Open-source Arduino PA Controller for Hermes-Lite 2 (GPS, CAT, SWR & Thermal Protection)

[Ricardo]

Hello HL2 community,

I'd like to share an experimental project that some of you running external amplifiers with your Hermes-Lite 2 might find interesting.

Full transparency first: this firmware was developed with heavy AI assistance (Anthropic's Claude), iterating through six versions of review, bug hunting and simulation. It has NEVER been flashed to real hardware yet. It compiles clean (g++ -Wall -Wextra, zero warnings) and passes 63 automated checks on a virtual test bench that simulates the pedal, relays, GPS, CAT traffic, SWR bridge and NTC sensor — but simulation is not the real world. Treat it as UNTESTED CODE that may contain errors. Do not connect it to an amplifier you are not willing to risk.

That said, here is what it attempts to do, all on a single $5 Arduino Pro Micro (ATmega32U4):

• PTT sequencing via footswitch with hardware interrupt: relay closes first, 20 ms settle, then MOX via CAT "TX;" — reverse order on release so the relay never hot-switches RF
• Automatic band decoding by polling Thetis over CAT (Kenwood TS-2000 protocol, "FA;" query), driving a 4-bit BCD output for LPF/band relays of salvaged PA modules — with 4 selectable mapping profiles (standard, 5-band kits, inverted BCD, custom EEPROM map)
• GPS time sync (NEO-6M/M8 + PPS) for FT8/FT4/WSPR window indication, fully offline, plus a "$TIME" sentence so your shack PC can discipline its clock without internet
• SWR protection: directional coupler on two ADC pins, latched TX abort above a settable limit (3 consecutive bad samples to reject SSB transients)
• Thermal protection: NTC on the heatsink, fan control and TX inhibit with hysteresis
• Boot-inhibit against spurious PTT pulses at power-on, band-change TX inhibit, anti-chatter, watchdog timer, and a serial command console (STATUS, PROT, CALIB, etc.)

The interface BOM is roughly US$25-35 excluding the PA itself — the whole philosophy is giving new life to power stages and low-pass filters pulled from scrap rigs.

The package includes the firmware AND the virtual test bench (plain C++ with Arduino mocks), so anyone can run all 63 checks on their own PC before flashing anything.

I'm sharing this openly for anyone who feels like experimenting with it — whether out of curiosity, as a starting point for your own amplifier interface, or as a base that could be improved, adapted and perhaps evolve into something genuinely useful for the community. The code is fully commented and the architecture is modular, so taking just one piece (the band decoder, the GPS sync, the protection logic) is perfectly viable too.

Reference projects and reading that shaped this work (all worth your time):

• EA3IGT's 70W PA for the HL2, the closest existing project: https://github.com/ea3igt/HL2-PA70
• N2ADR's official HL2 IO Board (Pico-based, I2C route): https://github.com/jimahlstrom/HL2IOBoard
• DL1BZ's ESP32 band-voltage interface for the XPA125B: https://github.com/dl1bz/ESP32-XPA125B-HL2
• Tech Minds' HL2 I2C band volts project: https://github.com/TechMindsYT/HL2-I2C-BandVolts
• radi8's Hermes-Lite filter/signaling controls: https://github.com/radi8/Hermes_Lite_Controls
• HL2 Getting Started wiki: https://github.com/softerhardware/Hermes-Lite2/wiki/Hermes-Lite-2.0-Getting-Started
• Thetis CAT protocol internals (TS-2000 emulation): https://deepwiki.com/ramdor/Thetis/5.1-cat-protocol
• W3PM's GPS-disciplined FT8/WSPR beacon: https://github.com/W3PM/40MHz-GPS-FT8-WSPR-MEPT
• Why PPS beats NMEA-only timing (~450 ms latency measured): https://github.com/00steve00/Super-Accurate-Arduino-Clock
• Portable GPS NTP server write-up: https://hackaday.com/2021/07/25/portable-gps-time-server-powered-by-the-esp8266/
• NEO-6M module fundamentals: https://randomnerdtutorials.com/guide-to-neo-6m-gps-module-with-arduino/
• W8JI on relay hot-switching damage (the reason for the band-change TX inhibit): https://www.w8ji.com/relay.htm

  The attached files are listed below:  

73 de Rick, PY2VOX