Resistor Equation

[odel1830]

I am having trouble deriving a general equation for calculating R3 (shown in the attached JPG image). V1, R1, and R2 values depicted in the image are constant values and will not be changed. I will be writing code for my microprocessor's ADC to measure Vout, so Vout will be known. I just need an equation to plug in to my code to calculate R3, can any one help?

[pebe]

According to Thevenin, you can replace R1 and R2 with a single series resistor (Rt) equal to R1 and R2 in parallel and fed by a voltage V1 x R2/(R1 + R2). That becomes 1.65V feeding into 50K.

So V1 now feeds a potentiometer consisting of Rt for the top section and R3 for the bottom section. Vout then becomes: 1.65V x R3/(50k + R3).

I hope that may help, but calculating R3 from that is beyond my limits.