Update:
…I went on a tangent… a deep one… Like I have a bad habit of doing
Long story about the escalation from a simple start following
bms hasn’t progressed much, so what have I been doing?Well record scratch… It all started when a colleague couple weeks ago went to go eat at a local mall on his lunch break. He then popped by an electronics/hobby/hardware/misc store (Clas Ohlson) on his way back, because they had a deal going there. For ONE euro you could buy this small, simple and cute DIY bluetooth speaker kit (shown below). He bought 5.
https://images.clasohlson.com/medias/sys_master/9630741168158.jpg
https://images.clasohlson.com/medias/sys_master/9630741299230.jpg
He bought one for the office and I’m sure I looked like an excited manchild when I was assembling it (we were all making good jokes and times about it). I thought it was pretty dope, as a concept and for the price one couldn’t complain. You could power it via USB or 3 AA batteries.
Here it starts… The sound was quite tinny, no doubt in large part for the cardboard case having more holes in it than a traffic sign in Alabama. Ok, I can make a better box for it to improve the low end. Hmm… 3 AA batteries should actually be operating pretty near lithium cell voltages, let me just try it with a single 18650 cell, oh it works no problem. How much power does it use?, does some current use measurements oh this thing should last like at least 15+ hours on a single cell with decent volume… This could actually be made into a pretty sweet portable boombox with some effort…
I even looked up the ICs on the control board. Bluetooth comms were handled by Beken BK3254 ( https://oss.aliyuncs.com/netmarket/29bc7cac-8266-45bf-b2b0-f152227ba546.pdf ) and the 2 drivers were then driven by 2 MIX2052 mono channel class D amplifiers ( http://www.pluschiptech.com/web/userfiles/download/MIX2052_Prilimineary_datasheet_V1.0.pdf )
After looking at how simple the class D amp circuit was, I went onto Digi-Key to check what they had. There are many low-power class D amp ICs that can work off of a single lithium cell. Hmm… I have always been interested in getting into some audio stuff, but it has always seemed just a bit too complicated to get started, but with a single IC and just 5 external components I could have the equivalent amplification as on the original board… So I then started designing my first audio amp board… There was also some escalation on the design too and in the end it ended up getting a op-amp low-pass circuit in front of the audio amp (haven’t really worked with op-amps before, so I saw this as a good opportunity to start tinkering with them). I also LTspice simulated the low-pass behavior.
But of course, things must escalate more… I then decided that I want to charge the 1S lithium cell battery via USB and it should be protected against overdischarge, so I then started designing a single cell BMS… Which ended coming out like this.
So… This 1 euro bluetooth speaker has already been escalated to 2 custom PCBs… Well we can go further than that! I then spent couple next days designing a new enclosure that fit all the original parts.
I then spent couple days 3d-printing the parts and shown below is the assembled package.
There was still a small problem, it was still sounding pretty tinny (all original parts still). Hmm… I remember that Digi-Key does have some decent Peerless speakers… I guess I’ll just have little look… Oh those ones are quite cheap (7,5 € ea.), how will they work in a small enclosure though… Guess I’ll boot-up Winisd and simulate their behavior with the speaker parameters… ( http://www.loudspeakerdatabase.com/Peerless/PLS-65F25AL04-04 ). I also spent quite a lot of time just reading about speaker parameters, trying to understand their impact on the speaker behavior in different enclosures and testing my expectations through simulation.
Oh these ones could go decently low in quite a small volume vented enclosure… Guess I’ll order couple of those! After looking at the first enclosure for some time I decided I wanted to make a better one. I also needed to optimize the printing process at the same time. I approached the new enclosure from a different perspective.
And you know what. I’m pretty darn happy with it at the moment and I think it’s pretty cool.
It has good low-end for being a very portable speaker and is quite loud. Not gonna burst ear drums, but loud enough for indoor use. Once I get my custom amp boards I can tame the top-end a little bit down. But only original part left in the speaker is the bluetooth and amp PCB + speaker wires. I even changed the volume knob.
But this was my escalation story about how a “1” euro cardboard bluetooth speaker kit turned into tens of hours of development, studying, simulation and design hours… But I feel content and satisfied… for now…
TLDR: I got captivated by a 1 euro bluetooth speaker kit and spent tens of hours making “improvements” to it.
On BMS end of things… The only real thing left to do is to test the CAN bus. Opto-isolator has been tuned and tested to work from 2.8 - 50 V control signals without leaving the front resistor dissipate any crazy amounts of heat.
I’m gonna write the testing CAN libraries on the weekend and then test the comms in the office with a PC CAN adapter.
We are essentially on the final straight in the development for the 0.1 HW iteration. I have quite a list of bullet points collected on things that need changing to 0.2 and I will start implementing them once the CAN is tested and confirmed working.