Mobile Speaker System S.A.R.A.H.

Super Awesome Rectangular Audio Hardware

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As an outdoor fan, especially camping and alpine huts, and a fan of music I sometimes face the problem of the lack of electrical power supplies.
The limitation of Bluetooth speakers regarding power, battery life and extra functionalities have brought me to the idea of (over-)engineering my own audio hardware.
The two planned operation sites will be a music festival and a alpine hut weekend. Both without electricity, higher volume and rough environment.

The main requirements:
- Portable
- Long battery life (>15 hrs)
- Strong housing
- High connectivity for music inputs (USB, Aux cable, Bluetooth)
- Decent maximum volume
- Budget around 150 EUR
- Additional features could include: USB outlets, lighting for tent/hut, FM radio reception


The housing was planned in OpenSCAD. It is 50 cm wide, 40 cm high and 35 cm deep.

It was made with plywood (10 mm) and has 6 outer boards and one internal board to seperate the batteries from the rest of the electronics. All inner edges are stabilized with 20x20 mm bars.

The result is somewhat heavy, but very robust! This fullfills some requirements: Made for rough enviroment and cheap.

I did not make this housing myself, but let my talented hobby-carpenter father do this (he also has better tools for woodworking than I).

The OpenSCAD files are also on GIT.


After deciding for the quick and cheap car radio solution, I scetched up some schematics with Visio:

This covers the important parts of the schematics:

  • Battery packs with fuses
  • Charge connectors with balance contacts
  • Main switch with power LED
  • Volt and Ampere meter
  • Small uC (MSP430) for a very simplified battery voltage check (I know, you should monitor every cell in series, but meh, I switch off at a fairly high voltage)
  • Status LED
  • Relay
  • Car radio with speaker
  • 5 V DC/DC for USB Connectors


The software for the MSP430F2274. I had a really small board laying around. The fairly short source code can be found in my bitbucket:


Now to the fun part, assembly! Just look at the pictures in the galery.

Open Items

  • Under Car Illumination
  • Paint job
  • Wheels (this thing is heavy!) --> Done
  • DC/DC for USB chargers --> Done


Mount for screw joints to connect battery pack poles

scad - 650.00 bytes - 05/10/2017 at 19:46


USB connector plate.scad

Small plate to fit the USB connector to a round hole

scad - 323.00 bytes - 05/10/2017 at 19:46



Complete Box in OpenSCAD

scad - 7.56 kB - 05/10/2017 at 19:45


  • 1 × Car Radio with Bluetooth JVC KD-X341BT
  • 2 × Speaker 16 cm - double cone JVC CS-V618
  • 2 × Speaker - old Found in parents storage - free!
  • 3 × Battery Pack of 18650 Li-Po Cells Each 15p4s
  • 1 × Housing Made of plywood

View all 13 components

  • Project Finished! (for the time being)

    Doubleyou07/23/2017 at 11:29 0 comments

    First of all, I added a BOM with links in the GIT repo. I landed below the goal of 150 Euro, but mostly because a lot of stuff was free or had already been lying around my workshop for years.

    As the two planned events for which SARAH was made came, she had proven herself worthy. The rough circumstances of a music festival did not do any harm to her. Battery capacity is a total overkill. From fully charged (16.8 V) to about 15.8 V after the festival, which is still well over the nominal voltage. Battery spec lists about 10 V as minimum, I programmed the MCU to switch of at 12 V.

    As there are no more events for SARAH planned this year, I will mark the project as finished. Maybe next year, before the next festival, I add things like LED lights and stuff.

    It was a fun little project. SARAH was very valuable on the two events! The best thing is that she became a humanized part of our group, everybody knows who SARAH is. :)

  • USB Connectors

    Doubleyou07/05/2017 at 21:18 0 comments

    One more of the targeted features is now done and working: The USB connectors.

    I used a configuration which should allow android systems to charge with a higher then standard current (up to 2.1 A depending on the cable). This means D- and D+ are shorted and on a voltage of 1.2 V (10k and 33k resistors).

    On the inside I used a USB cable usually used in PCs. Since these are meant to be mounted on thin metal, i had to drill a pretty large holes so that the USB cables fit. I 3D printed two plates to close the holes.

    The DC/DC is mounted next to the rest of the electronics, shown in the picture below.

  • The wheels on the box...

    Doubleyou06/15/2017 at 17:31 0 comments

    hopefully make my life easier!

    Since my experience at the Hack 'n Tell at the Munich CCC I know that the thing might be a little to heavy for longer transport with muscle power (alone). So I added wheels!

    2 standard wheels and 2 with integrated brakes my dad had lying around:

    Which gives me a new idea: under car illumination!

    I have some water resistant RGB LED stripes lying around, so next step including these with some nice color changes. This is now an absolutly important requirement for SARAH!

    Another add is an FM antenna for the radio:

    Also I added a DC/DC Converter for the car radio, I will update the description and schematics soon.

  • Skip logs, updated description and details

    Doubleyou05/10/2017 at 19:52 0 comments

    I was a little lazy with keeping up with the logs. A added a lot of details, photos and files!

    For the remaining open points I hope to remember to write build logs ;)

  • Design decisions

    Doubleyou05/02/2017 at 12:35 0 comments

    After collecting the above requirements, i started to plan on how to achieve these goals:


    It should be small enough, so one person can carry it. Since one destination will be a music festival, also two people carrying it alongside other stuff can be possible. So not wider than 50 cm and with handles on each side.

    Long battery life (>15 hrs)

    I do not have that much experience in scaling battery size. Therefore i will go with, the more the merrier.
    The budget limit would be very problematic, but fortunately i have several big battery packs from a dismantled Formula Student race car. More on that later., but it saved me the big bucks in Li-Po batteries.

    Strong housing

    Since my father is a hobby-carpenter and my girlfriend a hobby-painter, I outsourced the housing to him and painting the box at the end to her. I will however plan everything in CAD before starting any construction.

    High connectivity (USB, Aux cable, Bluetooth), Decent maximum volume, Budget around 150 EUR

    These together have puzzled me for some time. I made the conclusion that i good compromise will be a car radio. This way i do not have to build everything from scratch (no experience in audio amplifier construction).

    A nice new car radio with two or four speaker should be enough to cover all three requirements.

    Additional features could include: USB outlets, lighting for tent/hut, FM radio reception

    Throwing in some 5 V converter, a connector for some sort of LED lamp and an antenna for the car radio will not be that hard.

    That should be enough to get some parts and start drawing some schematics!

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