I’m building a device that, in part, uses two 9V batteries to power a computer fan which is controlled by a potentiometer. If the potentiometer is below 60% or so, the fan fails to move. I want to hook up an LED that is only on when the fan is spinning.
I tried using 2 AA batteries and a transistor to power and control the light, but that didn’t work. I don’t need all the calculations done for me, but I’d appreciate input on where to go from here.
The circuit is very simple. A push button controls on/off, and the potentiometer controls speed. I understand the two 9V batteries can't power the fan very long, but I will only need to run it for ~5 seconds at a time, very infrequently.
I think this diagram better represents what I'm working with. After looking into it I realized I've had the potentiometer wired as a variable resistor. 
Best Answer
Does the fan have a tach output? (3-wire fan). That’s the easiest way - the tach is open-collector and can drive the LED directly.