Saskatoon TechWorks Forum

ADDandy wrote:the Easiest way i can think of without extra/expensive hardware is to try and send lots of information VERY quickly, more or less hoping that at distance the SNR will make the transmission not work. once you are nice and close, the SNR is good enough that the highspeed communication Works, and thus opening the door.

And why not just ask the radio chipset what the power level is? I have never seen a radio chipset that you couldn't just ask for SNR or RSSI levels from. And I know that I can get the bluetooth signal strength on my laptop and and android phone. I would imagine that there is something similar for rfid chipsets.

I'm likely going to start buying parts soon. I'll likely grab an RFID reader for my Arduino, and then a USB reader that I can hook up to my wall wart plug computer.

I'm thinking the wall wart will be the more efficient solution, but I like the idea of the arduino as I can add a GSM shield onto it as well, and we can control the door via text message or calling as well. An alternative would be to use the wall wart, which could then open the door remotely via the internet, and anyone with a smart phone would be able to control the door.

Also, it seems that not many products actually specify their range, and antenna's will just give you the gain and power output rather than any meaningful indication of what sort of range extension you should expect. Would I be correct in applying the Friis Transmission Equation to RFID?

Ranges in RF are generally meaningless, there are just too many variables in the actual path loss. It all depends on the antennas, background noise, reflections, refractions, multipath etc.

You can go back to Friis equation for an estimate but it is just an estimate, I have heard "slope" factors of >50% for RFID being used even in controlled environments. Slope factor is a way of accounting for the numerous factors that can deteriorate the CINR but are not easily modeled. You won't have to worry about foliage or atmospheric ionization over such a short range. But multipath, interference and polarization are going to be more important and can cause huge changes in signal quality and are just factors of the environment you deploy in.

Basically make and estimate with a low and a high slope factor (probably 10% and 60%, based on my quick google searching of RFID) and get something that should cover the hole range. You can tune the Tx Power and fiddle with antennas to tune within your environment to get it just right.