Dead Simple LED Communication

Ideally for most devices, you want a small alphanumeric LCD to display quick information. However for some companies, they are too much of a cheapskate to even include one.

Concept: "Dead Simple" Visible LED for low speed but realiable communcation, to a smartphone via it's camera?

(Why not bluetooth or NFC? Because cost too much)

How is this LED hooked up to the microcontroller? There is three methods I think should be considered.

1. UART hooked to an single colour LED: Cheap but would require a UART port (Not easy to bitbang serial), and requires accurate timing.

2. SPI communication over RGB LED: Bit more expensive than first option and more PCB area to route (due to two extra LEDs). [Red, Green, Blue] = [SCLK, MOSI, SS] = [Clock,Data,Sync]

3. Custom protocol over 2 LED: Bit banging required, but is clocked via "pulses". E.g. PulsedRed=0, PulsedGreen=1, RedGreen(Yellow)=Sync , (The clock is implicit via pulses).

4. I2C over 2 LED: [SCL, SDA] on 2 reds. (Idea from lurch)

5. RGB LED: Red=Sync, Green=Bit1, Blue=Bit2

I would prefer (3), as it has the benifit of (2)'s clocked transmission, simplying the reciver's program. While (3) is not as "dead simple" as (2), it has the advantage of only needing 2 LEDs, Green and Red. And finally, its easier to record 1's and 0's from (3) manually via eyeball if needed, since 1 is green, and red is 0 (At low speed, e.g. error code).

Both would still be relatively easy to reverse engineer (sorta futureproofed?).

Forward Error Correction is definately recommended, but can be optional.

This will be very interesting if smartglasses become popular enough. Imagine walking though a server room, and seeing a real time serial feeds when staring at a mounted server.

-------------

TERMINOLOGY: (SPI:~Serial Peripheral Interface) (Clock:~ Indicates next bit of data) (Sync:~ Indicates that a character has transferred) (Data:~ current bit to transfer)