reading a DC voltage into an android phone microphone port
Another alternative is this: -

Feed the unknown DC voltage into an integrator. The integrator output voltage will ramp up at a rate determined by: -


A fixed value resistor
A fixed value capacitor
The DC input voltage


Use a comparator on the output of the integrator - when the integrator output voltage reaches a certain level, the integrator is zeroed by shorting the capacitor with a FET or BJT. Then the whole cycle repeats.

What you get is a voltage to frequency converter. Feeding this into your android and calculating the frequency should be do-able.

Here's a circuit I got off the internet that looks like it should work - it uses a 555 instead of a comparator but essentially it's doing the same thing: -



You should be able to replace the 741 with a RRIO op-amp and get it working from a single supply voltage.