Communication over 433Mhz links

I’ve bought, about a year ago, maybe more, some 433Mhz transmitter and receivers , so I could build a mailbox monitoring solution, loosely based on LOFI Project. The base idea was when someone put something in my mailbox I was notified. Anyway, the ESP8266 came along, and after some experiments, there is no way that the ESP8266 can be used for implementing the monitoring mailbox project I was thinking due to power consumption but mainly because the high distance across several floors and walls between the mailbox and my access point. So back to basics and to the original simpler idea of using plain 433Mhz link for transmitting data.

I’ve started to do some experiments using these devices:

eBay 433Mhz RX TX

All I can say about these is while the transmitter is ok and works fine across floors and walls (I can see the signal clearly using my SDR), the receiver is absolute garbage and useless for the intended purpose.

The receiver is only able to receive in line of sight with the emitter when there are no obstacles, and even in this scenario with both emitter an receiver with attached antennas, the maximum distance between the emitter and the receiver is 4/5 meters at maximum.

The solution is to use the much better and higher cost RXB8 receiver that has according to the datasheet -114dBm sensitivity, but it does work and is very good.

rxb8

The code for interfacing with this receiver is the same code that works with the cheaper receiver and is the Arduino Manchester encoding library.

Using the RXB8 with an attached 433Mhz antenna (I’m using an antena with 5dBi and a SMA connector), the results are simply superior, with the end result that the signal/messages when the emitter is located several floors down and across several walls, are received correctly and are able to be decoded by the Arduino.

Anyway, one of the interesting things on the LOFI project is/was the Hamming Error Correction implementation found here in the RobotRoom blog.

So I’ve forked the original mchr3k Arduino Manchester encoding library and added the Hamming Code Error correction support: Arduino Manchester encoding library with Hamming EC support.

The usage is quite simple, and can be seen on the examples Hamming_TX e Hamming_RX.

The two new functions are:

uint8_t Manchester::EC_encodeMessage( uint8_t numBytes, uint8_t *data, uint8_t *ecout)

where the input data buffer is provided by the data parameter with the buffer size also provided by the numBytes parameter. The output buffer ecout should at least be one third bigger than the original buffer since for every two input bytes an additional parity byte is added.

For receiving the buffer with the data and parity data is decoded and corrected if any errors are detected. Errors are corrected if possible.

uint8_t Manchester::EC_decodeMessage( uint8_t numBytes, uint8_t *ecin, uint8_t *bytesOut, uint8_t *dataout )

The input data is the size and the buffer with the data and parity, namely the numBytes and ecin input buffer pointer, and the output is returned on the dataout buffer, with the decoded size also returned on the bytesOut parameter.

Above these functions we can build some logical protocol for the received data, but that really depends on the way we want to use the library.

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