A lot of smartphones are losing their headphone jacks, which I think is the last remaining port that smartphones have for interfacing to the analog world. Now any kind of sensor, even a simple microphone, would need to go through the USB or Lightning port on the phone, which is entirely digital, right?
What are the general steps necessary to take, say, a simple photoresistor or microphone and turn it into a USB device?
Obviously you would need an A/D converter and possibly an amp of some sort.
A/D converters read the voltage of a signal, but what about a photoresistor where the resistance is read instead?
What about drivers that the Android, Windows, or Apple OS would use to run the device? Driver signing requirements?
Is it basically a huge can of worms?