For the demonstration at the Radio-frequency Identification course, I have built an elementary race timing device with one wireless checkpoint. The system consists of RC522 RFID module (ISO 14443 A), two MSP430 Launchpad development boards, each with nRF24l01 2.4 GHz transciever mounted on it. One board, playing the role of main timing unit, is connected to PC via serial port. Remote node with tag reader is equipped with batteries and is set “in the field”.
After main module boot up, the user has to press a button to activate the countdown to start. Three seconds later, main unit begins to count milliseconds and check for the packet from the checkpoint. If player’s tag appears in the proximity field of the reader, tag ID is read and sent to the timer, where it is time-stamped and sent to the PC. There is a special delay machine, which inhibits sending same tag several times, preventing spam to the timing system.
Total timing lag was measured to be about 1.5 ms, when most of the delay is caused by the RFID library during tag identification. Because this is a systematic error, final results can be corrected based on that data.