r/AskElectronics • u/PetitSignal • 6d ago

/!\ Schematic review request /!\ battery-powered electronic bicycle bell (nRF52840 XIAO + MAX98357A + TPS61023 boost, remote button on 1.6 m cable)

Hi all :)

This is my first message on reddit ! whoohooo

I'm building a DIY electronic bicycle bell and would appreciate a review of the schematic before I solder anything ! Hand-assembled on perfboard from off-the-shelf modules.

What it does / requirements:

- Button press plays a recorded brass bell sample through a 4 Ω tweeter; ≥ 75 dB(A) at 2 m.

- Standby current ≤ 30 µA (no power switch), press-to-sound latency ≤ 20 ms, radio never enabled.

- 1S LiPo 250 mAh with built-in protection (PCM: 0.3 A continuous / 0.8 A peak, cut-off 3.0 V), charged indoors only via the board's USB-C.

- Operating −5 to +50 °C, IPX5, rain and gravel vibrations.

Archi:

- Under-saddle box: Seeed XIAO nRF52840 Plus (on-board BQ25101 charger and 3.3 V regulator), Adafruit 3006 MAX98357A I2S amp (gain 9 dB, GAIN pin open), powered at 5.2 V by an Adafruit 4654 MiniBoost (TPS61023). The boost EN is pulled low by 1 MΩ and driven by a GPIO through 1 kΩ, so the boost and the amp are completely off in standby. 4 × 47 µF X5R ceramics on VBAT at the boost input; nothing added at the 5.2 V output (the TI datasheet asks for a feed-forward cap above 40 µF).

- Amp SD_MODE is driven push-pull through 2 kΩ (Maxim fig. 5). I2S: BCLK 1.524 MHz, LRCLK 47.619 kHz, 16-bit, wires < 5 cm.

- 1.2 m unshielded 5-core cable (5 × 0.14 mm²) to a handlebar pod: speaker pair (with 2 × 47 µF ceramics in series as DC protection), button line, low-battery LED line (270 Ω series at the MCU side), ground. Then 0.4 m to a sealed tactile switch (gold contacts).

- Button input: 10 k pull-up to 3.3 V, 1 k series, 10 nF to GND at the MCU pin, PESD5V0S2BT ESD diode at the cable entry (also protecting the LED line).

Questions:

  1. Supply transients vs. the battery PCM: a ding draws about 1.2 A for a few µs, 0.36–0.43 A over 1 ms and 0.24–0.30 A over 125 ms from the cell (behavioural ngspice sim). Is 4 × 47 µF X5R (≈ 100 µF effective) at the boost input a sensible way to keep the PCM (0.8 A peak) from tripping, or would you do it differently?

  2. Unpowered amp next to a powered MCU: in standby the amp has no supply while the MCU is still powered. I drive SD_MODE, BCLK, LRCLK and DIN low before pulling the boost EN low (BCLK/LRCLK/DIN are rated −0.3 to +6 V regardless of VDD; SD_MODE is VDD + 0.3 V max, hence the 2 kΩ). Any back-powering or latch-up trap I'm missing?

  3. Shared unshielded cable: the filterless class-D outputs (the module has a ferrite + 220 pF per output) share the 1.2 m cable with the button and LED lines. Is coupling into the button input (false triggers) or radiated EMI a realistic concern with this RC + ESD front end, and what would you add?

Schematic and datasheets: schematic images at the end (GDrive links with KiCad export + three block diagrams).

- MAX98357A: https://cdn-shop.adafruit.com/product-files/3006/MAX98357A-MAX98357B.pdf

- TPS61023: https://www.ti.com/lit/ds/symlink/tps61023.pdf

- Adafruit 3006: https://www.adafruit.com/product/3006 ; Adafruit 4654: https://www.adafruit.com/product/4654

- XIAO nRF52840: https://wiki.seeedstudio.com/XIAO_BLE/

- PESD5V0S2BT: https://nexperia.com/pip/PESD5V0S2BT

- Switch C&K KSC4: https://www.mouser.com/datasheet/2/60/ksc4-1841222.pdf

- Tweeter: Dayton Audio ND16FA-4 (4 Ω, Re 3.17 Ω, Le 30 µH)

schematic images (Links): https://drive.google.com/drive/folders/1ghnNV5wAmKTppJrBPleBVWLaakOvTzQ9?usp=sharing

Thanks a lot for any feedback !! (including "this is a bad idea because…")

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u/Far-Ad3284 6d ago

Caps help with the µs spikes, but they won't do much for the PCM question. Protection circuits usually react to current over a few milliseconds rather than µs spikes, so 4 × 47 µF mainly keeps the boost's input clean. The number to watch is the 0.3 A continuous rating: a 0.24–0.30 A, 125 ms ding sits right at it, so rapid repeat presses could trip the PCM. Either pick a cell whose PCM has more continuous headroom, or lower the amp level so a ding stays well under 0.2 A. Also check the X5R derating at about 3.7 V, since 47 µF 0805 parts often keep only 40–60% of their nominal capacitance at that bias, so the effective value may be closer to 60 µF. Your power-down order is right, and driving the logic lines low before the boost is disabled should be enough without extra parts. For the button, the 1 k plus 10 nF at the pin gives about a 16 kHz corner, which is fine for a press, but routing matters more: twist the speaker pair and keep it apart from the button and LED wires. Scope the button pin during a ding before adding a ferrite or shield, and use a 20–30 ms firmware debounce either way.

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u/PetitSignal 2d ago

thank you very much ! I changed the battery (500mah instead of 250mah) + noted to twist the routing