Devotchi

Hardware Design

PCB

Device Variants

Portable WiFi+BLE (current)

• ESP32-S3-MINI-1 with integrated PCB antenna
• HS20HS072RX 2.0" color TFT LCD (ST7789, 320x240, IPS)
• LiPo battery, USB-C charging
• 3 buttons (approve, reject, action)
• Status LED, vibration motor
• Connects to server via WiFi, or to phone via BLE

Portable Cellular (planned)

• Same form factor as WiFi+BLE version
• Additional cellular modem (MCU TBD — may use same ESP32-S3 + SIM module, or different MCU)
• SIM/eSIM support (provider TBD)
• Works standalone anywhere with cell coverage

Desktop (planned)

• USB-powered (no battery needed)
• Potentially larger display
• Same ESP32-S3-MINI-1, connects via USB serial or WiFi

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Parts

See purchase-list.md for the complete BOM with LCSC part numbers and sourcing info.

MCU: ESP32-S3-MINI-1-N4R2

Datasheets: Module | SoC

Why N4R2: PSRAM needed for display framebuffers — 320x240 RGB565 = 150KB per frame, double buffering needs 300KB. Without PSRAM, internal 512KB SRAM can't fit framebuffers + WiFi stack + application.

Display: HS20HS072RX 2.0" Color TFT LCD

Datasheet: HS20HS072RX datasheet

Source: LCSC/JLCPCB C5329582 — $3.70 (1+), $2.49 (100+), 1687 in stock.

Why this part:

• Full RGB color — expressive Tamagotchi character (red angry, green happy, etc.)
• 320x240 resolution — excellent detail for pixel art and text
• IPS viewing angle — looks good from any direction (critical for handheld)
• Runs entirely on 3.3V — no boost converter needed (unlike OLED panels needing 12V VCC)
• ST7789 has first-class ESP-IDF support via built-in esp_lcd driver
• $2.49 at volume with 1600+ stock — cheap and available
• 12-pin FPC is simple to wire (vs 30-pin for OLED panels)
• Backlight can be PWM-dimmed via MOSFET for power saving

FPC pinout:

Backlight Dimming Circuit

Backlight driven from +BATT (3.7-4.2V) through 22Ω resistor + AO3400A N-ch MOSFET, PWM-controlled by ESP32.

Why +BATT instead of 3.3V: LEDs have Vf = 3.0V. Only 0.3V headroom from 3.3V — poor regulation. +BATT gives 0.7-1.2V headroom.

Circuit: +BATT → R (22Ω) → LED anode → LED cathode → MOSFET drain → GND, gate driven by DISP_BL GPIO.

Current at different battery voltages:

• 4.2V (full): ~55mA | 3.7V (nominal): ~32mA | 3.4V (low): ~18mA

Firmware: PWM at 1kHz+. Sleep: SLPIN (0x10) + duty=0. Wake: duty>0 + SLPOUT (0x11), wait 120ms.

Firmware Graphics Library

Recommended: ESP-IDF native esp_lcd + LovyanGFX

• esp_lcd provides the built-in ST7789 panel driver for SPI initialization
• LovyanGFX provides sprite-based graphics, text rendering, and animations
• LovyanGFX supports ESP-IDF natively (not Arduino-only)
• Supports DMA transfers for background rendering
• ST7789 sleep commands (SLPIN/SLPOUT) supported for power management
• Alternative: LVGL if more complex UI widgets are needed later

Power Chain

USB-C → TP4056 (charger) → DW01A+FS8205A (protection) → ME6211 (3.3V LDO) → ESP32-S3 + display

Datasheets: TP4056 | DW01A | FS8205A | ME6211

Key design notes:

• TP4056 charge current set by R205 (3kΩ → ~330mA charge rate)
• DW01A+FS8205A: standard LiPo protection pair, back-to-back MOSFETs in battery ground path
• ME6211 LDO: 100mV dropout — at low battery (<3.4V) output sags below 3.3V, acceptable for MVP
• USB-C CC1/CC2 pull-downs: 5.1kΩ each for proper USB-C detection

Buttons

3 tactile switches with hardware debounce (10kΩ pull-up + 100nF cap per button). Active low, pressed = GND. Any button press wakes from deep sleep.

Feedback

• Debug LED (green 0603) — GPIO direct drive through 150Ω resistor. Dev/debug only.
• Vibration motor (6mm coin, 3V) — AO3400A MOSFET driver + 1N5819W flyback diode. PWM-capable. Connected via solder pads.

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Pin Assignment

Strapping pins to avoid: GPIO0 (boot mode), GPIO3 (JTAG source, floating), GPIO45 (VDD_SPI), GPIO46 (boot mode). These are sampled at reset and should not have external loads that could affect their default state during boot.

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Power Budget (estimated)

Real-world usage will be a mix — display on for a few seconds when events arrive, then back to sleep. Expected practical battery life: 2-5 days with typical AI coding session activity.

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Power Management Strategy

1. Normal operation: WiFi connected, display off. ESP32 in light sleep between events.
2. Event arrives (via WiFi): Wake up, turn on display, show status/approval, vibrate if needed.
3. Approval request: Display stays on with countdown timer until user responds or timeout.
4. No activity for N minutes: Enter deep sleep. Wake on any button press.
5. Deep sleep: Everything off. Battery lasts weeks/months.
6. Display auto-off: Display turns off after 30 seconds of no new events (configurable).

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PCB Design Notes

• Single PCB — power management and logic on one board
• Power section (TP4056, ME6211, USB-C, battery connector) on one side, logic section (ESP32, display connector, buttons) on the other
• Ground plane between sections
• ESP32-S3-MINI-1 antenna keepout zone respected (no copper/ground under antenna area)
• Display connects via 12-pin 0.5mm FPC cable (AFC01-S12FCA-00 connector)
• USB-C connector: 16-pin mid-mount type with D+/D- connected to ESP32-S3 USB OTG pins for firmware updates and serial communication
• Battery connector: JST-PH 2-pin SMD (standard for small LiPo, LCSC C7433648)
• Test points: 3.3V, +BATT, GND, UART TX/RX

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Enclosure

• 3D printed (material TBD: PLA, ABS, or PETG)
• Size determined after PCB layout and LiPo selection
• Rough estimate: ~50 x 40 x 20mm (similar to original Tamagotchi)
• Cutouts for: display window, 4 buttons, USB-C port, LED light pipe
• Optional clip or lanyard hole for portability