Double functionality on Micro

[Skylimit]

I have a issue where due to lack of i/o lines i need to double up on one i/o line. As an A/D to read light sensor and output to drive the LED.
The light sensor output should go from 0 to 2.7V. However, it is being clamped to around 2V (forward volt drop of LED) i think it may be due to the 220R and leakage current of the LED.

My possible solution was to use an Zener instead of the 220R or a diode instead of the 220R. I have no other i/o lines available.

Any help would be appreciated.
Thanks in advance.

[Berni]

Well many ways to skin a cat. Often its a better idea to dual purpose digital pins rather than analog

But you can do things like change resistor values around your light sensor to make it produce a smaller voltage. Or you can even use LEDs as light sensors. They work like solar cells and produce a small voltage when light shines in them so you might be able to use the ADC to read the voltage the LED is producing.

[woofy]

Drive the LED via a MOSFET.
Then high gate impedance should not affect the A/D.

[Kleinstein]

One could get a little more voltage with a diode in series to the LED. Still the LED will be some loading to the analog signal, so it is a compromise on accuracy.

[Whales]

N.B. if your other pins are driving lots of LEDs: https://en.wikipedia.org/wiki/Charlieplexing