Unit 11: Bluetooth Speaker¶
A rechargeable stereo Bluetooth speaker in a wooden box we built, tuned to sound as good as a $75 store-bought one.
Sessions: 6–8 · Cost: ~$70–90 · Badges: Tiny Parts, Heat Manager (for the Li-ion safety talk), (new) 🔊 Sound Engineer Prerequisites: Units 7 and 9 (and 6 if we etch the carrier board)
What we're building¶
| Block | Choice | Why |
|---|---|---|
| Bluetooth receiver | ESP32-WROOM-32 devkit (the original ESP32) | Only the original ESP32 has Bluetooth Classic (A2DP). The S3, C3 and C6 can't do it. |
| Firmware | Phil Schatzmann's ESP32-A2DP + arduino-audio-tools |
A few lines of code; see firmware/speaker/speaker.ino |
| Amplifiers | 2 × MAX98357A I2S class-D breakouts (L and R) | A digital input means no DAC and no hiss; ~3 W into 4 Ω at 5 V each |
| Drivers | 2 × 2.5–3" full-range, 4 Ω (Dayton Audio ND65-4 / PC68-4 class) | Small, efficient, good midrange |
| Bass | 1 passive radiator (optional, v1.5) | How the small commercial speakers get their bass |
| Battery | 1S Li-ion 18650, protected cell, + a USB-C charger/protection board + a 5 V boost | Simple and safe |
| Enclosure | Plywood or MDF box, or a 3D print, ~0.5 L per driver sealed | The kids' woodworking project |
⚠️ Lithium safety¶
The parent handles bare cells.
- Use protected 18650s, or a board with DW01-style protection.
- Never short a cell: a short can start a fire.
- Charge on a non-flammable surface while someone is home.
- A puffy, dented, or hot cell goes in a metal can outside, then to battery recycling.
- Explain why to the kids. It's a great conversation about energy density.
Block diagram¶
flowchart LR
phone([📱 Phone]) -- Bluetooth A2DP --> esp[ESP32-WROOM-32]
esp -- "I2S: BCLK 26, LRCLK 25, DATA 22" --> ampL[MAX98357A<br/>LEFT]
esp -- I2S --> ampR[MAX98357A<br/>RIGHT]
ampL --> spkL((🔈 L))
ampR --> spkR((🔈 R))
usb[USB-C 5V] --> chg[Charger +<br/>protection]
chg <--> cell[(18650<br/>3.0–4.2 V)]
chg --> sw[Power switch] --> boost[5 V boost<br/>≥ 2 A]
boost --> esp
boost --> ampL
boost --> ampRHow it works (kid version)¶
- Your phone sends music as numbers over radio (Bluetooth).
- The ESP32 catches the numbers and passes them to the amplifier chips, 44,100 numbers per second for each ear.
- The amp chips flip the power on and off super fast, hundreds of thousands of times a second. The speaker cone is too heavy to follow the flipping, so it just follows the music. (That's "class D".)
- The box matters as much as the electronics. A speaker cone pushes air out the front and the back. Without a box, those two waves cancel each other and the bass vanishes.
How it works (grown-up version)¶
MAX98357A channel select. The SD_MODE pin voltage picks the channel (datasheet):
| SD_MODE voltage | Mode |
|---|---|
| > 1.4 V | Left |
| 0.77–1.4 V | Right |
| 0.16–0.77 V | (L+R)/2 |
| < 0.16 V | Shutdown |
- The chip has an internal 100 kΩ pulldown. The Adafruit breakout adds 1 MΩ to Vin, which gives 5 × 100/1100 = 0.45 V, so it defaults to mono (L+R)/2.
- Left amp: tie SD directly to Vin.
- Right amp: add 470 kΩ from SD to Vin. In parallel with the 1 MΩ that makes about 320 kΩ, giving 5 × 100/420 ≈ 1.19 V.
- Check your breakout's schematic. Clones differ.
Gain. GAIN pin: 9 dB is the default. 12 or 15 dB is louder but clips sooner at 5 V.
Power budget.
| Load | Current at 5 V |
|---|---|
| 2 × 3 W peaks | ~1.4 A (music averages far less) |
| ESP32 with BT | ~0.25 A |
At the cell, ÷ 0.85 boost efficiency × 5/3.6 V gives ~2.2 A peak. Use a boost rated ≥ 2 A continuous, and a good cell (e.g. a Samsung 35E or Molicel P28A). A 3000 mAh cell gives roughly 6–10 h at normal volume.
Enclosure. - Sealed is easiest and most forgiving. Aim for about 0.4–0.6 L internal volume per driver. - Verify with the chosen driver's Thiele-Small parameters in WinISD (free). It's a great 11 y.o. graph-reading exercise. - Line the box loosely with polyfill, and seal every joint (air leaks whistle). - A passive radiator (a weighted cone with no magnet) extends the bass. It has to be tuned to the box, again with WinISD.
Parts¶
| Qty | Part | Notes |
|---|---|---|
| 1 | ESP32-WROOM-32 devkit (e.g. ESP32-DevKitC) | Not the S3/C3/C6 |
| 2 | MAX98357A I2S amp breakout (Adafruit #3006 or clone) | |
| 2 | 2.5"–3" full-range driver, 4 Ω | Parts Express, Dayton Audio |
| 0–1 | Matching passive radiator | v1.5 |
| 1 | Protected 18650 + holder | Or a 1S pouch pack with its own protection |
| 1 | USB-C Li-ion charger + protection board (TP4056 + DW01 type) | Not the version without protection |
| 1 | 5 V boost converter, ≥ 2 A | |
| 1 | Latching power switch, status LED | |
| 1 | 470 kΩ resistor | Right-channel select |
| 1 | 1000 µF 10 V capacitor | Across 5 V at the amps: handles bass transients |
| — | 1/2" plywood or MDF, wood glue, polyfill, speaker grille cloth or metal mesh, rubber feet | |
| — | 3.5 mm aux jack (level-up) |
Jobs¶
| Step | 8 y.o. | 11 y.o. | Parent |
|---|---|---|---|
| Breadboard: ESP32 + one amp + one driver on the bench supply | Pairs the phone, picks the test song | Flashes the firmware, wires I2S | |
| Box design | Draws what it looks like | WinISD volume, then a cut list | Checks it |
| Box build | Sands, glues, finishes (paint, stain, stickers) | Measures, marks, glues | Saw cuts, driver holes (hole saw) |
| Electronics | Solders the switch, LED, speaker wires | Solders the amp headers, the SD resistor, power wiring | Battery and charger wiring |
| Tiny Parts badge | Watches | Solders the 470 kΩ as an 0805 on a breakout, or does the SOIC practice board | Coaches |
| Listening test | Blind test vs a store speaker | Records the comparison | Referee |
Steps¶
- Firmware first. Install the Arduino IDE with the ESP32 board package, plus the
ESP32-A2DPandarduino-audio-toolslibraries. Flashspeaker.ino. Pair the phone to "Workshop Speaker". - One channel on breadboard, from the bench supply at 5 V with a 1 A limit. Music!
- Add the right channel with the 470 kΩ select. Play a left/right test track to check.
- Box: design it, cut it, glue it. Test-fit the drivers. Seal it.
- Battery power: the parent wires the charger and cell. Then check:
- it charges from USB-C
- it runs from the battery
- the voltages at the boost output hold up under load at high volume (use the Unit 8 scope)
- Final assembly: hot glue or standoffs for the boards; polyfill; grille.
- Blind listening test against a store-bought speaker. Be honest in the build log!
Testing and troubleshooting¶
| Symptom | Likely cause | Check |
|---|---|---|
| Won't show up in Bluetooth | Wrong ESP32 variant (S3/C3) | It must be the original ESP32 |
| Both speakers play the same thing | SD_MODE not set per channel | Measure the SD pin voltage: ~5 V (L), ~1.2 V (R) |
| Crackle at high volume | Supply sagging | Scope the 5 V rail; add the 1000 µF; a bigger boost |
| Resets at loud bass | Boost current limit or cell protection trip | Better cell or boost; lower the gain |
| Thin sound, no bass | Air leak or polarity | Seal the joints; check both drivers are wired + to + |
| Hiss or whine | Ground loop from charging while playing | Normal-ish on cheap boosts; add an LC filter on 5 V |
Level-ups¶
- Passive radiator, tuned in WinISD. The difference is obvious in a before/after test.
- Aux input: the Unit 4 Punk Console plays through it!
- Battery gauge: read the cell voltage with the ESP32's ADC (through a 100k/100k divider) and blink the LED.
- v2, more power:
- a PCM5102A I2S DAC + a TPA3116D2 amp on a 3S pack, for 15–30 W per channel.
- The protection gets serious at this level: a 3S BMS and a balanced charger.
- Custom board: etch a carrier board for the ESP32 + amps (Unit 6 skills), or send it to a fab.
Talk about it¶
- Why does the speaker sound so much worse without its box?
- Where does the energy go when the music is loud? (Some to sound, a lot to heat. The amps are ~90 % efficient; the speaker cone only about 1 %!)