Open Source Mesh and Bluetooth Motorcycle Intercom
website | discord
OpenHelmet is an open-source, motorcycle intercom system designed to outperform current proprietary solutions in latency, scalability, transparency, and hackability.
- Open protocol and open firmware
- Commodity, off-the-shelf hardware
- Use any microphone and any speakers from your existing helmet setup
- Mesh networking for 4-8+ riders
- Real-time full-duplex voice
- 8-16 hours of active riding per charge
- Interoperability with legacy Bluetooth intercoms (Cardo, Sena, etc.) via standard profiles
This project explicitly avoids reverse-engineering proprietary intercom protocols. Interoperability is achieved only through standard Bluetooth audio profiles.
- Cardo/Sena mesh protocols are proprietary and closed
- True mesh interoperability with them is impossible today
- Voice communication requires deterministic latency, not best-effort networking
As a result, OpenHelmet is designed around custom real-time audio transport.
- ESP32-S3
- TDMA mesh protocol over ESP-NOW (2.4 GHz)
- Opus low-bitrate voice
- Analog microphone via headphone jack
- Speaker output via headphone jack
- VOX for voice activation
- Configure ESP32 tx power to 20 dBm (100 mW) for ESP-NOW
- Silence suppression (Opus DTX) - no radio TX during silence
- ESP32-S3 + Nordic nRF52 52840
- TDMA mesh protocol over ESB radio with custom PHY control
ESB runs at 250 kbps with 8 dBm TX power (
OMI_ESB_BITRATE/OMI_ESB_TX_POWER_DBM). - SPI audio & control bridge between MCUs
Needed to make the SPI handling to take place only in 1 core of the ESP32
- Noise supression, echo cancellation
- Allow user to choose between ESP-NOW and ESB
ESP-NOW is not interoperable with nRF52' ESB.
- Add nRF21540 RF FEM to boost range
- FSC-BT1026D (Qualcom QCC3034) BT5.1 audio SoC
- Phone interop via HFP profile
- Audio mixing for legacy intercom bridging
- PCB design with integrated audio codec and power management
- Battery power with charging circuit
- USB-C for firmware flashing
- Buttons
- Open-source manufacturing files
- Testing on real motorcycle rides
- Good configuration menu buttons
- LED status indicators
- Channels for multiple groups
Channels are represented as an LED color, e.g. green=1, red=2, blue=3, etc.
- High latency
- Unpredictable scheduling
- Poor scaling for continuous audio
These technologies are unsuitable for real-time group voice.
OpenHelmet uses a hybrid MAC design:
- TDMA for voice frames
- CSMA for control traffic
- Fixed time slots per node
- Deterministic latency
- Zero collisions
- Linear scaling with rider count
Frame structure (20 ms frame, 8 riders):
| Slot 1 | Slot 2 | Slot 3 | ... | Slot 8 | Control | Guard |
| 2 ms | 2 ms | 2 ms | ... | 2 ms | 2 ms | 2 ms |
- Voice slots: 8 × 2 ms = 16 ms
- Control window: 2 ms (CSMA for join/leave/sync)
- Guard time: 2 ms (absorbs clock drift)
Motorcycle groups are small and topology changes slowly, making TDMA practical.
Used for:
- Joining/leaving the group
- Topology updates
- Slot renegotiation
- ESP-IDF v5.5+
- ESP32-S3-DevKitC-1-N8R8
- (Optional) XIAO nRF52840
- USB-C data cables
- Audio Hardware Wiring Guide
- (Optional) KiCad
# Activate ESP-IDF environment
source ~/esp/esp-idf/export.sh
# Build firmware
idf.py build# Find your serial port
ls /dev/ttyACM* /dev/ttyUSB*
# Flash firmware (replace PORT with your device)
idf.py -p /dev/ttyACM0 flash# Option 1: Using idf.py (interactive, requires TTY)
idf.py -p /dev/ttyACM0 monitor
# Option 2: Using stty + cat (non-interactive)
stty -F /dev/ttyACM0 115200 raw -echo && cat /dev/ttyACM0Exit monitor: Ctrl+]
I (xxx) omi: ========================================
I (xxx) omi: OMI - Open Motorcycle Intercom
I (xxx) omi: Boot time: 0 ms
I (xxx) omi: Opus version: libopus x.x.x
I (xxx) omi: Free heap: 275432 bytes
I (xxx) omi: ========================================
I (xxx) audio: Initializing audio subsystem
I (xxx) audio: Audio task heartbeat: loops=0, encoded=0, decoded=0
| Issue | Solution |
|---|---|
| Device not found | Use a data cable (not charge-only), try different USB port |
| Garbage characters in monitor | Wrong baud rate, use 115200 |