Sadly, since most smartphone magnetometers seem to have a sample rate of 100/s, this will not be applicable to Americans and everyone else with a 60 Hz grid frequency, the 50 Hz were already at the Nyquist–Shannon limit.
The trick should work fine, but you may confuse the 60Hz signal with a 40Hz signal [1] [2].
This should work for higher frequencies too, but if the frequency is toooo high the problem is that the magnetometers averages a short period of time (or use a window) instead of being an actual an instant measurement.
[1] Calculated using my fingers moving in the air. 60=50+10 -> 50-10=40. I think it's 40Hz, but I would need a pencil and paper to be sure.
The hack you can do is to additionally sample at, say, 97Hz as well as 100Hz. A 97Hz sampling rate will then see a 60 Hz aliased to 40Hz signal go from 40Hz to 37Hz, showing that your signal is probably actually really 60Hz and not 40Hz. If it was 40Hz at 100 and 97Hz sampling frequency, then it's probably actually 40Hz.
(It's been a looong time since signals class though.)
In Germany phyphox is quite popular in physics education.
However on android the sampling rate of the acceleration sensor is limited to 50/s. At least if you install through the official app store.
[1] https://iopscience.iop.org/article/10.1088/1361-6552/aac05e
My understanding is that it’s the same even on iOS (or at least on my iPhone SE 2020). More specifically, the output only measures till 50hz (but the sensor sampling rate is actually 100hz - Nquist, you need double the measured frequency as sampling frequency, yada yada.)
The sensors have analog lowpass filters that can be adjusted in order to avoid aliasing.
In general, with more bandwidth you can do more intrusive things. But if you want to tell wether two people ride in the same car, 50 Hz should be sufficient anyways.
Phyphox has a smartphone sensor database:
Edit: no, it can't have. Then the phone sensor database would show that since it is built from submissions within Phyphox: https://phyphox.org/sensordb/
I'm not sure what problem you're running into (perhaps a very unusual phone that has only a 50 Hz accelerometer) but Android/Phyphox can do way more than 50 Hz
But I see Google indeed introduced another permission for this: https://developer.android.google.cn/develop/sensors-and-loca...
Curious that the database shows good rates but you're not seeing it in your instance. The device is from 2024 so it'll have shipped with this new restriction; the database submissions can't be from an older android version. Phyphox must declare that permission or else it shouldn't be in the database like that, but then you should also see higher rates in the app. So I don't get it but Android still can do more it seems, if an app is granted the new permission
I dont know if others have experienced this, but there's always that one kid in the science fair who builds like an entire satellite dish or something waaaaaay over the top.
I don't necessarily have anything against it, it's just a pattern I've recognized.
(I've been meaning for ages to write a piece of software that's able to extract change over time data from a video of a 7 segment display, like on a balance or a digital thermometer or something)
https://github.com/arturaugusto/display_ocr?tab=readme-ov-fi... and https://github.com/Shreeshrii/tessdata_ssd which may be more fitting (or more modern?) for your use case. Lots has changed in ~15 years!
or are there some weird historical archives full of video's of segment displays?
Back in the olden days of hardware RSA tokens, there were stories of people bridging out the TOTP values via setups like this. I've got an amplifier in the garage hooked up to a smart outlet that'll turn it on, but I have to go over there manually and rotate the knob from 20db to 50db.
I've been tempted to hack up some sort of "RC-Car Wheel + WebCam" to be able to remotely control/nudge the volume, but luckily my lack of free time prevents me from going off the rails like that. My other option is some sort of arduino and hacking at the rotary encoder circuit directly from the inside, but again (luckily) my lack of free time has kept the box closed.
The world's bigger than what you've seen so far, I'm guessing!
If one doesn't have free time, consider half the budget for the RPi+camera, then just find the closest makerspace or kid with an interest in electronics and ask them if they would like to hook it up for you, you saved half your expense, the kid has some more experience and some money to show for it.
Same with the amplifier knob, just needs a cheap wireless microcontroller, and sufficient low pass filtering on a PWM signal, perhaps a quad op-amp IC for the filtering and the voltage buffer...
The interface is more polished, but the information is less technical than Phyphox (as the app is geared towards being a survival toolkit).
Most recently I used it to check light levels at home in different rooms, to determine where we need to boost or replace LED strips. Sure, there's million Lux meter apps, but Phyphox is better than all of them and demonstrates why these things shouldn't be dedicated apps in the first place. In the past I also made use of EM and vibration frequency displays to troubleshoot hardware.
A complement to that is https://play.google.com/store/apps/details?id=org.intoorbit.... which, once upon a time, helped me track down a source of rage-inducing, late-night high-frequency beeping that was driving us insane - down to specific apartment in a block on the other side of the street. I ended up friends with those neighbors, after teaching them how to disable the alarm clock on their Bluetooth radio when they go away for a weekend.
My parents have a sound bowl, and I wanted to know the resonance frequency. Took an audio spectrum, zoomed in on the first peak, read the frequency (iirc it was around 208 Hz).
