Is there a way to communicate with smartphone remotely via a sensor without using internet? - signal-processing

Question

I want to detect and send/recieve data from a smartphone in some vicinity without using internet.

Answer 1

I've always thought it would be fun to do this with audio. Most modern ways of modulating a signal (like OFDM) will sound like a white noise hiss over audio, and you should be able to get a few KB/s in a normal room environment if the phones are close to each other.
It also has the benefit that the user can always tell when it's transmitting.

Answer 2

Multiple methods are possible.
You could use a private (isolated) local area network that is not connected to the internet. Either ethernet cabled or over WiFi.
Airdrop might not require an internet connection (a WAN connected access point).
Bluetooth BLE communication doesn't require an internet connection. You could use an ESP32 or Raspberry Pi to read sensor data and have a mobile device connect over BLE to the ESP32 or Pi (or another mobile device).
You could use audio. Play FSK tones or Morse Code on one device and receive and decode the audio modulations on another device. (I've tried both of these methods successfully.) Or you could use a speech synthesizer on one device and a voice transcription app on another.
You could use light. Flash the flashlight (or LED) on one device, and receive and decode the light pulse sequences using the video camera another device. (There may be apps in the App store that can do this.) Or display a bar code or QR code on one device and use the camera on another to decode the data in the bar code or QR code.
You could use MIDI. Bluetooth MIDI over BLE from device to device. Or with MIDI cables, using a bunch of Lightning to USB and USB to Midi adapters.
You might be able to use vibrations from the Taptic engine on one device, and detect the vibration sequences using the motion sensor API on another device.
With many Android devices, you can connect a USB to serial port dongle, and use a long RS232 serial cable between devices.
With an iPhone, you could use a Lightning to Ethernet adapter, plus a fiber optic media converter, and send signals over several kilometers of (private) fiber optic cabling. etc.

Answer 3

You might want to use the IR sensor on your phone by using an IR sensor library. (Search it on a search engine). If the does not have that, you can use a QR code generator library (Search it on a search engine) to transfer your data.

Answer 4

You could use a raspberry pi (for example) to take readings from your sensor and store them. Make it run a webserver and create its own wifi network (not connected to the www) where you can access a webpage that displays the readings. Or you can set it up so that the Pi logs into the wifi hotspot from your phone whenever available and then uploads the data or sends it in an email or whatever.
You can use an internet module, for example the FONA 800 or 808 by Adafruit to let your Pi talk with the internet, via a SIM card from hologram.io for example. The Pi can talk to the FONA in Python. But to be honest that doens't really answer your question with the proximity thing - but if I were you I would drop that and do the following:
Read the data from the sensor and save it to a csv file on the Pi
Once every hour (or whatever), connect to the internet via FONA/hologram.io SIM
Insert the data from the previous hour to a remote mysql database
Use PHP or something to display the data from the database nicely and access via your phone
That way, you can have as many sensors as you want and access all from your phone. As I said the proximity thing is not relevant for me, it's easier imho to go through cellular (+ I wouldnt know how to do it over lets say bluetooth)