Comment by SilentM68

That's a great design and idea. Have you considered designing a light-based Morse code device? The light would be invisible to the naked eye, requiring special glasses/detector to translate either mechanically or digitally. The source light (e.g. neon, lamps, something new) would be encoded with encrypted messages, so it doesn’t look like obvious Morse code by using aspects of steganography where encrypted data would be hidden in pseudo-random light intensity/pulse patterns mimicking candle flicker, noise, etc.). It would require a custom protocol, error correction, obfuscation. All made on the cheap.