Two ESP32 boards can exchange data through visible light using one ordinary LED on each board. In the SecurePair demonstration, each LED both emits and detects light; a separate library handles pairing and encrypted messages, while PacketLED provides the optical communication link.
How do two ESP32s communicate using one LED?
An LED is normally used as a light source, but it can also respond to incoming light and act as a basic light sensor. PacketLED uses that two-way behavior to send data between boards: one LED flashes a signal and the other detects the changes, then the direction can reverse. The LEDs are brought face to face, so the link depends on a clear line of sight.
This bidirectional use of LEDs has precedent in published work: Mitsubishi Electric Research Laboratories documented low-cost sensing and communication with bidirectional LEDs in 2003. SecurePair is a separate, newer project; the historical report does not establish that it uses the same implementation.
What SecurePair adds to the LED link
PacketLED supplies the light-based transport. SecurePair is the pairing and encrypted-message library that can use that transport. The project also supports ESP-NOW and LoRa, so its security and pairing functions are not limited to LED communication.
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Pairing by matching blink codes
- Start pairing on both boards and bring their LEDs close together, facing one another.
- Watch the synchronized sequence of 20 short and long blinks on both boards.
- Compare the codes. Confirm on both boards only if they match; a mismatch is rejected.
The comparison gives the people setting up the boards a way to check that both sides display the same code before accepting the pairing. The project says the resulting key persists across resets and that messages use authentication and replay protection.
Cryptography and its limits
SecurePair’s documentation describes X25519 key agreement, HKDF-SHA256 key derivation, and AES-256-GCM encryption and authentication. These are claims about the project’s implementation, not an independent assessment of it. The README states: “The protocol has not yet been reviewed by an independent cryptographer.” Do not treat the demonstration as independently validated or assume it is suitable for production-sensitive use.
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What you need to try the LED pairing example
- Two ESP32 development boards running Arduino-ESP32 core 3.x.
- One LED and a suitable resistor for each board.
- A button or other yes/no input on each board to confirm or reject pairing.
- PacketLED version 1.1.0 or later.
Follow the project’s circuit instructions for LED orientation and resistor selection; the README’s requirements do not specify a universal resistor value. Avoid assigning a button to a boot-strapping pin: holding such a pin during reset can put a board into download mode.
The SecurePair README reports hardware testing on ESP32, ESP32-C3, and ESP32-C6 boards using Arduino-ESP32 core 3.3.12. That is a project-reported compatibility statement, not a guarantee for every board or later software combination. Its documentation also identifies ESP32-S2, S3, C3, C5, C6, H2, and P4 as having the HMAC peripheral required for its encrypted-storage option; check current project guidance and your exact board before relying on that feature.
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How the communication options compare
The following distances and message sizes are specifications reported by the SecurePair project README, not independent measurements or benchmarks. LED distance depends on the LEDs and requires line of sight.
| Transport | Project-stated range | Documented message size | Important caveat |
|---|---|---|---|
| PacketLED over visible light | A few centimeters to 2 m, line of sight | 35 bytes | Pair LEDs a few centimeters apart; achievable distance depends on the LEDs. |
| ESP-NOW | Tens of meters | 217 bytes | Boards use a shared Wi-Fi channel. |
| LoRa | Kilometers | 216 bytes | The README lists SX1276/78 modules, but says LoRa has not been tested on hardware. |
The README says the project tested LED pairing up to 2 m apart and tested pairing and messaging over ESP-NOW on hardware. Its LoRa figures remain documented specifications rather than demonstrated results. For a simple build, the smallest example pairs two boards over light but sends no messages; other examples send messages over LED or ESP-NOW, and another adds an SX1276/78 LoRa module.
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Project status and where to check details
SecurePair was documented as beta version 0.5 in the project repository accessed October 7, 2026. The project’s compatibility, example wiring, and transport specifications can change, so consult the SecurePair project repository before assembling a current build. The project README is also the source for the reported testing and limitations; Jenny List’s October 5, 2026 Hackaday report covers the demonstration. For background on earlier bidirectional LED communication, see the 2003 Mitsubishi Electric Research Laboratories report.
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