Not expecting a complete solution just wanted to know if something like this already existed.

I made a small circuit that gives me about 3VDC , 9mW from my cellphone audio port and I would like to use that voltage with sensors. The idea is to generate a frequency or PWM that goes back in to the microphone input. The android app would have configurable units and would allow me to configure what measured frequency corresponds to what value, e.g. 20Hz = -20 Fahrenheit and 20kHz = +200 Fahrenheit etc. Something like a configurable relationship between frequency and what I want to measure.

I totally suck at programming, I wonder if somebody has seen such an app.

Thanks

 

A quick goolge reveals that there are frequency and spectrum measuring apps out there - here's one discussion which may help you get started
http://stackoverflow.com/questions/12721254/how-to-calculate-sound-frequency-in-android

but I doubt that an app to do exactly what you want exists

 

And there are a *ton* of apps for tuning musical instruments.