2 layers76.2 × 132.3 mm Stm32 calc, the printed circuit board in 3D

Stm32 calc

A mechanical-key calculator that also works as a numpad, designed and sold as a kit.

Size: 76.2 × 132.3 mm
Copper layers: 2
Thickness: 1.6 mm
Parts: 85
Nets: 101
Pads: 357
Tracks, vias: 575, 41
Pours: 3
Routed: Yes
KiCad: KiCad 7 (file 20221018)
Created: 2 Dec 2023
Updated: 13 Aug 2026

Overview

Stm32 calc is an open-source printed circuit board by georgsnarbuts, from the georgsnarbuts/kalkulators repository on GitHub. It is a 2-layer board of 76.2 × 132.3 mm with 85 parts on 101 nets, routed with 575 tracks and 41 vias.

It is built around the STM32F072CBT6, TM1638 and USBLC6-2SC6, with the XC6206P332MR, HC-TYPE-C-16P-CH2.58-3A-02 and SS-1290 among its other parts. Its parts include 19 diodes, 16 capacitors, 10 resistors, 6 ICs and 3 connectors. It belongs with the STM32 boards of this library.

Main components

README

README.md

Kalkulators

A mechanical-key calculator that also works as a numpad, designed and sold as a kit.

The finished calculator held in one hand, seven-segment display lit The back plate, embossed with a scattered pattern of maths symbols

It runs standalone off a single AA cell as an eight-digit calculator. Plug it into USB-C and it enumerates as a keyboard instead, working as a numpad or macropad with every key remappable through VIA. The firmware watches VBUS to decide which of the two it should be.

Specification


MCU	STM32F072
Bootloader	stm32-dfu
Display	Two 4-digit 7-segment modules on a TM1638
Matrix	4 x 5, 17 keys, COL2ROW
Switches	MX-compatible
Power	One AA cell, or bus power over USB-C
Firmware	QMK, with VIA support

Repository layout

text

firmware/            QMK keyboard definition: calculator logic, TM1638 driver, keymaps
hardware/
  pcb/               Main PCB, KiCad project plus fab package
  switch-plate/      Switch plate
  back-plate/        Back plate, embossed with maths symbols
  graphics/          Artwork used on the plates
docs/images/         Renders and assembly views

Firmware

firmware/ is a QMK keyboard definition, not a standalone project. Drop it into a QMK checkout before building:

git clone https://github.com/qmk/qmk_firmware.git
cp -r firmware qmk_firmware/keyboards/kalkulators
cd qmk_firmware

There are two keymaps, default and via:

make kalkulators:default
make kalkulators:via
make kalkulators:via:flash

The board uses the STM32 DFU bootloader, so put it in bootloader mode before flashing rather than expecting a reset key to work from the calculator side.

New to QMK? Start with the build environment setup and the make guide.

How the two modes work

VBUS_CHECK_PIN is wired to B5, and NO_USB_STARTUP_CHECK is set so the firmware will boot without a USB host present. On battery it stays in calculator mode and drives the TM1638 display directly. On USB it comes up as a keyboard. calc.c holds the arithmetic and the display formatting, tm1638.c the display driver.

Hardware

Three separate KiCad projects, each with a JLCPCB package ready to order.

Board	KiCad project	Fab package
Main PCB	`hardware/pcb/stm32_calc.kicad_pcb`	`hardware/pcb/jlcpcb/production_files/`
Switch plate	`hardware/switch-plate/calc_plate.kicad_pcb`	`hardware/switch-plate/jlcpcb/production_files/`
Back plate	`hardware/back-plate/back_plate.kicad_pcb`	`hardware/back-plate/jlcpcb/production_files/`

Each production_files/ folder holds the gerber archive, the BOM and the pick-and-place CSV. STEP exports of the final revision sit alongside each project for mechanical fit checks.

The populated PCB with switches and both display modules fitted, before the case went on The bare PCB from the back, next to a pair of calipers

Licence

See LICENSE.

Category and platform

STM32

Project files

Show this board on your page

https://pcbkit.io/boards/georgsnarbuts/kalkulators

The board's own page, for a message or a post.

Source

Repository: georgsnarbuts/kalkulators
Board file: hardware/pcb/stm32_calc.kicad_pcb
Commit: cb2b264
Licence: Not stated

Parts

Qty	Value	Footprint	References
17	1N4148	Diode_SMD:D_SOD-123	D3, D4, D5, D6, D7, D8, D10, D11, D12, D13, D14, D15, D16, D17, D18, D19 +1
14	SW_SPST	georgs_lib:geo1.00u_PCB	SW_clear1, SW_divide1, SW_eight1, SW_five1, SW_four1, SW_minus1, SW_mutiplicate1, SW_nine1, SW_one1, SW_point1, SW_seven1, SW_six1, SW_three1, SW_two1
7	MountingHole_Pad	MountingHole:MountingHole_2.2mm_M2_DIN965_Pad	H1, H2, H3, H4, H5, H6, H7
5	100n	Capacitor_SMD:C_0402_1005Metric	C1, C3, C6, C9, C11
3	100pF	Capacitor_SMD:C_0402_1005Metric	C15, C16, C17
3	10K	Resistor_SMD:R_0402_1005Metric	R5, R6, R7
2	0.1uF	Capacitor_SMD:C_0402_1005Metric	C8, C10
2	10k	Resistor_SMD:R_0402_1005Metric	R1, R8
2	1u	Capacitor_SMD:C_0805_2012Metric	C19, C20
2	4.7u	Capacitor_SMD:C_0402_1005Metric	C2, C7
2	5k1	Resistor_SMD:R_0402_1005Metric	R10, R13
2	CC56-12GWA	georgs_lib:7seg_sr410361n	U3, U4
2	SW_SPST	georgs_lib:geo2.00u_Vertical_PCB	SW_equals1, SW_plus1
1	100	Resistor_SMD:R_0402_1005Metric	R2
1	10n	Capacitor_SMD:C_0402_1005Metric	C5
1	1k	Resistor_SMD:R_0402_1005Metric	R12
1	1n4148	Diode_SMD:D_SOD-123	D2
1	1u	Capacitor_SMD:C_0402_1005Metric	C4
1	2k	Resistor_SMD:R_0402_1005Metric	R11
1	Battery_Cell	Battery:BatteryHolder_Keystone_2460_1xAA	BT1
1	C5307759	Connector_USB:USB_C_Receptacle_HRO_TYPE-C-31-M-12	J3
1	Conn_01x03	Connector_PinHeader_2.54mm:PinHeader_1x03_P2.54mm_Vertical	J2
1	Conn_02x03_Odd_Even	Connector_PinHeader_2.54mm:PinHeader_2x03_P2.54mm_Vertical	J4
1	LOGO	georgs_lib:logo_w65	G***
1	LOGO	georgs_lib:soy	G***
1	Reset	Button_Switch_SMD:SW_SPST_TL3342	SW3
1	SS-1290	georgs_lib:ss-1290	SW2
1	STM32F072CBT6	Package_QFP:LQFP-48_7x7mm_P0.5mm	U1
1	SW_SPST	georgs_lib:geo2.00u_PCB	SW_zero1
1	TM1638	georgs_lib:SOP28 tm1638	IC1
1	USBLC6-2SC6	Package_TO_SOT_SMD:SOT-23-6	U2
1	XC6206P332MR	Package_TO_SOT_SMD:SOT-23	U5
1	batter_switch	georgs_lib:SK12D07VG4_slider	SW1
1	p channel mosfet sot 23	Package_TO_SOT_SMD:SOT-23	Q1
1	schottky diode	Diode_SMD:D_SOD-123	D1

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Questions

How big is the Stm32 calc PCB?

It measures 76.2 × 132.3 mm and has 2 copper layers, 1.6 mm thick.

What parts does the Stm32 calc use?

85 parts, built around the STM32F072CBT6, TM1638, USBLC6-2SC6 and XC6206P332MR. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.

Where can I download the Stm32 calc KiCad files?

From the georgsnarbuts/kalkulators repository on GitHub, at the commit this page was made from: the board is hardware/pcb/stm32_calc.kicad_pcb and its schematic hardware/back-plate/back_plate.kicad_sch. PCBKit opens the board in the browser, with nothing to install.

Can I use the Stm32 calc design in my own project?

Its repository states no licence, so its author keeps all rights: ask georgsnarbuts before reusing the design.