Testing a Faraday Bag with AirTags

Among my many gadgets I have a Faraday Bag. Faraday bags are essentially a flexible version of a faraday cage. Such devices contain metalic content and prevent the passage of radio signals. You have probably seen various applications of this, such as wallets or envelops designed to prevent an NFC credit card from being read, or the metalic grid in the door of a Microwave oven that prevents the microwave radiation from getting out.

I won’t get into the physics of how these work. But it is worth noting that a Faraday cage may only work for a range frequencies. A cage that prevents one device from getting a signal might not have the same effect on another that uses a different frequency. While I’ve seen that my Faraday Bag has successfully blocked WiFi and cellular signals from reaching my phone and tablet, I wanted to see if it would work with an AirTag. For those unfamiliar, the AirTag is Apple’s implementation of a Bluetooth tracking device. Another well known Bluetooth tracker is from Tile. The fundamentals of how these devices work is essentially the same.

AirTags on top of Faraday Bag

The trackers are low-energy Bluetooth devices. If the tracker is near your phone, the phone detects the signal and the ID unique to the tracker. The phone takes notice where it was located when it looses signal to the tracker and generally assumes that the tracker is in the last place that it was when it received a signal. That isn’t always the case. The tracker my have been moved after the phone lost the signal (think of a device left in a taxi). The next method of locating that these devices use is that other people’s phones may see the tracker and relay the position. For the Tile devices anyone else that has the Tile app on their phone effectively participates in relaying the position of tiles that they encounter. For the AirTag anyone with a fairly recent iPhone and Firmware participates. My expectation is that that the ubiquity of the iPhone will make it the location network with more coverage. As a test, I gave an AirTag to a wiling participant and asked that they keep the device for a day. When I checked in on the location of the Device using the “Find My” app on the iPhone, I could see the person’s movements. On a commute to work, other iPhones that the person drove by on the Interstate reported the position. I could see the person’s location within a few minutes of them arriving at work.

There are some obvious privacy concerns with these devices. Primarily from an unwilling party having an AirTag put in their belongings. Apple is working on some solutions for some of the security concerns, though others remain. I thought about someone transporting a device with an AirTag that may not want their location located. One way to do this is to remove the battery. Another is to block the signal. Since I already have a Faraday Bag I decided to test out this second method.

I found that my Faraday Bag successfully blocks the AirTag from being detected or from receiving a signal. You can see the test in the above video. This addresses one of the concern for such trackers, though not all of them. This is great for an AirTag that one is knowingly transporting. For one that a person doesn’t realize is in their belongings, a method of detection is needed. For iPhone users, the iPhone is reported to alert a user if there is an AirTag that stays within their proximity that is not their own. Results from others testing this have been a bit mixed. The AirTags are also reportedly going to play an alert sound if they arenot within range of their owner for some random interval between 8 and 24 hours.

Presently, Android users would not get a warning. Though Apple is said to be working on an application for Android for detecting lingering AirTags. In the absence of such an application, I’ve tried using Bluetooth scanners on Android. The Airtag is successfully detected. The vendor (Apple) can be retrieved from the AirTag, but no other information is retrievable. I’ve got some ideas on how to specifically identify an AirTag within code for Android, but need to do more testing to validate this. This is something that I plan to return to later on.

I purchased this Faraday Bag some time ago. The specific bag that I have is, from what I have found, no longer available. But other comparable bags are available on Amazon.

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Faraday Bag for Phones

Faraday Bag for Tablets and Phones.

Silicon AirTag Case

Silicon AirTag Case

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HDMI Capture on the Raspberry Pi

Back in January I tweeted about an HDMI capture device that for the Raspberry Pi. I’ve only recently have gotten a chance to use it. The device, known as the “HDMI to CSI-2 module”, works with the Raspberry Pi. Overall my experience was positive, though I found that this device has limitations that, if not previously known, can result in some frustration. The device connects to the CSI-2 camera interface and presents itself as a camera. The utilities and scripts that you may have used with the Raspberry Pi also work with this device without modifications. Along with the HDMI capture module the package contains the cable needed for connecting it to the full size Raspberry Pi and a second cable for use with a Raspberry Pi Zero.

One of the first uses that came to mind with this device is that I could use camera options beyond the official Pi cameras. The camera that I have about the house produce clean HDMI signals. They already have a range of lenses, ranging from some macro lenses for pictures of small items close-up and a 2132 millimeter Schmidt–Cassegrain for astrophotography.

My smallest lens next to my largest lens. Both of which are not available for use on the Pi through my digital camera.

The first time I tried to use the capture device with one of my cameras, it didn’t work. I received a non-descriptive error that is primarily associated with non-working or improperly installed cameras.

mmal: mmal_vc_component_enabled: failed to enable component: ENOSPC
mmal: camera component couldn't be enabled
mmal: main: Failed to create camera component
mmal: Failed to run camera app. Please check for firmware updates

Thankfully, this isn’t indicative of an actual hardware failure. The capture device works with a limited set of resolutions and refresh rates. For 1080p video signals, the maximum refresh rate is 25 fps.

ResolutionRefresh Rate (fps)
720p50
72060
1080i50
1080p24
1080p25
Supported Resolutions

After making adjustments to the output settings of my camera, I was successful in using it with the HDMI capture.

The camera was the first device that came to mind, but it could work with non-camera HDMI sources too. I connected a Nintendo Switch to the device and it captured from the switch just fine. Provided that the signal is within the resolution and FPS range and is not an encrypted (HDCP) signal, it works.

Comparing the HDMI capture device to the Raspberry Pi cameras, there were a few differences to note. While it may be easy to assume that the digital photo camera paired with this device is better than the Raspberry Pi cameras, that isn’t necessarily the case. “Better” is a matter of what satisfies the requirements for a solution. If that solution requires high physical portability, the photo camera’s size could be a disadvantage. Using an external camera also ads to external power needs; the external camera will need to have it’s own battery or power supply. The official Raspberry Pi cameras run off of the Raspberry Pi’s power.

HDMI to CSI-2 Module next to Raspberry Pi Camera

The Pi cameras offer some higher resolutions than one can capture with the HDMI capture device. Resolution is an attribute of quality, but not the only metric for quality. I hesitate to label the higher resolution as higher quality because there are cases where a lower resolution camera may be rated better on other quality metrics, such as clarity or dynamic range, or may have attributes that make it a better fit for a specific application, such as a different shutter angle.

The Raspberry Pi HQ camera (recognizable from it’s C-mount for attaching a lens) can capture still photographs of up to 4056×3040 pixels. The Raspberry Pi Camera v2 captures stills at up to 3280×2464 pixels. For video, all of the cameras have the same resolution. Keep in mind though at these higher resolutions since the device is receiving stills and not video frame the rate of capture will be much lower.

ResolutionFrame Rate (fps)
1080p30
720p60
480960/90
Raspberry Pi Camera Framerates

How did it work? After trying it on a Raspberry Pi with a Nintendo Switch I would rate the capture device as being okay. It isn’t stellar, but it isn’t bad either. It provides a way to interface with HDMI sources. During the process of recording, it appeared there were frames that were dropped. The playback confirmed this. I was wondering if the dropped frames were due to the speed of the memory card in the Pi or from some computational limits on its ability to encode the video to .H264. The next thought that came to mind was to try it with the Jetson Nano. Sadly, while the Jetson Nano uses the CSI-2 interface, at the time of this writing it is not compatible with the Jetson Nano.