I'd imagined an antenna under the carpet reading a short distance, or to the side of the step, reading a longer distance. The scenario is a young boy with epilepsy, and other complications which also affect his walking. The stairs in his home are steeper than normal. A stair lift has been fitted for him, but he prefers not to use it unless he is really struggling.
His mum generally knows when he's going up and down the stairs and can just keep an extra eye on him in case he has seizure part of the way up, but the position of the stairs is such that he can easily get to them without actually being noticed sometimes. To complicate matters, the stairs are in a small hallway, where he will pass between two ground-floor rooms without actually going up.
I am a volunteer engineer for a charity in the U.K. that custom makes equipment for people with disabilities, where there is no commercial solution, and I was asked by the mum whether I could come up with something that would alert her if the young boy (as opposed to anyone else in the house) were to put a foot on either the top or bottom tread of the stairs. As there is another child in the house, a button that suppresses the alert for everyone else to use isn't really feasible, as the boy in question may then suppress it.
I can see how a passive tag on the sole of the shoe would work, and he does apparently wear the same special shoes around the house, but I'm unclear yet whether it would be feasible to attach the tag beneath. I have seen an RFID antenna in the form of etched PCBs, which would probably cope with going under the carpet and being trod on, but I have never seen an antenna wide or narrow enough to pick up the varying foot position on the step. Certainly, in the 125 kHz and 13.56 MHz RFID equipment, the antennas seem to be small loop types and the range is relatively small—say, an inch or two, at best. That's fine if he always puts the same foot in the same position, but we can't guarantee that.
My preference would be to go with UHF and a tag tied to the shoelaces, but RFID technology seems to be one extreme or the other—low in cost and a very low range, or high-cost commercial equipment for long-range tracking of assets. There doesn't seem to be anything that is cheap and with a range of, say, a couple of feet, which would be ideal for this purpose.
Yes, this is certainly possible. A passive HF RFID transponder in the sole of the shoe and an antenna in the stair tread would work better than on the side, but you could use a passive UHF transponder on the shoe and a reader to the side as well. You would need to be sure that the antenna under the carpet was not damaged by people walking on the stairs. You would also need to be sure you were not infringing the privacy of people going up or down the stairs.
—Mark Roberti, Founder and Editor, RFID Journal