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FLARM USB Updater — REV B
USB-to-RS232 adapter with 12V boost converter for updating FLARM firmware without external power.
Plug into a PC USB port → connect RJ45 cable to FLARM → run firmware update tool. No 12V bench supply required. Also supports FLARM display firmware updates via the same connector.
Tip — connection order: connect the RJ45 to the FLARM before plugging the adapter into the PC. If you hot-plug the FLARM while it's already on USB, the COM port may briefly disconnect and reappear — that's harmless inrush (charging the FLARM's input capacitors through the boost momentarily dips the USB 5 V rail and re-enumerates the FT232), not a fault.
Status: REV B built and validated — bench-tested and confirmed updating real FLARM hardware over USB (boost, 12 V rail, and full serial path all verified). Fixes the REV A boost topology bug (12 V rail was dead on all REV A boards). See Archive/REV_A/ for the REV A schematic, rework guide, and bring-up log.
Disclaimer: This is an unofficial, community-built project. It is not affiliated with, authorized, sponsored, or endorsed by FLARM Technology Ltd. "FLARM" and "PowerFLARM" are trademarks of FLARM Technology Ltd.; they are used here only to describe device compatibility. Firmware updates are performed using FLARM's own official update tool — this project supplies power and a serial connection only. Use at your own risk; you are responsible for any device you connect it to.
🛒 Want one ready-made? Pre-built, tested boards for fellow pilots — ~€40, shipping excluded (cost-share to fund the next batch). Start a Discussion or email flarm@rvdb.dev.
Features
- Powered entirely from USB (5V)
- 5V → ~12V boost converter (MT3608) supplies FLARM via RJ45 pins 1 & 2
- FT232RL USB-to-UART bridge — uses Microsoft built-in usbser.sys, no driver install on Win10/11
- MAX3232 RS232 level shifter, powered from the 3.3V rail (±5V signaling, RS232-compliant with all FLARM RS232 ports)
- B5819W Schottky boost freewheel diode — also provides backfeed protection into live 12V aircraft bus
- USB ESD protection (USBLC6-2SC6) on D+/D−
- USB-C connector with proper CC pull-downs
- Power, TX and RX LED indicators
- Dedicated AMS1117-3.3 LDO — 3.3V supply for FLARM accessories (RJ45 pin 3), up to 800mA
- R6 (10Ω) isolates AMS1117 output from FLARM's internal 3.3V on pin 3
Hardware
Designed in EasyEDA Pro for direct JLCPCB SMT assembly ordering.
PCB REV B: 53.4 × 30.5 mm (2102 × 1200 mil), 2-layer FR4, 1oz copper. Single-sided top routing with a solid bottom GND plane, 54 stitching vias.
Note on C-numbers: IC and connector C-numbers are confirmed. Passive C-numbers (resistors/caps) are from the JLCPCB Basic parts library and should be verified in EasyEDA before ordering — search the value + package if a number doesn't match.
Component List (REV B)
| Ref | Description | JLCPCB # | Class |
|---|---|---|---|
| U1 | FT232RL USB-UART SSOP-28 | C8690 | Extended |
| U2 | MAX3232CSE+T RS232 transceiver SOIC-16 | C7258 | Extended |
| U3 | MT3608 boost converter SOT-23-6 | C84817 | Extended |
| U4 | AMS1117-3.3 LDO regulator SOT-223 | C6186 | Basic |
| L1 | 22µH shielded inductor 1A (SMNR4020) | C135264 | Extended |
| D1 | B5819W SL Schottky 40V/1A SOD-123 | C8598 | Basic |
| D2 | USBLC6-2SC6 USB ESD clamp SOT-23-6 | C7519 | Extended |
| J1 | USB-C receptacle TYPE-C-31-M-12 | C165948 | Extended |
| RJ1 | RJ45 SMD 8P8C (RJ-064-1) | C7205199 | Extended |
| J3 | 1×3 PTH header 2.54mm (unpopulated) | — | — |
| LED1 | Green LED 0805 — power (KT-0805G) | C2297 | Basic |
| LED2 | Yellow LED 0805 — TX (KT-0805Y) | C2296 | Basic |
| LED3 | Yellow LED 0805 — RX (KT-0805Y) | C2296 | Basic |
| R1 | 100kΩ 0402 — MT3608 FB top | C25741 | Basic |
| R2 | 5.1kΩ 0402 — MT3608 FB bottom | C25905 | Basic |
| R4 | 5.1kΩ 0402 — USB-C CC1 | C25905 | Basic |
| R5 | 5.1kΩ 0402 — USB-C CC2 | C25905 | Basic |
| R6 | 10Ω 0402 — RJ45 pin 3 isolation | C25077 | Basic |
| R7 | 680Ω 0402 — power LED | C25130 | Basic |
| R8 | 680Ω 0402 — TX LED | C25130 | Basic |
| R9 | 680Ω 0402 — RX LED | C25130 | Basic |
| C1 | 22µF / 25V 0805 — MT3608 Vin | C45783 |
35 components (34 SMT + 1 PTH unpopulated) — 7 Extended parts ($21 JLCPCB setup fees)
Net Names
| Net | Description | Trace class |
|---|---|---|
| VUSB | USB 5V power rail | Power (≥20mil) |
| GND | Ground (pour both layers) | pour |
| SW | MT3608 switching node (L1 → U3 SW → D1 anode) | default |
| V12_OUT | After Schottky D1 (~12.0V) to RJ45 pins 1,2 | Power_12V (≥15mil) |
| V3V3 | AMS1117-3.3 output | default |
| V3V3_OUT | After R6 to RJ45 pin 3 | default |
| FT_3V3 | FT232RL 3V3OUT (pin 17, decoupling only) | default |
| UART_TX | FT232RL TXD → MAX3232 T1IN → J3 | default |
| UART_RX | MAX3232 R1OUT → FT232RL RXD, also J3 | default |
| RS232_TX | MAX3232 T1OUT → RJ45 pin 6 | default |
| RS232_RX | RJ45 pin 5 → MAX3232 R1IN | default |
| UD+, UD− | USB data, FT232 side (diff pair DP1) | 6/6 mil |
| UD_IN+, UD_IN− | USB data, USB-C side (diff pair DP2) | 6/6 mil |
| CC1, CC2 | USB-C configuration channel | default |
| TXLED, RXLED | FT232RL CBUS0/CBUS1 (active-low) to LEDs | default |
| FB | MT3608 feedback node | default |
| CP1P, CP1N, CP2P, CP2N | MAX3232 charge pump | default |
| VS_POS, VS_NEG | MAX3232 V+/V− supply | default |
| LED1_K, LED2_K, LED3_K | LED cathode nodes | default |
FLARM RJ45 Pinout (8P8C)
| Pin | Signal | This device |
|---|---|---|
| 1, 2 | +12V | From boost via D1 (~12.0V) |
| 3 | FLARM 3V3 | From AMS1117-3.3 (U4) via R6 (10Ω) |
| 4, 7, 8 | GND | PCB GND |
| 5 | FLARM TX | RS232 RX input (MAX3232 R1IN) |
| 6 | FLARM RX | RS232 TX output (MAX3232 T1OUT) |
| 9, 10 | Shield | PCB GND copper pour |
The pinout is silkscreened on the back of the board:
SMD RJ45 note: The RJ-064-1 connector uses reversed pad numbering (pad 1 = RJ45 pin 8). The PCB routing compensates for this — the netlist pad references and the RJ45 pin numbers in this table are intentionally different.
Pin 3 design note: FLARM outputs 3.3V on pin 3 (up to 200mA per FLARM manual) to power external accessories. The AMS1117-3.3 on this board also outputs 3.3V on pin 3. R6 (10Ω) isolates the two 3.3V sources:
- FLARM update: Both regulators output ~3.3V. R6 limits mismatch current to ~10mA max. AMS1117 handles this safely (no protection diode needed with small output caps).
- Display update: Display draws power from AMS1117 through R6. At 20mA → 0.2V drop (3.1V), at 50mA → 0.5V drop (2.8V). Most displays work fine at 3.0V+.
- R6 can be replaced with 0Ω if display voltage drop is an issue and FLARM backfeed isolation is not needed.
Display updates — TX/RX crossover cable: For display firmware updates, TX and RX are swapped relative to FLARM updates. Use a crossover RJ45 cable (pins 5 and 6 swapped) labeled "DISPLAY". RS232 is non-damaging if crossed — receivers are high-impedance.
Breakout Header (unpopulated)
J3 — TTL UART (1×3, 2.54mm)
Exposes the TTL-level UART signals between the FT232RL and MAX3232. Not assembled by JLCPCB — solder a pin header when needed.
| Pin | Signal | Description |
|---|---|---|
| 1 | TX | 3.3V TTL TX from FT232RL |
| 2 | RX | 3.3V TTL RX to FT232RL |
| 3 | GND | Signal reference |
VCCIO note: The FT232RL VCCIO (pin 4) is tied to 3V3OUT (the FT232RL's internal 3.3V regulator), so J3 outputs 3.3V TTL and is safe to connect directly to 3.3V MCUs (nRF, RP2040, ESP32, STM32, etc.). FT232RL VCC (pin 20) remains on USB 5V — that powers the USB interface and does not affect the I/O logic level. The MAX3232 is also powered from the 3.3V rail, so the FT232RL↔MAX3232 interface is level-matched on both directions with in-spec margin.
LED Indicators
| LED | Colour | Signal | Resistor | Current | Behaviour |
|---|---|---|---|---|---|
| LED1 | Green (C2297) | VUSB → R7 → GND | 680Ω | 3.2mA | Always on when USB connected |
| LED2 | Yellow (C2296) | VUSB → R8 → TXLED# | 680Ω | 4.3mA | Flashes during TX |
| LED3 | Yellow (C2296) | VUSB → R9 → RXLED# | 680Ω | 4.3mA | Flashes during RX |
TXLED# and RXLED# are active-low, default CBUS0/CBUS1 factory config — no EEPROM needed.
What Else Can It Do?
With the FLARM cable connected, this board provides full RS232 + power — not just firmware updates:
- IGC flight log download — open FLARM Tool, download flight logs for OLC/competition submission.
- FLARM configuration — change aircraft type, competition ID, NMEA output sentences, baud rate, and other settings via the serial link.
- Live NMEA diagnostics — open a serial terminal (PuTTY, Tera Term) at 19200 baud to watch GPS, traffic, and error sentences in real-time.
- General-purpose RS232 adapter — make an RJ45 cable with only GND (pins 4/7/8), TX (pin 5), and RX (pin 6) connected. Leave pins 1/2 (12V) and 3 (3.3V) unconnected. The boost converter idles at ~1mA with no load.
Cable Configurations
| Cable | Wiring | Use |
|---|---|---|
| FLARM (straight) | All pins 1:1 | Firmware update, config, IGC download, diagnostics |
| DISPLAY (crossover) | Pins 5 ↔ 6 swapped | Display firmware update |
| RS232 only | Pins 4, 5, 6, 7, 8 only | General RS232 (no 12V/3.3V) |
FT232RL EEPROM Customization (optional)
Use FTDI's free FT_PROG tool (Windows) to program the FT232RL's internal EEPROM:
- Device description: set to "FLARM USB Updater" — shows in Device Manager instead of generic "USB Serial Converter"
- Serial number: unique per board — helps identify the right COM port when multiple FTDI devices are connected
One-time USB programming step per board. Not required for basic operation.
Boost Converter
Standard boost topology (fixed in REV B):
VUSB ── L1(22µH) ── SW ── D1 ── V12_OUT ── RJ45 pins 1,2
│
U3 SW pin
Vout = 0.6 × (1 + R1/R2) = 0.6 × (1 + 100k/5.1k) ≈ 12.35V After B5819W drop (~0.35V): ≈ 12.0V at RJ45 — within FLARM 8–36V input range.
| Parameter | Value |
|---|---|
| Output voltage | 12.35V (12.0V after diode) |
| Duty cycle | ~60% |
| Inductor ripple | 113mA peak-to-peak |
| Peak inductor current | 304mA (vs 1A L1 rating) |
| Output ripple | <1mV estimated |
| Output cap | C3 = 22µF at 25V (2× voltage margin) |
Power Budget
| Consumer | Current (mA) |
|---|---|
| FT232RL | 15 |
| MAX3232 | 5 |
| USBLC6 | 0.5 |
| AMS1117 quiescent | 5 |
| LED1 (power) | 3.2 |
| LED2 (TX, avg) | 2.2 |
| LED3 (RX, avg) | 2.2 |
| MT3608 boost @ 85mA load | 247 |
| Total USB draw | ~280mA |
USB 2.0 limit: 500mA → 56% used. A regular FLARM runs on any standard USB port (bench-tested with a Flarm06).
A PowerFLARM draws more (~165mA @ 12V → ~510mA on USB) — fine on USB 3.0 / USB-C ports, which supply well above 500mA. On an older 500mA USB 2.0 port that's right at the limit, so if it doesn't power up, use a USB-C charger (1.5A+).
Cost Estimate (JLCPCB SMT assembly)
| Qty | PCB | SMT | Extended fees | Components | Shipping | Total | Per board |
|---|---|---|---|---|---|---|---|
| 5 | $2 | $8 | $21 | ~$37.50 | ~$15 | ~$83.50 | ~$16.70 |
| 10 | $4 | $8 | $21 | ~$75 | ~$15 | ~$123 | ~$12.30 |
Changes from REV A
REV A was ordered, received, and bench-tested. RS232, USB, 3.3V rail, and LEDs all worked. The 12V rail produced 0V on every board due to incorrect boost topology. Rework instructions are preserved in Archive/REV_A/REV_A_Rework_Guide.md.
Fixed in REV B:
- Boost topology — L1 moved from SW↔output to VUSB↔SW. D1 moved from output↔V12_OUT to SW↔V12_OUT. Now a standard boost.
- Removed R3 (10kΩ EN pull-up) — EN tied directly to VUSB.
- Removed C2 (100nF boost input HF) — not required by MT3608 datasheet.
- Removed C4 (4.7µF boost output aux) — not required by MT3608 datasheet.
- VBOOST net eliminated — SW→D1→V12_OUT is one path.
- LED2 colour — corrected from green to yellow (KT-0805Y / C2296) to match physical BOM.
- MAX3232 supply — VCC (U2 pin 16) and its bypass cap C5 moved from VUSB (5V) to the 3.3V rail (AMS1117 V3V3, regulator side of R6). At VCC=3.3V the MAX3232 driver input threshold is 2.0V, which the FT232RL TXD (2.2V min at VCCIO=3.3V) meets in spec; previously the 5V-powered MAX3232 needed 2.4V and was out of spec worst-case. Both ICs now share the 3.3V rail (level-matched). J3 is therefore 3.3V TTL — safe for any 3.3V MCU.
Design Verification (REV B)
Netlist + BOM review, re-verified 2026-05-15 against the REV B EasyEDA project export (programmatic netlist parse against component datasheets):
- FT232RL: all 28 pins verified (TEST→GND, CTS/DSR/DCD/RI→GND, OSCI/OSCO NC, VCC on VUSB=5V, VCCIO on FT_3V3=3.3V)
- MAX3232: VCC on V3V3 (3.3V rail); C5 VCC bypass; C6/C7 charge pump flying caps; C8/C9 V+/V− storage. Ch1 UART↔RS232. T2IN (pin 10) tied to GND.
- MT3608: VUSB→L1→SW→D1→V12_OUT. Feedback 100k/5.1k → 12.35V. Boost topology correct (REV A bug fixed).
- USB-C: CC1/CC2 pulldowns 5.1kΩ, D+/D− through USBLC6, diff-pair rule active (DP1, DP2, 6/6 mil), both orientations
- AMS1117-3.3: V3V3 → R6 (10Ω) → RJ45 pin 3, adequate headroom
- LED circuits: active-low CBUS, correct polarity, 3.2–4.3mA current
- RJ45 pinout: matches FLARM IGC specification (accounting for SMD connector pad reversal)
- GND copper pour on both layers, 54 stitching vias
- Power budget: ~280mA within USB 2.0 500mA limit
- All capacitor voltage ratings: ≥1.8×, all resistor power ratings: <40%
Bring-up Checklist (before connecting to FLARM)
- Visual inspection: USB-C orientation, RJ45 jack seating, no tombstoned 0402s
- USB plug-in: LED1 (green) on. Host enumerates as FTDI serial device.
- DMM: 5V on VUSB test points, 3.3V on V3V3, 12.0V on V12_OUT (RJ45 pins 1,2)
- Loopback test via
flarm_query.pyorflarm_monitor.py— TX/RX LEDs flash - Connect FLARM on bench supply first (not live 12V aircraft bus) for initial test
Files
- EDA/USB_FLARM_UPDATER.eprj — EasyEDA Pro project (REV B)
- EDA/ProPrj_USB_FLARM_UPDATER_2026-05-29.epro — REV B as-built source export (the fabbed design)
- EDA/Gerber_PCB1_2026-05-27.zip — REV B Gerber for JLCPCB
- EDA/BOM_Board1_PCB1_2026-05-27.csv — REV B BOM
- EDA/PickAndPlace_PCB1_2026-05-27.xlsx — REV B SMT pick-and-place
- EDA/Netlist_Schematic1_2026-05-27.tel — REV B netlist (for parser audit)
- EDA/silkscreen_back_REV_B.svg — back silkscreen artwork
- EDA/USB_FLARM_UPDATER_backup/ — dated project snapshots
- EDA/3DPCB.step — board 3D model (STEP) for enclosure / mechanical design
- Images/ — board photos used in this README
- Docs/FLARM_USB_Updater_Design_Document.docx — design document (BOM, pinout, power budget, bring-up)
- Component datasheets — see each part's LCSC code (C-number) in the BOM; enter it at lcsc.com for the datasheet
- Enclosure/ — 3D-printable case:
box+lid(STEP/3MF/STL) andenclosure-assembly.step(board-in-box fit reference) - Archive/REV_A/ — REV A schematic, PCB, BOM, Gerber, rework guide, bring-up log
Support the project
This is fully open hardware — anyone can build the board from the Gerbers in EDA/ and the BOM. If the design or documentation saved you time, or you'd like to back continued development:
- ☕ Buy Me a Coffee — one-time thanks, any amount
- 🛒 Pre-built, tested boards (~€40, shipping excluded) — see the note at the top, or reach out via Discussions / flarm@rvdb.dev
Buying a board doesn't get you anything the open files don't — you're paying for assembly, testing, and the time saved. The repo is the canonical source either way.
Questions & Contact
- 🐛 Bug or technical problem → open an issue
- 💬 Question, build help, or interested in a board → GitHub Discussions
- ✉️ Private / direct → flarm@rvdb.dev
License
Copyright Rik Vanden Boer.
This source describes Open Hardware and is licensed under the CERN-OHL-S v2 (CERN Open Hardware Licence Version 2 — Strongly Reciprocal). You may redistribute and modify this source and make products using it under the terms of CERN-OHL-S v2 (LICENSE).
This source is distributed WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY, SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE. Please see the licence for the full text.
As a strongly reciprocal licence: if you make and distribute a (modified) board based on this design, you must make the complete source (schematics, PCB, BOM) available under the same licence.
"FLARM" and "PowerFLARM" are trademarks of FLARM Technology Ltd. and are not covered by this licence — see the disclaimer at the top of this document.