This seems like a stupid question, but I can't figure it out. Say I have a potentiometer with a 3 pin connector on it (VCC, GND, and the variable output which will go to an ADC pin on the Arduino), is there any way to detect whether it's connected or not, with just those 3 pins?
Attached is a diagram to clarify, as well as how I would do it using a 4 pin connector and a digital I/O pin with its internal pullup resistor turned on. But it seems like it should be doable with just the 3.
To be clear, I don't mind using another Arduino pin. It's the connector that I'm trying to keep minimal. Also, the fixed resistor could be moved to the Arduino side if that makes any difference.
Going with the top circuit I would make the connections that go to the + supply and ground to I/O pins.
Then make the ground I/O pin an input and the + supply I/O pin an output and set it high.
If a device is not connected then the ADC input should read close to 1023.
Then make the ground I/O pin an output and set it low and the + supply I/O pin an input.
If a device is not connected then the ADC input should read close to 0.
If both those conditions are met then the chances are their is no device fitted.
Well, if you put a very high resistance (at least ten times the value of the pot) from the input pin to ground, then you can read the input in both INPUT and INPUT_PULLUP modes.
If the pin reads zero in INPUT_PULLUP mode, then clearly the pot is holding it at zero. Conversely if it reads something other than zero or near to it in plain INPUT mode, then the pot is again, controlling it. If it reads 1023 in INPUT_PULLUP mode, or something close to it and zero or near to it in plain INPUT mode, then the pot is not connected.
The exact parameters depend on the value of the pot and that pull-down resistor, given that the internal pull-up is very approximately 45 k.
