Trinketcore

TrinketCore: the all-in-one supercapacitor trinket board

TrinketCore puts everything a small rechargeable gadget needs on one 40 × 60 mm board, ready to order assembled from PCBWay:

• Room for two capacitors. They lie flat on the back, wired in parallel.
• USB-C charging, with a red "charging" and a green "full" LED.
• A socket for an OLED or colour TFT module. The microcontroller reads the capacitor's charge and can show it on the screen.
• An RP2040 microcontroller with 16 MB of flash for code, sounds and pictures.
• A speaker amplifier, two buttons, a user LED, a wake-up clock and an expansion header for LEDs, buttons and a lid switch.

The power side is CapCore's proven circuit. It cuts off before the capacitor over-discharges, uses a soft power switch that turns the whole board off, and has a buck-boost converter that keeps 3.3 V steady as the capacitor drains.

Top	Bottom (capacitor bay)

Renders of the actual Gerber layers. The display plugs into the socket along the top edge. The capacitors solder into the pads on the left and lie flat inside the outlines on the back.

What's on the board

Which display?

The socket fits the common 0.96–1.54″ modules, so you can try several and pick per product. A screen is the second-biggest power draw after a speaker, so the firmware uses it in short bursts: wake, show, dim, switch off.

Yes, colour TFTs work well. The backlight is most of the extra power, so dim it and switch off after a few seconds. On two 220 F capacitors a colour charm shown for 10 s, 30 times a day, lasts about four weeks. Played continuously, a colour game lasts about two hours. See the design method for the numbers.

Module pin orders vary. Modules labelled GND VCC SCL SDA RES DC CS BLK plug straight into J4. Seven-pin OLEDs (GND VCC D0 D1 RES DC CS) use the first seven positions. Others, including many GC9A01 modules (VCC GND ...), need short jumper wires. Check that GND and VCC line up before plugging in.

E-ink works too, and suits anything that should stay visible: the image costs no power at all once drawn, so the board can switch fully off. E-paper modules have a BUSY output where TFTs have a backlight input. Wire them to J4 by function (VCC → 3V3, GND, DIN → MOSI, CLK → SCK, CS, DC, RST → RES) with BUSY on the BLK pin, which the RP2040 reads on GP13. The oracle example shows a random tarot card this way.

Runtime per charge

LIC build, 85 % converter efficiency, 5 µA standby (tools/energy_calc.py):

* Beyond a month or two the capacitors' own self-discharge dominates.

Fitting the capacitors

The two capacitor positions sit along the left edge. Square pad = +. Each position takes a 5.0 mm lead pitch (10–12.5 mm cans) or 3.5 mm (8 mm cans). Bend 16 mm cans' 7.5 mm leads in slightly. Bend the leads 90° so each can lies flat on the back inside its outline (CAP 1, CAP 2), with its leads at the left edge. Stick a strip of Kapton tape under each can, and glue or strap large cans.

• One capacitor is fine: fit either position.
• Two capacitors must be the same type (both LIC, or both EDLC on an EDLC build). Sizes may differ.
• Good choices: Eaton HS1225-3R8127-R (120 F, 12.5 × 25 mm) or HS1625-3R8227-R (220 F, 16 × 25 mm). See choosing a capacitor.

Never fit a 2.7 V supercapacitor to a board built with the LIC values. It would be charged to 3.71 V and can vent. Tick the build box on the back, and measure the charge voltage before fitting capacitors (see Bring-up).

Pinout

J4 display socket (top edge, pin 1 = square pad, left): GND · 3V3 · SCK (GP10) · MOSI (GP11) · RES (GP12) · DC (GP9) · CS (GP8) · BLK (GP13 via 100 Ω)

J5 I²C OLED socket: GND · 3V3 · SCL (GP17) · SDA (GP16). J8 Qwiic: GND · 3V3 · SDA · SCL. The same bus as the clock (address 0x51), with 4.7 kΩ pull-ups.

J6 expansion (right edge, pin 1 at the top): GP7 · GP6 · GP5 · GP4 · GP3 · GP2 · GP1 · GP0 · GP29 · GP28 · WAKE · VSYS · 3V3 · GND. The order matches the RP2040's pins, which keeps the tracks from crossing; the back silkscreen labels every pin.

• WAKE: pull to GND to switch the board on (lid, tilt, reed or vibration switch). For a lid that may stay open, put a 1 µF capacitor in series with the switch and a 22 MΩ resistor across that capacitor. Opening the lid then sends one wake pulse, and the board can still switch itself off.
• VSYS: the capacitor voltage (2.6–3.7 V), on only while the board runs. Use it for vibration motors or LED strips that tolerate it.
• GPIO pins drive LEDs directly, up to ~8 mA each through a resistor.

J7 speaker: 8 Ω, 0.5–1 W. GPIO map: see the firmware README.

LEDs and power states

With USB plugged in the board also switches on, so a gauge can show charging progress. Cut jumper JP1 on the back if plugging in should only charge. Bridge JP2 for gadgets without a power button (always on above the cut-off).

Ordering from PCBWay

Everything is in fab/:

1. On pcbway.com choose PCB Instant Quote → Quick-order PCB and upload trinketcore-gerbers.zip. It should read 40 × 60 mm and 4 layers.
2. Board settings:
   • Material FR-4, thickness 1.6 mm, any solder mask colour.
   • Min track/spacing 6/6 mil, min hole size 0.3 mm.
   • Surface finish: ENIG is recommended because the RP2040 has 0.4 mm-pitch pads. HASL lead-free usually works too.
   • Order number: the back has a WayWayWay placeholder where PCBWay can print it. Tell them in the notes, or pick the remove-order-number option.
3. Tick Assembly service: turnkey, top side only, about 102 SMD parts (44 unique lines), no through-hole parts. Upload pcbway-bom-lic.csv and pcbway-centroid.csv.
4. Paste this note: "BT1, BT2, J4–J7, JP1–JP2: do not populate (customer fits capacitors and connectors). J8 and all other parts: assemble as per BOM. Please confirm pin 1 of U2, U3, U4, U5, U6, U7, U8 and the LED polarity against the silkscreen before assembly."
5. PCBWay's engineers review the files, source the parts, and email if a part needs a substitute. Resistors and capacitors may be any brand with the same value, size and tolerance. Approve their placement confirmation photos.

Rough cost (prices change, so trust the quote): about US$25–50 for five 4-layer boards, plus assembly (setup ~US$30–50, parts ~US$8–12 per board with the RP2040, flash and amplifier). Five assembled boards typically land around US$100–180 before shipping. Most of that is fixed, so ten cost little more.

The same files work at other assembly houses (the BOM has manufacturer part numbers, and LCSC numbers where known).

Bring-up

Before fitting the capacitors:

1. Plug in USB-C. The red and green LEDs may flicker (nothing to charge yet), and the board switches on.
2. Measure between the square BT1 pad and GND: about 3.7 V is the LIC build, 2.6 V the EDLC build.
3. The computer should show an RPI-RP2 drive (blank flash starts the bootloader). Install CircuitPython and try the charge gauge with a display on J4.

Then unplug USB, fit the capacitors (square pad = +), and plug USB back in: red while charging, green when full. Unplug and press A: the board runs for about 5 s, or longer if firmware raises HOLD. A new capacitor that arrives below the cut-off keeps the board off until it has charged past it.

Design files

As with CapCore, the board is generated from code:

cd hardware/trinketcore/scripts
python3 gen_footprints.py
python3 gen_schematic.py && python3 check_netlist.py
python3 check_place.py
FREEROUTING_JAR=/path/to/freerouting-1.9.0.jar python3 route_pcb.py --autoroute
python3 check_netlist.py --pcb && python3 export_fab.py && python3 render.py

netlist.py is the single source of truth: parts, values, manufacturer part numbers and every connection. Autorouting needs Java and Freerouting 1.9.0 (under xvfb-run on a headless machine); without --autoroute the board is rebuilt from the committed trinketcore.ses. The crystal and the power fan-outs are drawn by hand in preroute.py, and a small grid router (maze.py) finishes whatever Freerouting leaves open.

Verification status: the schematic and PCB are checked pin-for-pin against netlist.py (378 pins). KiCad DRC with PCBWay's limits reports no errors and no unconnected items, only one warning: the USB-C footprint differs from the library copy because its silkscreen was trimmed at the board edge. The routing is automatic, so some tracks take detours a hand layout wouldn't. The board has not been built and tested yet: treat the first order as a prototype run and follow the bring-up steps.