RFID system for proximity detection with data logging

Question

I am wondering if anybody has ideas for developing an Arduino-based system that would use RFID to log occurrences when two people are in close proximity to each other (e.g., <= 1 meter). Ideally, I would like to construct something that would operate in a manner very similar to the OpenBeacon system -- and, preferably, the events/data would be transmitted via wi-fi in real time.

My ideas are as follows, so any feedback or alternative solutions would be greatly appreciated. (Please keep in mind that I am not an engineer!)

I am thinking of using a wi-fi enable microprocessor (e.g., Arduino Yun) that would serve as the reader or detection system. For beta-testing, this device could be set in an enclosed case and attached to a belt. I presume it would require a fairly robust battery supply. The other person would be wearing an RFID tag that has a range of <= 1 meter and would log the event. Ideally, a reading would take place roughly every 30 seconds, and the captured event would be transmitted over wi-fi.

Here are a few other things to keep in mind:

1. The people using the hardware should not have to do anything other than simply wear the constructed devices.

2. The reader itself should be as small as possible.

3. My ultimate goal is the data logging of the proximal events.

I am certainly open to other technologies, as my ultimate goal is simply the proximity detection with data logging (preferably via wi-fi).

Thanks in advance! Brian

Answer 1

Consider using two basic XXMhz transceivers:

-Master loops through used ID's and transmits them one at a time.
-If a belt receives an ID other than it's own, it remains quiet.
-If a belt receives it's own ID, it transmits "Marco" (quietly) to the other belts.
-For a receiver to hear the very quiet signal, it will need to be very close.
-The RSSI signal can be monitored for signal strength to determine close(enough) proximity.
-If sufficient proximity is detected, the event is reported to the Master.
-If it receives no response, it informs the Master.
-Master proceeds to the next ID.

In this setup, you only need to provide networking (WiFi/Ethernet) to the master. (cheaper)

So that each belt can identify the others, "Marco" could be the transmitter's ID, but you'll need to add some sort of "protocol" so the belts know if the ID they hear is from the Master or another belt.

If ID is only 7 bits, the "protocol" could be as simple as a bitmask:

Server = Send(ID &  (1 << 7)); //1XXXXXXX
  Belt = Send(ID & ~(1 << 7)); //0XXXXXXX

Belts only reply to their ID if MSB is 1

This example has excessive range for your purpose (250m); these radios will be screaming at each other and the RSSI signal is unlikely to provide meaningful data. http://www.seeedstudio.com/depot/nRF24L01Module-p-1394.html

Using RFID, you are wanting (very)long range from an (intentionally) short-ranged system.
Using ISM, you are wanting (very)short range from an (intentionally) long-ranged system.

Neither is 100%, but both should work.

Answer 2

This is a cheapo 125khz reader from Seeed Studio (Shenzhen): http://www.seeedstudio.com/wiki/index.php?title=Electronic_brick_-_125Khz_RFID_Card_Reader

The range on it is very limited and I'm not suggesting you buy that specific one. Instead, just notice the pinout, +,-,TX,RX. You will find many, many readers with that same pinout. When a tag is detected, the reader blindly transmits the ID via UART (serial). You will also find many WiFi modules with similar TX/RX pins.

In your situation, the Arduino is actually wasteful since it's only purpose will be to mirror the reader's data from it's RX to it's TX. It's better to cut out the middle-man and just connect the (voltage-compatible) reader directly to the WiFi module to save power.

Many wireless modules are capable of automatically going low-power when there is nothing to do. Part of the receiver circuit will activate for transmitting, but since you are only transmitting sporadically from the belts, neither the receiver or transmitter needs to be powered constantly. I think you will be pleasantly surprised by the battery life.