Battery Free Sensors interesting for anyone?

Hello I am new here and looking for some kind of feedback from Smart Home enthusiasts:

Currently I am working with a few colleagues on designing a class of battery free devices (Think RFID, but as Sensor). Currently this is mostly done for industry applications. Passive sensors are used wherever electronic/batteries do not survive long enough or add to much weight.

Main idea for me is that I in the past kind of disliked the reliability of battery driven SmartHome devices, while at the same time cabling everything is not an option to me in a rented flat. I believe reliability has improved a lot over the last years (ZigBee ftw), but I kind of dread the idea to put sensors on every door and Window, as it would increase maintenance of my system.

So our idea is to put one cabled reader device in the middle of the room, i.e. like screwed to the ceiling next to the light where your smart lightbulps are and where there is electricity and use this to read out passive sensors (door/window-contacts, buttons, temperature). The middle position is kind of enforced as we do not expect the tehnology to have much more than 4 meters of range.

I am meanwhile wondering if this is just me who wants that/sees value in it or if there is more people out there who think the same?

Also, if yes: What would you be willing to pay to exchange all sensors in one room to be passive and have just one active cabled device? (200€,500€,800€?)

Is there features you think this device would definately need. e.g. (Wifi+mqtt,REST api, or Thread+Matter support)


What RF source would be able to provide enough power for that?? If Wi-Fi now dense would the deployment need to be?

The RF source would need to be close as said in the middle of a room. That would be the cabled device.
Using 2.45 Ghz ISM band.

So it would conflict with 2.4 GHz smart Wi-Fi & possibly Zigbee devices. What are you planning for data access? Z-Wave?

I would say the read out is in the microsecond time range… meaning if you read out Buttons 10 times per second, doors 5 times and windows once… you might block the medium only for such a small time that you will never notice. (3Windows +2 Doors + 2 buttons would for example block the medium 1/30000 th of the time)

I would say the device itself should be connected per wifi with your smarthome… but IEEE 802.15.4 (zigbee/thread) may be a possibility.

Wi-Fi has high overhead, not well suited for low power devices. Zigbee was designed to minimize battery power usage.

But the idea is that the central device is not battery powered…it has wifi and talks to your smarthome…

Overhead does not matter too much as the device will not be on battery.

THe passive sensors use their own system… that has nothing to do with wifi or zigbee…

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I assume this would be another proprietary ( closed, non-free) protocol. That minimizes usefulness in an open community IMHO.

Nah that would be not per se a protocol that you want to have … but more like the protocol on the chip in your zigbee/zwave device that you don’t care for… as long as the device itself speaks mqtt/zigbee/zwave in the end with your system.
Think of it as I2C … everyone can use it… but as enduser nobody will except if you want to solder areound in your own device, but then it is actually not hard.

I guess I’m not clear on the definition of “passive sensors.”

You suggest these go on things like doors and windows. What powers them? Are we talking Nikola Tesla stuff here?

Obviously powering all the sensors we love to install introduces compromises. New options are always welcome.

Yeah Nikola Tesla is the idea… that is why I said RFID. You have a reader device on the ceiling that gets power from the grid. It sends out a radio signal to a very simple sensor device free of electronics, batteries and what not. This resonates in the frequency and sends a weaker answer back and measurements are done by observing the frequency of the resonance, which could be changed by a magnet (door window sensor) or temperature etc.

While in a metal faraday shield electrical box?

I had this idea a few years back, when I was looking for temperature and window contact sensors. But I didn’t find an effortable solution. 500€ for a single reader was too much… I also didn’t want to buy 1000 temp sensor tags :grin:
However, I would love to see such a system.

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I like the idea but the cabled device per room is bulky to me unless it simply plugged into AC and utilized zwave/zigbee/wifi to communicate back to HA server. Right now it sounds like adding another hub type device and I set my max at (2) hub with WiFi being #1, so not much room after that.

I love idea of rfid door/window/temp/button but would prefer if the per room hub device utilized local AC for power and existing comm to get back to HA.

40€ per room for hub+2 sensors i would pay maybe

Pragmatically, if it works with Matter (or an existing, widespread, standard) you’re more likely to have an interest. If this is yet another custom protocol… :man_shrugging:

Yes wifi +mqtt would be the way to integrate this with different open sourse solutions…
zigbee is pretty much going to be “Thread+Matter” in the future … so this is the other alternative for integration that may be interesting.

While 40€ per such a hub + sensor is impossible… I currently doubt that 200€ would be possible for the hub… but sensors could be cheapish in the 20€ per piece range…

Either Wifi+MQTT or Thread+Matter I guess is the future for this… I am not sure which…
It feels wrong to use a 802.15.4 standard with a powered device, but would probably make it easier for many…

Not too sure about this, a few questions if you don’t mind.

  • You compared this to RFID, but then you mentioned it would be electronics free and rely on analyzing the reflected interference patterns. This confuses me. Passive RFID uses an actual MCU that is powered by RF induction and actively sends back digital processed data. Yours doesn’t, so your system doesn’t really have anything to do with RFID at all then ?

  • If it only relies on reflected pattern analysis, how reliable is this ? This is not an industrial controlled environment, but a home where things can move around and randomly change, you get changing signal reflections with multi path returns, etc. Would you have to recalibrate the system every time you move around (metal) things in your room ?

  • You said it works on 2.4GHz. That band is ultra crowded. How well would it handle continuous stream interference, for example from video streaming / big data downloads over wifi or similar ? Being passive and relying on very short pulses, your system does not have active error correction, CCA or ARQ requests to handle interference from other devices. Won’t that lead to massive reliability problems in crowded and changing RF environments ?

  • Oh and another thing. What about encryption ? A passive reflection only system cannot use any type of advanced bidirectional encryption and would be very prone to signal replay attacks. Not good when talking about door / window sensors or similar.

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That is likely why there is not already something like this in the market.

If the system runs on a 900MHz band instead of 2.4, the transmission range from the sensor to the device should be increased, and there will be less interference from already existing IoT devices