This document describes a smart RFID tag designed to detect moisture or water. The tag uses a passive HF RFID system with a grounding plane near the tag. When dry, the grounding plane reduces the tag's read range, but when moisture is detected, a material absorbs water and increases the distance from the grounding plane, allowing the tag to be read again. The final design packet uses a 45mm tag with a 26mm read range and detects moisture in adult diapers. A display shows readings from two RFID readers and alerts users when moisture is detected.

1. Smart RFID Tag for Moisture
Detection
Introduction
RFID is a system that provides contactless unique
identification of a transponder embedded in RFID
Tag from a RFID reader. The reader is able to power
the tag through electromagnetic induction meaning
that the tags can be fully passive.
In recent years smart RFID systems have been used
for material detection and other functions. Smart
RFID systems exploit radiation patterns of tags to
give the reader an indication of the RFID tags
environmental conditions. For this project a Smart
RFID tag will be designed that is able to detect a
threshold level of water.
The RFID system used for this project is Passive
with High Frequency (HF) range.
Figure 1: Basic Passive HF RFID System Diagram
The purpose of this project is to use a standard
RFID tag to detect a certain threshold volume of
water. A packet will be created that could be
embedded into an adult diaper that can indicate to a
RFID reader when water is present.
Aims and Objectives
Requirements of the Project
- Test how RFID tags perform in wet environments
- Design a Smart RFID Moisture Detection Packet
that is able to read at approximately 30mm
- Design a module that can detects multiple RFID
reader outputs and display results to a end user
Moisture Detection Packet
The moisture detection packet will use a grounding
plane (Silver Foil) in very close proximity to the RFID
Tag, while the gap between the two is very small the
radiation efficiency of the RFID Tag will be reduced
making the Tag undetectable by the RFID reader.
Once water is introduced to the packet Sodium
Polyacrylate powder will absorb and expand. This
will increase the distance between the tag and the
grounding plane reducing the effect on the Tag
radiation efficiency allowing it to be detected by the
reader again.
Figure 3: Diagram of how the moisture detection packet functions
Packet Detection Display
For this project 5
different sized HF RFID
tags were purchased for
testing. Each was tested
for read range while at
certain gap distances
from a grounding plane.
After these tests two
provided the best results
a 35mm Tag and a 45mm
Tag these would both be
tested in the final moisture
detection packet.
The display panel needed to input readings from
multiple RFID readers, for this project the display
would read from two readers. The display will
indicate on a LCD screen which RFID reader had
been triggered and also what time the tag was
detected. The display will have a reset button built in
to clear the screen once
the tag has been
attended to also a LED
will be built in to clearly
get the attention of
users. The display uses
a Arduino Mega as the
Microprocessor.
Moisture Detection Packet Tests
Final packet was designed
to be 50x50mm in size with a
45x45mm perforated silver foil
grounding plane. Ten of each
Tags packets were made to
ensure reliable results. The
45mm packet results were far
more consistent so the final
design would use the 45mm tags.
Figure 4: Read Ranges with ground plane
Figure 5: Average Packet Results
Conclusion
Figure 2: Final Packet Detection Display
The aims of the project were met the final 45mm
moisture detection packet was able to read with a
range of 26mm while embedded into an adult diaper.
The final packet design seen in figure 6. The
hardware also meets all of the original aims being
able to display multiple RFID readers detections with
the capability of further
Development, Final display
seen in figure 2.
Figure 6: Final Packet Design
The read range could be
increased if a higher grade
RFID reader was used. It
may also be increase if a
UHF RFID system was used.
Jack Soldano - JS702
Dr John Batchelor