A while back I did a review of some of Ananiah Electronics‘ RFID modules and readers. These are simple RF devices that fulfill the baseline requirement of an RFID device (periodically transmits an identifier)… but the great thing about these modules is the price. You can get them for a steal of a deal, which makes proof of concept applications, prototyping, or just plain home brew projects able to be done on the cheap.
The only down side at the time of my original review was the lack of an RSSI value in the reader data stream. An RSSI value is basically a signal strength indicator that tells you how well the reader is receiving a signal from an RFID tag. The upside to that is, many people use value as a poor man’s distance meter. True distance or location can never be surmised using RSSI alone, but using multiple readers placed in key locations around an area you want to monitor can give you a darn close approximation.
Well just recently Ananiah Electronics released their new RF9315R reader module that reports 8 bit RSSI data! That means you get a high resolution RSSI value between 0 to 255, depending on how strong the tag’s signal is. This is great because until now I’ve not been able to find an active RFID reader with a decent RSSI data stream for under $40, but these guys have done it.
As you can see, the tag’s ID is given, then the RSSI value, followed by a space character (ASCII 32). The only thing I might have done differently would be to make the RSSI value a fixed length field, so 008 or 092 instead of just 8 and 92. But, parsing the data is easy enough as all Ananiah tags at this point are 4 bytes long, and the “end of line” delimiter is a space. Anything between is RSSI
For the test above I didn’t move the tag at all because I wanted to see what the RSSI variation would be of a stationary tag. I was pleased to see that the RSSI value didn’t change much while everything was sitting still. After picking up the active tag and moving it further away, the RSSI value went down as expected. However the same effect could have been achieved by simply curling the reader antenna under itself or interfering with the signal in any number of ways. Because its easy to interfere with the signal, RSSI values alone can not be used to accurately judge distance from a reader, or location between multiple readers. Even low batteries could be a factor when it comes to RSSI values changing.
Even so, the bottom line is; the RF9315R is a great and very affordable active RFID reader.