Power board v1

N-Pulse Workshop Board

A teaching board for the N-Pulse electronics workshop: an ESP32-S3 platform with a complete battery power path, four sensors, and a heart-rate (PPG) front end — designed so that new members can learn hardware design by reading a real, working board rather than a toy example.

What this board is for

Most beginner projects are one circuit. This board is deliberately the opposite: it puts one worked example of nearly every subsystem you meet in real hardware onto a single PCB, so that "how do I charge a battery?", "how do I protect a USB port?" or "how do I amplify a microvolt sensor?" all have an answer you can hold in your hand and probe with a scope.

It is a reference, not a first project. Read it, measure it, copy from it.

Every design decision is documented in DESIGN_NOTES.md, which also contains general guidance on how to draw a good schematic and how to lay out a good PCB.

Features

Power

• USB-C input with ESD protection and a TVS on VBUS
• MCP73833 Li-ion charger (1 A) with charge / done / power-good indicators
• BQ29728 cell protection with back-to-back MOSFETs
• TPS631000 buck-boost — holds 3.3 V across the whole 4.2 → 3.0 V cell range
• TPS2116 priority mux — runs from USB when present, battery otherwise
• TPS2068 load switch — firmware can cut power to the peripherals
• Two rails: +3V3_ESP32 (always on) and +3V3 (switched)

Sensing

• SHT40 — humidity and temperature (I²C)
• LIS2DW12 — 3-axis accelerometer with interrupts, on the always-on rail so the board can wake on motion
• MCP9700A — analog temperature
• PPG pulse sensor — LED + photodiode into a two-stage bandpass amplifier (0.34–5 Hz, gain ×121)

Interfaces

• ESP32-S3-WROOM-1, programmed over native USB — no bridge chip, no drivers
• microSD card slot with mechanical card detect
• Two RGB LEDs and a piezo buzzer
• I²C expansion header
• 17 labelled test points

Physical: 70 × 80 mm, 2-layer, 142 components, four M4 mounting holes.

Using the board

Powering it. Plug in USB-C and it runs, regardless of the ON/OFF switch. On battery, slide the ON/OFF switch (bottom edge, marked ON/OFF). With USB connected the board runs from USB and charges the cell at the same time.

Charging. The three LEDs on the left edge are, top to bottom, CHRG (charging), DONE (complete) and PG (USB power good).

Programming. Connect USB-C — the board enumerates directly as an ESP32-S3. Normally you can just flash it. If you need to force download mode: slide the BOOT0 switch (right edge) to the boot position, press ENABLE to reset, flash, then slide it back and reset again. The LED beside the switch shows its position.

Reading your pulse. Rest a fingertip on the fingerprint symbol in the middle of the board, covering the LED and photodiode, with light steady pressure. The amplifier needs a second or two to settle. Signal quality depends a lot on contact pressure and ambient light — that is part of what makes it a good teaching circuit.

I²C expansion. The 4-pin header on the right edge is 3V3 / SDA / SCL / GND (labelled on the silkscreen). It is ESD-protected and shares the bus with the on-board sensors, so remember the address space is shared.

Test points. Every rail and every interesting node is brought out: +5V USB, 3V3 USB, 3V3 BATT, 3V3 ESP32, 3V3, +BATT, -BATT, GND, plus FB, PR1, MODE and PWR SW for the power circuits.

Opening the project

Built with KiCad 9/10. Clone and open Electronics_Workwhop_PCB.kicad_pro.

All custom symbols and footprints live in the repository — N_pulse.pretty/ for footprints and Library/ for vendor symbol libraries — so the project should open with no missing parts. Nothing needs to be installed globally.

Electronics_Workwhop_PCB.kicad_pro   project file — open this
├── Architecture.kicad_sch           top-level sheet
├── ESP32.kicad_sch                  MCU, SD card, LEDs, buzzer, buttons
├── Power_management.kicad_sch       USB-C, charger, protection, regulators
├── Sensors.kicad_sch                SHT40, accelerometer, temperature, PPG
└── Electronics_Workwhop_PCB.kicad_pcb
DESIGN_NOTES.md                      why the circuits look the way they do
docs/                                images and documentation assets

Pin map

See Appendix A of DESIGN_NOTES.md for the full table, generated from the netlist.

The one rule worth memorising: analog inputs must be on ADC1 (IO1–IO10). ADC2 cannot be read while Wi-Fi is running.

Contributing

KiCad files cannot be merged. If two people edit the same sheet at the same time, git produces a conflict that no one can resolve by hand.

Before you start work, say which sheet you are taking, and pull before you open the project. Commit with a message describing the change, not just "update".

The pre-fabrication checklist in Appendix B of DESIGN_NOTES.md should be run before any board is ordered.

"What I cannot create, I do not understand." — Richard Feynman

(also printed on the board)