameters I need to send data over I2C to it, which I think would be better to just send those commands from off board (like from an arduino or any other mcu). As I'm aiming for a low cost there isn't really a need to have a mcu on board, especially not for version 1.
After a few hours of research and going through datasheet I decided to go with this stack- FM Transmitter: KT0803L This is an overall good chip, its a low power FM transmitter that takes in analog audio. It can be controlled by writing to its memory over I2C. USB Audio Chip: PCM2704C Acts as a USB audio device, has stereo analog output which can be used by our transmitter. No firmware needed
And well that's basically it for version 1. Just two little chips!!
My current plan is to have a really really basic version 1 with USB A, and then look at what I did right and what I did wrong. Then create version 2 with a mcu which can be programmed over usb, and look at what I did right and what I did wrong. And finally, for version 3 fix all the previous mistakes and add USB-C to it.
Time Spent: 2 Hours
May 21st - Schematic Finished
Today I printed out the datasheet for the KT0803L and the PCM2704C and read them and researched a bit more. I made a schematic for the TX chip using the example schematic provided in its datasheet. I did the almost the same thing for the PCM2704 Chip, but its example circuit sucked (you had to read the values from like a paragraph of text) but luckily somebody made a project using it so I follow that schematic. It was pretty much the same except you could actually see the component value in the schematic (link to that project).
NOOO, I just realized that the transmitter chip (KT0803L) is not in stock on jlcpcb or lcsc. Also I just uploaded my unfinished pcb to jlcpcb to get a rough estimate of how much it would cost and its a more than what I initially anticipated. It costs around around $60 CAD to get 2 PCBs assembled, which I feel like is way too expensive for such a simple project.
I think instead of getting them assembled from jlcpcb I'll solder them myself, this would be my first time ever reflow soldering a board. Its gonna be fun!!
Schematic that I made today-
Time Spent: 5 Hours
May 22 - PCB Design
Since I am now gonna be soldering the board on my own I decided to use 0805 footprint for the capacitors and resistors. I've also decided to now just source my parts from aliexpress, its more expensive that parts from jlcpcb but hey I'm saving a ton of money by not using PCBA.
Here are some changes I did in my design: - Changed the LDO, now using a MIC5205 - Added an ESD protection module on board - Changed up a lot of footprints - Now using a 12Mhz crystal for both the audio chip and USB chip
I made a rough PCB layout to sort of picture how I'm gonna place the components, and I did not like the layout I made
As you see in the images above its a pretty 'Chonky' board. Its not even that big its just Chonky and wide which I don't like. Plus there's a lot of empty space on the PCB which means the design can be compacted more. Which is exactly what I'm gonna do.
Time Spent: 5 Hours
May 23 - Finished Version 1!!
Making a rough PCB layout yesterday really helped me visualize how I was gonna fit everything together on a board. To actually figure out how my new layout should look like I drew a layout on paper (not to scale, just eyeballed the sizes)
After this I actually started on the new layout, It took quite a while to fit everything properly into a small board. Then after I was finally happy with the layout I started routing traces, the routing was very sketchy
is basically all I have to say.
3D Render of the PCB (The 3D models are a bit messed up and not the correct size)
Time Spent: 4 Hours
May 24 - BOM
I finished my final BOM today!! All the components and tools are gonna be ordered from aliexpress. My overall cost for this project is coming to be around 100 USD. Which I know is pretty expensive but a big portion for this is the PCB shipping price which is probably around 17% of the budget. I think I probably spent about 80% of the time today just on finding cheap parts from reputable sellers on aliexpress, I got most of my items for pretty cheap with free shipping. Only the main two chips (PCM2704 and KT0803L) had shipping fees, but It was not too overpriced so it was alright.
I also wrote a readme for the project today, formatted everything nicely, added source files in the main repo
Looking back at my initial goals I think I pretty much achieved everything I wanted except for the USB C part but its alright because soldering USB C would be practically impossible for me to do on my own and getting it fabricated would be super expensive.