Periodically, including today, my wife heads off to work without her phones, and when she gets to work she either has to come back home, about nine miles in heavy traffic, or if I haven't left yet I detour on my way to work to deliver them, or my son happens to be home from college (like today) he delivers them.
What I want to do is to have a device that sounds an alarm when the car is started (USB charging port turns on) but no device is plugged in.
So I want to detect the current flow from the USB +5 to the phone and if there is no flow, sound the alarm. There is always some flow upon power-up, even if the phone is charged, then it stops if the phone is already topped off.
There are current shunt monitors, i.e. the TI INA233 which would probably work, but does anyone have a better idea.
I'm thinking a using an Arduino Uno (%V analog inputs) with a shunt resistor between two analog pins, reading the values of each analog pin, and activating a speaker if the analog values are the same (no current flow) so she'd have to plug in the phone to stop the speaker. Plenty of analog pins so I could do one with a Lightning cable for her Android phone (personal) and one for her company phone (iPhone).
If I wanted to get really fancy I could add a Wave Shield and output some speech.