We are gonna have a bad time with E-Waste with all these smartphones going outdated soon.
Most of them have the commonality of having an audio port. And sometimes a usb port.
So why not repurpose these smartphones as OCR scanners, for barcodes like QR codes.
Just output the code over a serial port via audio or usb. As for the display, you could display QR codes, if needed as well.
The cable could be a typical usb that simulates a serial host, or an audio port that outputs serial signal.
The serial signal can then be read directly for custom/embedded systems, or can simulate a usb keyboard for easy entry of data for the common user.
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Keeping everything as 'serial', also will reduce vectors for exploits.
Could allow wifi connection, but might be safer to just disable it entirely.
Serial :- Old school UART-- mofosyne, Oct 06 2014 I'm all for recycling but smartphones aren't designed to last, and definitely aren't designed for industrial use. I had an old Android phone that I was using as an OBDII scanner for a while but 6 months after I got a new phone it was already hard to keep the software up to date and the hardware working on the old phone.-- DIYMatt, Oct 06 2014 Old smartphones are/will be valuable for their rare Earth metals. They're less rare in phones than the Earth...-- bs0u0155, Oct 06 2014 Once more, with feeling!-- RayfordSteele, Oct 06 2014 random, halfbakery