I’m not sure if this even is possible, but I’ve been thinking about it so I figured I’d post it here and see if anyone who knows more than I do has thoughts about the feasibility. Would it be possible to build a RFID sensor to determine if a door was open or closed. The tag would be placed between the door and the wall. At the most basic level it would sense a magnetic connection between the door and tag. If there when the tag is excited by the reader it would respond high. Otherwise it would respond low. I’m not 100% sure if it is possible to add/replace to the onboard memory that is typically transmitted when the reader scans. Maybe there could be two memory components on the tag and which is used is based on whether the door is open or close. Maybe two tags could be used together one on the door one on the frame and the connection between the two if it exists does something. Like I said I just started thinking about this, but if it is possible it would create a sensor that does not require a battery. The reader would handle the zwave communication. That seems like the easier part. If this is possible I think the possibilities would be endless.
What advantage would this have over a battery-powered zwave reed switch door/window sensor?
The way I understand it, RFID (and NFC) sensors are actually powered by the reader. The reader sends out a wireless signal that excites a coil in the RFID tag, providing it with power (similar to how wireless charging works). Once powered, the RFID tag sends out a separate signal that is read by the reader. In order for this to work, the RFID reader would have to be close enough to the tag to both power it and to receive the reply signal.
Without relatively large tags or pumping out a high number of RF Watts, I think range would also be a significant limiting factor. Dont RFID tags typically only have a range of about a foot? If every tag needs a nearby reader, and that reader needs constant power, it seems like alternate solutions would be easier.
From what I’ve read RFID can have ranges of 10m+. The biggest benefit I see is for looks. The cylinder door sensors are kinda an eyesore when it comes down to it. They’ve made some that you drill into the door, but I’m not super inclined to drill a 1in diameter hole into my doors. GE made a hinge but that is bulky. With these the reader can be positioned somewhere else. The actual door would have a very small tag somewhere that isn’t visible. There wouldn’t be any batteries to worry about etc.
I am not sure about the size requirements for longer ranges.
Just as an example.
https://learn.sparkfun.com/tutorials/simultaneous-rfid-tag-reader-hookup-guide?_ga=2.157980222.503987762.1613154603-1168779790.1613154603
I think with this reader and these tags we are talking like 16 feet. The tags should be super cheap to create in bulk. I’d imagine significantly less than $1. If you could sell the reader with zwave for $30-$40 it’d be way cheaper for the user. One plugged into a outlet on my 2nd floor could cover every door in the hallway.
I did some more research. It appears people are working on RFID sensors now, but it looks like long range scanners are a bit expensive still.
Okay let’s talk conceptually for a moment.
RFID tags are (almost all) powered by the device that reads them. The reader is transmitting the interrogation signal as electromagnetic waves of a particular frequency. The tag has a tuned antenna and circuitry that can both decode this signal, but also harvest the energy present in the signal itself. This energy is then used to power the chip, and when a response is required, formulate and transmit the response.
The downside to this is the range- for the tag to be able to harvest enough energy to transmit, it must be within the near field of the antenna (inches). As the range increases, transmit power of the reader must increase. Because at a greater range the power is spread out over a larger area, even a small increase in range requires a large increase in transmit power.
The RFID tags that can work over several feet are, for the most part, active tags. The active tag has a small battery embedded within it. It still gets energy from the reader, but not enough to power a radio transmission. That’s what the battery is for. Even with the battery, the tag needs a directional antenna pointed directly at the tag.
Now let’s apply this to a window.
You could certainly have an RFID tag that also had a GPIO pin of some kind, when interrogated the tag would return a unique ID as well as the pin state (open or closed). However the problem remains the same- the tag either needs a reader very close to it (in which case there’s little benefit over a standard magnetic contact), or a powerful antenna pointed at it (inconvenient if you have several), or it needs a battery and a large nearby antenna (which removes the tiny size benefit).
So I guess my question is, what is the benefit vs. a standard magnetic contact?
I agree that it is not a great idea if it is impossible to read multiple tags within a 10-15 foot range. Things I read online suggested this is possible with a UHF passive tag, but I did not see that a directional antenna was required which would obviously make this not that feasible.
The goal of this is to avoid having to put a cylinder on every door you want sensed. Also to avoid having to drill into the door or to have a bulky hinge. These are all the solutions that I know about besides the flat ones that are extremely expensive and you can’t replace the battery.
Speaking conceptually this is not impossible, but that 10-15’ range is in open air with few or no obstructions.
To design a hypothetical such system-
Each room would need a reader device mounted on the ceiling. This reader would use a phased array antenna- that’s a matrix of several antennas together, slightly different versions and phases of the signal get transmitted by each, so when they combine they form a beam of signal much like a directional antenna. This would have to be ceiling mounted, however it could be mostly flat and painted so it wouldn’t stick out.
The sensor would need a magnetic reed switch, so it’d need about 2-3mm thickness at that point. However the rest of the sensor could be essentially a sticker, containing the antenna.
The magnet for this could be about 10mm diameter and 2-4mm thick, maybe smaller. Also, think sticker. On a wooden window both the magnet and the reed sensor could be countersunk, leaving an essentially flush installation.
The advantage of this- it’ll run forever with no battery changes, and it’d actually be somewhat more secure than a normal RF contact sensor- the RF sensor only transmits when opened, whereas the RFID sensor would probably be interrogated 1-3x per second. It’d also be an almost invisible install, that could be easily painted over. And it’d be a great option for rooms with a lot of windows and doors.
The disadvantage- cost. The reader would take a lot of engineering, the sensor tags would be a custom design, and once that’s all done you have to hope some alarm company buys it, which they may well not because alarm companies don’t want complication, they want to install alarms quickly and move onto the next customer.