Sunday, November 11, 2012

Teardown of Wireless Sensor Tags

The Wireless Sensor Tag is a smart tag system by CAO Gadgets.

The Wireless Sensor Tag
(Credit: CAO Gadgets)
The system is based around an Ethernet Tag Manager, a small box that connects directly to your home router and manages the all the associated tags. Basically it acts as a bridge between the wireless tags themselves and the Cloud backend which manages the tags.

The Tag Manager
(Credit: CAO Gadgets)
Each tag has motion and temperature sensors and can be configured to notify you via tweet, email, push notification, or via a URL callback (either to an Internet facing host or directly to an internal IP on your home network) when the tag is moved, or when the temperature goes out of a user defined range, or if the tag itself is moved out of range of the tag manager. Each tag also has a piezo electric buzzer and an LED which can be triggered using the same ruleset.


I actually ordered my system back in August, only to be told that the sensor tags had not been CE certified, and they weren't quite prepared to do that just yet. I could either cancel my order, or wait until they had enough orders to that it would make "economic sense" for them to obtain CE marking at which point they'd ship me my order.

I have to admit I wasn't that impressed by that. If you're offering something for sale internationally then you should have the items in stock and you should be able to ship it out of the country. If they weren't prepared to ship into the EU, they shouldn't have processed my order, or billed my credit card.

They also seem to be having supply chain problems as their current stock is sold out and their store has a note on the front page saying that the lead time for a new order is currently one to one and half months between order placement to fulfilment.

However it looked like someone had put a lot of thought into this problem space, and despite the number of similar systems that are starting to appear, the system should be a neat solution. So I hung on, and I'm glad I did, as I I'm actually quite impressed with the build quality of the hardware and how it hangs together as a system...


My system arrived in a plain USPS box. Inside was the tag manager, an ethernet cable, a USB power adapter and cable, ten tags with velcro strips, and somewhat oddly, seven spare batteries.

The Wireless Sensor Tag System
Setting up the system is a fairly simple procedure. Plug the tag manager into your router and let it grab a DHCP address and announce itself to the server. Theoretically the tags included in the tag manager in your shipment should come pre-associated, but at least for me this didn't seem to be the case, but that said associating them is no big deal. Pull the battery tab, and if you have a flashing LED that means the tag is unassociated, go to the web app and click on the button to add a new tag. The tag then shows up in the list on the web backend and you can go ahead and configure the triggers for that tag.

The Software

Unfortunately despite initial promise, I'm not finding the system amazingly reliable as yet. Tags that are within a few feet of the tag manager are being periodically being reported as "Out of Range" by the web interface. However it's possible that the hardware is fine and I'm just not understanding the software.

Update (12 Nov): Just got an email from the manufacturer saying that they've "fixed some bugs on the server" and these spurious "Out of Range" notifications shouldn't happen any more. It's a bit soon to tell one way or the other, but certainly I'm not seeing them at this point, so this might well be fixed now.

The web application used to manage the tags on your system
The web application is confusing, the interface basically looks like someone has exposed the backend database schema in software without much thought as to how the user is going to interact with it. At the moment the interesting hardware is being let down badly by the back end software, there should be preset options, e.g. "this tag is attached to a door", "this tag is attached to a moveable object", that would auto-configure the tag to common presets.

As it is there are numerous settings to go through for each and every tag, and most of these aren't well explained, and the buttons to set the options are not self explanatory or just flat out confusing. 

I'm sure it makes sense to the programmer that built it, as they understand exactly how the system works and how the components interact, to the rest of us, at least initially, it's confusing. 

I've been playing with the system for about an hour now and I still can't figure out how to get a tag to start bleeping when moved, and then keep on bleeping until manually reset. It's something you'd commonly want to do for a tag that's attached to something that might be stolen, and the system should be able to do it, but I can't figure it out. It's probably obvious once you know how, but...

They need to get a good UX person in, along with a designer, to overhaul the backend. That could make a big difference to the software and its usability for the average person. The system itself is really elegant, and easy to get up and working, the software to configure the tags is letting it down. 

Another frustration for me is that the iOS app that allows you to monitor the tags, and receive push notifications to your phone when one of your triggers is tripped, is only available in the US App Store. The ability to get push notifications was one of the major reasons for me to buy this system, so I'm hoping this is going to be resolved quickly. Again, if you're going to ship outside the US you need to make sure your system is ready to go there.

Update (12 Nov): The iOS app is now available in the UK store, which resolves one of my main problems with the system. Although unfortunately the software UX problems extend to the iOS application. It's comprehensive, but not easy to use.

The iOS Tag Manager application running on my iPhone 5
The Hardware

There is actually very little information about how the system hardware works, and I'm not in favour of "just magic", so I wanted to take a closer look at the board to try and figure out what's going on under the hood.

The rubberised enclosures are more-or-less indestructible and amazingly stretchy so they're fairly easy to take off, which is a good thing as you'll need to do that before pulling out the battery tab to activate the tag as they tend to break off if you try and do this with the enclosure still on...

Wireless Sensor Tag
The tag is powered by a single CR2032 button cell Lithium battery which they're claiming will last anywhere between two months and seven years depending on the sensitivity and polling interval you choose for the tag.

The onboard processor is a Microchip PIC16LF720 micro-controller, an interesting choice that draws about a fifth of the current than the Amtel ATmega micro-controllers used in the familiar Arduino boards. Wireless operations for the tags is provided by the Microchip MRF 49XA radio, which operates in the sub-GHz MHz ISM bands. While this can be changed, the tags ship and by default operate at 436 MHz. Unlike the US this is not an ISM band here in the UK, and although it is part of the amateur band, a license to operate is required and the primary user of the band is the MOD.

The choice of the sub-GHz ISM band is an interesting one, most competing products on the market use the 13MHz RFID bands, or more commonly the 2.4GZ band used by both Wi-Fi, Zigbee and Bluetooth LE devices. I'm guessing that they went with the sub-GHz bands to keep power usage to a minimum and extend the battery life of the tags themselves and provide a good range.

Motion sensing is provided by an Honeywell HMC5883L sensor, a three-axis magnetometer, again an interesting choice as most of the competitors are using linear accelerometers. I'm presuming they used a magnetometer to get good angular measurements, which is a perfect fit for one of the main uses cases for the tags; checking whether doors are open or closed. 

Finally I'm guessing temperature measurements are provided by a fairly anonymous chip marked with "AXWB." While that's just a guess, the word "TEMP" is the silkscreen legend next to it, so it's probably a decent one. I haven't been able to track down the data sheet for this chip or the manufacturer, although the specifications page gives an operating range of -40°C  to 85°C with an typical accuracy of ±1°C and a -2 to +4°C maximum deviation, with a sensor quantisation of around 1°C. 

Developer SDK

The system isn't "open hardware" but that's just fine, not everything has to be open, people have to eat and keep a roof over their head and that takes money. However I was pleased to see that there is some developer documentation available for the web service API of the backend server, along with the the source code for the management software I was complaining about earlier. The source code for the iOS and Android applications is also available if you email the company.

Update (12 Nov): I emailed the manufacturer and they want to know why I want the source code, and for me to sign an NDA before they release it to me, which wasn't exactly what I was expecting from the API documentation. I'm not sure what to think about that...

The system does crucially rely on a back end server provided by the manufacturer, but the FAQ tells us that if the tag manager can be flashed to point at a different server, and it's at least theoretically possible to run your own. The vendor then talks about licensing their own server side software, and promises that if their server gets shut down that they'll release the code and allow you to update the tag manager to point at your own server. So I'm happy enough at that point.


Just as I thought when I initially placed my order, someone has thought long and hard about this problem space, and it really shows in the quality and flexibility of the hardware if not the software. 

I'm fairly sure the software issues are going to get ironed out eventually, at least for me the existence of some sort of documentation would probably solve most of the problems I'm having. Although I do think that for the average user the interface needs a good UX expert and a redesign. 

I also hope to have my hands on the iOS app so I can use the tags as I originally intended, and enable push messaging to my phone, as that'll vastly increase the utility of the system for me.

From poking around on the manufacturer's website it seems that there are probably more tag types coming, including current (for home energy monitoring?) and moisture (for detecting water leaks) sensor tags. I'll certainly go ahead and purchase those when they arrive.

Despite some of the criticism above, I am impressed with this system and would recommend it.

Update (23 Nov): My tags were suffering from spurious "out of range" and "reconnected" messages. So I have just sent all my tags back to CAO Gadgets for a firmware update.


  1. This is very interesting post..and nice review about this article.You doing great job..thanks for sharing here.Wireless sensor systems

  2. The blog post Teardown of Wireless Sensor Tags was very interesting! Lot of interesting knowledge which can be supportive in some or the other way,

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  3. I ordered a system (base and 7 tags) back in Dec 2012.. I haven't had any communication with them since placing my order.. nothing.

    Do you have any contact information other than the phone number and email listed on their mytag site?

    Very frustrating.

  4. Can you do a tear down of the WiFi unit

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