Alright, so, my background is more mechanical engineering and computer science in nature, and it's been quite a long time since I've done anything EE related. So, know that I know the basics, but beyond that, I'm pretty rough, here.

I guessthat the easiest way to do this is to write out the logic in pseudocode (that's the CS major in me). Suppose I have a bunch of AA batteries, and based on how many there are, I want to direct the output to a different circuit. Assuming a AA battery is 1.5V:

if (Vin <= 1.5V)
Vin >> circuit1
if (1.5V < Vin <= 3V)
Vin >> circuit2
if (3V < Vin <= 4.5V)
Vin >> circuit3

And so on. So, if I put in one battery, the voltage is directed to a circuit that, say, lights a bulb. If I put in two batteries, a second bulb (and only the second bulb) lights up. If I put in three batteries, a third bulb (and only the third bulb) lights.

Assume I don't know the resistance of the circuits; all I know is Vin, and I want to switch between circuits based on these thresholds. Is this possible?

 

Using some relays/transistors could get this going, though you will run into a problem in that a battery's voltage will not always be 1.5V, effectively hosing anything you do...

You could, run a supervisory circuit, which would use fixed voltage references and enable/disable devices according to voltage ranges that would coincide with the ranges you'd expect from these batteries...

 

This will do it

LM3914 LED Bar/Graph generator. Scale the input with a voltage divider.. Use the DOT mode.

 

Beautiful!

 

It is beautiful, but won't save you from the inherent problem of trying to count batteries based on their combined voltage. 4 cells run down to 1.2V each will still look a lot like 3 fresh cells at 1.5V each. If all you really care about IS the combined voltage, then you're good to go.

You could also just dedicate a cheap multimeter to the chore. They're cheaper than the parts you'll need to build your LM3914 circuit. No pretty lights though.