Introduction: The Unthinkable Portable Speaker(Under 35€)

About: Hello everyone! I Ser S. 19 Y/o. I'm a college student in electronics. I spend most of my time working on my projects, and when not, I'm always in good company with friends that help me!. Always loved to tear …

Hello everybody!! In this instructable I'm going to show you how to build a cheap but loud enough portable speaker.All the parts involved can be easily obtained through internet shops or in your local store. Here are the main specs;

  • Bluetooth 4.0 connectivity.
  • Battery runs for 10 hours at 90dB.
  • Decent audio power output 25+25 W.
  • Universal 19v charger (PSU).
  • 2 4"speakers.
  • Very compact compared for its sound output.

Price ~

  • less than 5€ for the battery.
  • 9.80€ pair of speakers.
  • 10€ for the amplifier (this may vary depending on your likes)
  • 6.70€ for a pair of passive radiators (You can buy only one passive radiator if your speaker doesn't have a high excursion *Xmax)
  • less than 4€ on electronic components (However, I had everything laying around)

Step 1: Gathering Tools & Materials Needed

Here are all the parts you will need. I have choosen Aliexpress, but you can order from Amazon, ebay and so on.


  • 4S Battery protection board
  • Class D amplifier w/ built in BT conectivity(2*25w. TDA7492P)
  • A special charger for 4s betteries (Or later on, we will be making one with the an LM317)

-LM317 charger

1-Diode 1N4007
1- 240 ohms resistor 1/2W.
1- Variable/Adjustable resistor in the 5K ohm range
1- 1000uF electrolitic capacitor
1-100nF ceramic capacitor
1- 5 ohm resistor 1W


  • Batteries (If you buy them from china, i recommend Panasonic or Samsungs 2700mAh to 3500mAh.ones (Dont be fooled by the 9800mAh capacity some sellers argue)
  • A power supply which has to be ~18.5v up. (Also you can stick to any laptop charger lying around, or buy one)
  • A power jack, a switch, and nice-looking buttons (If desired)


When choosing a speaker, there are some technical specs. you may take care of. First, the power handling, both RMS and peak. However the RMS value is the important one .With the amp I am using, a 10w RMS or 15w RMS is more than enough. Also be aware of the sensitivity, do not get a speaker which has less than 85dB 1watt/1meter, because in that case, you would need a lot of power (meaning less battery life) to achieve same performance.

-Tools, materials And equipment

If you are experienced with electronics, you problably have all of these lying around.

  • Soldering iron.
  • Drill.
  • A rotary tool will come in handy.
  • Sandpaper.
  • Wire stripper
  • Hot glue gun.
  • A tester will come in handy.
  • Glue.
  • Scissors.
  • Tape will come in handy too.

Step 2: Prepare the Electronics

You would want to organise the stuff you are working with, so lay down everything on your table. At this point you should do some detailed research about what you have on your hands. For example; make sure all your batteries are ok and charged at 3.4v to 4.2v. Also it is the right moment to hook up the amp (with an external power supply, not the battery) and write down the average current it draws(at 16v 4ohms in this case) so we can afterwards calculate the average battery life. Another great idea is to test the bluetooth conection between your phone and the amplifier.

-How to calculate the battery life-

Let's say we have a battery rated with 12000mAh which are 12Ah and our device draws 500mA. With the formula: Battery capacity [mAh.] / Current consuption[mA.]

So 12000 mAh. divided by 500 mA. makes 24h. This means our battery will last 24hours ,in the greatest case, before we need to charge it. Note: In audio, there will be never a constant current draw as it highly depends on the music.

Step 3: Making the 4-cell Battery

    For this particular amp, the manufacturer recommends 15V. to get the max output at 4 ohms. Thus not to exceed 16v. I have chosen to build 4 batteries in series so I will be able to get 16.8v (4 * 4.2V.) fully charged and around 15.5 while working.

    First we need to make the solder stick to the battery. In order to do that , scratch with some sandpaper the surface of the cells and then clean the dust made with a cloth. Then we are going to connect all of them in series positive to negative and so on until we reach the 16v.

    The following step is to solder the respectives wires from the batteries to the protection circuit as shown in the diagram. Make sure the B+ is facing towards the positive side of the cell and the B- is facing to the negative side respectively.

    Finally wrap up the battery with some tape and ensure it is secured in place. The battery is now finished.


    Lithium batteries are dangerous when misused. Be very conscious about what you are doing and never leave them unattend, especially at charging process.Also be careful not to exceed heat when soldering them together.

    Step 4: Prepare Your Box (Enclosure)

    I know there are great box calculation software out there. Nevertheless, while we are building a portable speaker, we need to fit all the elements as tight as possible. If you want to have calculated boxes and deep powerful bass , you should have a look at custom made boxes.

    When building portable speakers, the most common box type is the sealed one. In this one, air is kept inside the box and cant move in and out. The second one in portable sytems is having passive radiators. This one must be also sealed but the radiator will act as another speaker, enhacing the low frequencies.

    After having all the electronics working, find a small but rigid box. This can be made of wood or plastic. Take care where are you sitting the passive radiators so they dont touch anything, even a wire counts.

    Then measure the speaker and passive radiators and mark their positions. Then add the rest. When having everything layed down, cut the necessary holes (Speakers, passive radiatos, power source, screws, etc.).

    Step 5: Sanding

    I think this step is self explainatory ...

    Step 6: Mount Speakers, Passive Radiators and Miscellaneous.

    The next step is to fit in place the speakers and secure them airtight with hot glue(or any silicone glue).If you are doing the sealed box, I recommend using silicone.

    Then, mount the 6 buttons(PD: At the end, I didnt mount the buttons becuase I wanted only the bluetooth conection), the power jack, switches and attach the amplifier. Check over the seal is airtight, especially on the power jack (some aren't sealed)

    Step 7: Painting and Finish.

    When building a good speaker , the finish is the key to succeess in speaker boxes. With that being said, go ahead grab your sanding paper and work your way up until it is nice and smooth. Remember to end with a high density sanding paper, just before the paint.

    I went with white paint. Clean, shiny and seems professional ,tho requires about 3 layers to get its look.

    If you are looking for a clean edge and dont want to mess, stick some tape on the wood where you dont want the paint. Once the paint has dried, remove the tape and it should end with a perfect straight line.

    For the front view, where the speakers are mounted, I decided to wrap it in some kind of black rubber foam as it makes a good contrast with the red speakers.

    Step 8: Charging Circuitry & Final Assembly

    The final step is to make the charging circuitry. We all know batteries need a special charger and li-ion (or Lipo) are different from Lead acid or Ni-Cad. Li ion cells are fully charged when they reach 4.2v. In order to achieve this, we could simply hook them to a voltage regulator set to 4.2v and let them charge. However, when the cells reaches 4.2v there will be still a current flowing to the cell and may explode due to overcharging. To avoid the cells overcharging, we need to control the current flow in this way; The more voltage the cell has, the less current it recieves, so when the cell is at 4.2 the current will be barely 1mA.

    Component list

    • C1= Electrolitic capacitor 1000uF
    • C2= ceramic capacitor 100nF
    • D1= Universal diode 1N4007
    • T1= BC548 or BC547 or any equivalent.
    • R1= 240 ohm.

    Rv and Rc must be calculated as those values may vary depending on your battery.

    Rv controls the voltage going to the cell(s) so it is a good idea to use a potenciometer or an adjustable resistor when developing the circuit. The voltage target must coincide with the cells at max charge (4.2v. For example to charge one cell, set the voltage to 4.2v and the circuit will do the rest. However, If the cells are charged full, they will have less operation life. So it is a good idea to charge them to 4.1v or less and extend its life. In my case, a 4s batt. , I'm setting the voltage to around 16.3v (4.1v * 4=16.4v) The desired resistence turned out to be 3.3K ohms.(remeber to set the voltage without connecting the cells)

    Rc controls the current flow and drops it when the cells are fully charged. Calculating Rc is a lot easier than Rv and it is independient to voltage. The resistance is given by the formula Rc=0.95/max. curren. To be safe, he charging current should be 25% of the total battery capacity. For example, For a 1Ah. cell, 250mAh is fine. I didnt want to stress my betteries so I went with 200mA current and Rc resulted in 5 ohms.

    Once you gathered all the electronic components, It is time to grab a perforated pcb and solder them. Double check the polarity of the capacitor, diode and the pin out conections of the transistor. Remember: Input ground and output ground aren't shared, they carry different current , do not bind them.

    The input voltage (unregulated) must not exceed 25V,because the lm317 may get very hot (and surely it will.)That is why is a very good idea to use a laptop charger (they are around 19v.)

    Finally, attach a large plate of aluminum (or anything that disipates heat well). Glue the circuit to the box and connect its input to the jack and the output directly to the cell. Then connect ground wire to the amplifier and do the same for the positive and solder a swith in between. Check it works and assemble the whole thing together. Now you are ready to rock!

    I reccommend not using the speaker while it is charging to avoid stressing the charging circuitry.

    Step 9: Improvements & Final Thoughts

    Homemade boomboxes usually outperfom comercial ones and this one surely it does for the price. Nevertheless, this one could improve. For instance, adding to the speaker a grill or speaker cloths. Also, the box could have been designed more wisely to accomplish greater bass reproduction. Adding more cells to the battery would improve its play life and making a smaller box would help with the form factor, because in the end, this is just a speaker you can carry. If you have the time patience and the equipment needed to calculate the box dimensions, go ahead, you wont regret. And yes, I'm forgiving about the aux conecction :p

    No matter what, this is an easy proyect and very satisfactory which everyone can put through. Go ahead and make!

    To end this project, I hope it has inspired you to make something ,or at least, you've learned something new :).Have a good one!

    "An expert is a person who has made all the mistakes that can be made in a very narrow field"

    -Niels Bohr