Wallet dmm

wallet_dmm

A multimeter the size of a credit card: DC volts, resistance, continuity and diode test, shown on a 0.91" OLED, with three buttons, an RGB LED and a piezo beeper. It runs from a small LiPo that charges over USB-C and is built around an STM32L051K8, with Rust firmware and a simulation of the whole thing.

This is revision 3. Rev 1 (the October 2024 JLCPCB order) is commit a01ea68; rev 2 is the v2 branch.

Changes from rev 1 and rev 2

• Power. The CR1025 (rated 0.1 mA continuous) could not run an OLED. It is replaced by a LiPo charged from USB-C (MCP73831), a 1 µA 3.0 V LDO that stays on while the MCU sleeps in Stop mode, a slide switch that disconnects the battery, bulk decoupling, and a load switch (Q1/Q2) that cuts the OLED module's supply.
• Piezo. The always-running 555 and its two MOSFETs are gone. The MCU drives the piezo push-pull from TIM22 CH1/CH2 on PB4/PB5, which gives 6 Vp-p from 3 V and draws nothing when silent.
• Programming. SWD is on PA13/PA14 with NRST, with nothing else on those pins. USART1 (PA9/PA10, also the ROM bootloader) is on test pads.
• Measurement. Separate V and Ω inputs. The V input is a 9.87 MΩ divider biased to a mid-rail reference, with a hold capacitor because the ADC wants a source below 50 kΩ. The Ω input has two reference ranges; its drive nodes are clamped to ground and the battery, so a voltage on it pushes at most 2.5 mA into the cell instead of into the MCU or the 3.0 V rail. What the MCU pins' own clamps still pass would lift the rail while the MCU sleeps (the LDO cannot sink current), so a TL431 (U4) holds it below 3.32 V.
• Everything else. A resistor for each LED colour, with the anode on the battery so green and blue have headroom. The MCU's calibrated internal sensor replaces the TO-92 MCP9700. Decoupling sits at the pins. Every part is stocked by JLCPCB and carries its LCSC number. All libraries are in the repo.

Layout of the repository

• hardware/: KiCad 10 project (schematic, board, project rules, footprints for the RGB LED and the OLED module, 3D models for every part)
• scripts/: generators for the schematic, board and 3D models, the autorouter wrapper, the fab-file export and the renders
• fab/: Gerbers, drill files, BOM and pick-and-place for JLCPCB, schematic and assembly PDFs
• firmware/: Rust firmware and its simulation (see firmware/README.md)
• docs/: renders of the board and the simulation results

Building

Everything runs in the dev shell (nix develop, or direnv).

./scripts/build.sh          # schematic -> ERC -> board -> DRC -> fab/

The schematic and board are generated by scripts/gen_schematic.py and scripts/gen_pcb.py; edit those rather than the KiCad files, or the next build will overwrite the change. Tracks come from hardware/routes.json. After moving parts or changing the netlist, route again with Freerouting:

kicad-cli sch export netlist --format kicadsexpr -o build/wallet_dmm.net hardware/wallet_dmm.kicad_sch
python3 scripts/autoroute.py build/wallet_dmm.net

The build stops on any ERC or DRC error and checks that the board matches the schematic.

The renders in docs/renders/ and a GLB of the populated board (build/wallet_dmm.glb, for any glTF viewer) come from:

./scripts/render.sh

Most parts use KiCad's own 3D models; the switches, buzzer, USB-C socket and RGB LED use the manufacturers' models (via EasyEDA). The OLED module and the battery are drawn by scripts/gen_models.py, which lights the OLED's pixels with the firmware's volts screen:

nix develop .#models -c uv run --with cadquery python scripts/gen_models.py

Ordering from JLCPCB

Upload fab/wallet_dmm-gerbers.zip: 2 layers, 0.8 mm, 1 oz, any colour, HASL lead-free or ENIG. For assembly, choose top side and upload fab/bom-jlcpcb.csv and fab/cpl-jlcpcb.csv. In the placement preview, check the orientation of U1 (pin 1 dot), U2, U3, Q1, Q2, D2, D3, D1 (cathode) and D4 (anode corner, the dot in the silkscreen) before confirming.

Solder these by hand afterwards:

• J3: a 0.91" 128×32 SSD1306 I2C module, flat on the board, header pins cut short. Match the module's pin labels to the silkscreen (GND VCC SCL SDA).
• J2: a 3-pin header or the probe leads (COM, V, Ω).
• J5: the battery wires from the back, BAT+ to the square pad.
• J4 stays empty; hold a header in it for an ST-Link, or solder one while developing.

Battery

A LiPo pouch cell with its own protection circuit, up to 30 × 40 × 4 mm, taped to the marked area on the back (no pins poke through there). Charging runs at 100 mA, so use 150 mAh or more; R3 sets the current (1000 V / R3).

Bring-up

1. Before connecting anything, check that BAT and 3V0 (TP3, TP4) are not shorted to GND.
2. Connect the battery and slide SW5 to ON. TP4 reads 2.94–3.06 V. If it reads 0 V, the switch is the other way round from the silkscreen: fix the labels in scripts/gen_pcb.py.
3. Plug in USB-C: the CHG LED lights while the cell charges and goes out when it is full.
4. Flash over SWD (ST-Link on J4: 3V0, SWDIO, SWCLK, NRST, GND). Holding RESET while connecting works if firmware has put the MCU to sleep.
5. With the firmware running (firmware/README.md): short V to COM in DC V and hold C for 3 s to store the zero; do the same with the Ω leads in Ω mode. Then put 5–30 V on the V input, measure it with another meter, and send cal <volts> over the UART (TP2) to set the gain.

Firmware

The firmware is in firmware/: Rust and Embassy, with all the maths and the UI in a host-tested crate. Its README covers the buttons, calibration, flashing and the simulation. The pins:

Simulation

firmware/simulate.sh runs the front end in ngspice, feeds it through a model of the ADC into the firmware's own maths, and runs the release binary in Renode on an emulated STM32L051. Across 200 simulated boards with 1 % parts, calibrated volts are within ±4 mV up to ±5 V and ±33 mV at ±30 V, and ohms within ±0.34 % from 1 kΩ to 100 kΩ. The details and plots are in firmware/README.md and docs/simulation/.

Renders

Front	Back, with the battery