Introduction: Arduino Encoded and Modulated Laser and Infrared Serial Communication
Hey there! Do you want to learn how to communicate wirelessly between two computers using lasers / LED's and photo-receivers? Need to send a signal that is robust to noise? Here is the solution for you.
This is an arduino library which enables you to use a simple laser or LED to transmit characters between two arduinos (transmitter arduino with the laser to receiver arduino with a photodiode). Each byte of data is encoded (but not encrypted) to add some robustness to noise during the transmission and then modulated. I originally wrote this as part of a netduino robotics project for uni, but I've since modified it to work with an arduino. I've tested it and it works with an LED also, but the range is slightly better with the laser.
If you want to learn how the code works, read step 1. If you just want to test that it works or incorporate it into one of your projects, skip to step 2.
What you need can be found on eBay:
Step 1: How It Works (In Chronological Order):
1. Hamming(7,4) Encoding:
- each byte of data (in ASCII: D7 D6 D5...D1 D0, e.g. D3 is 3rd digit of the byte) typed into the serial window is split into its most significant and least significant 4 bits, AKA nibbles: D7...D4 and D3...D0
- each nibble is encoded via hamming encoding, producing an unsigned 8 bit integer. The extra 4 bits contain information about the original nibble to help detect errors and recover the original nibble in case some of the data was scrambled due to noise in the transmission. The encoding / decoding algorithm has its limits on how much unscrambling it can do but it is certainly more efficient than sending multiple times to improve accuracy.
- the encoding is as follows: y = x G, where x = (D0 D1 D2 D3) and G is a Hamming(7,4) matrix, producing y = (H0, H1, H2, D0, D1, D2, D3) and then we add an additional bit of even parity at the front y' = (P0, H0, H1, H2, D0, D1, D2, D3) where the parity bit P0 is just the XOR sum of the other bits.
- the two encoded nibbles (now 8 bits each) are concatenated, least significant byte first to product a 16-bit integer
- the 16-bit integer is then split into is least and most significant bytes
- each byte is modulated via manchester modulation: 2 start bits to let the receiver know it is about to receive actual data and not just stray signals, then the byte followed by 1 stop bit, producing 11 bits. Each of those 11 bits is represented with two half-bits: first half-bit is the original XOR'ed with 1 and the second is XOR'ed with 0 instead, this adds a clock pulse into the signal. This produces 22 bits for each byte.
- the pair of 22 bits are joined end on end, least significant first, producing 44 bits of data
The 44-bit data is sent via the laser or LED, 1's are on and 0's are off
4. Manchester De-Modulation:
The 44-bit signal is received by the photo-detector and manchester-demodulated (reverse algorithm of modulation) to reconstruct the 16-bit encoded signal
5. Hamming Decoding:
The 16-bit demodulated signal is then hamming decoded to reproduce the original byte
6. Printing Message:
The byte is then printed on the serial window
Step 2: Install the Library
The library I wrote, LumenWire, can be found on my github repository here. Download it to your computer, you won't need the README.md file though. Just add the HT_LumenWire folder to your libraries folder in the arduino folder on your two computers.
If you come across bugs in the code or have simple recommendations on how to improve the performance or add extra functionality / features, you're more than welcome to let me know in the comments section. Thanks!
Step 3: Test the Setup:
- open the arduino application
- open File -> Examples -> HT_LumenWire and select either the photoreceiver or transmitter examples
- plug in the arduino and select the appropriate COM port, then upload the transmitter / receiver sketches to their respective arduinos
- connect the laser as follows: laser VCC pin to pin 6, - pin to ground
- connect the photodiode as follows: photodiode signal on pin 7, power pins as normal
- open the serial window (spyglass symbol in the IDE) on both computers
- point the laser or LED to the photodiode
- enter a character or phrase in the serial window on the computer doing the transmitting and press enter to send
- you should receive the message on the receiver serial window automatically
Now that you've verified the setup works, what you can do with it is up to your imagination. Do you want to further refine or modify the library to add extra features? Are you planning to incorporate it into a bigger project? Have fun.
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