We found, with my 6 y.o. kid, a 9V battery and an old
CPU fan power supply fan, and we were happy to see the following circuit 1. working.
Then he asked: "Can we add a switch and a LED?" For sure we can, but how... I'm not sure. Among 2., 3., 4., which one is the more appropriate? How to calculate the relevant resistor value?
Note: Please correct me if I'm wrong but I think we can assume this standard LED we found has a forward voltage ~ 2V. How to use this to derive the resistor value in case 2., 3., 4.?
Note 2: In circuit 1., I measured, with a multimeter, a voltage of nearly 9V (a little bit less when motor was running), and a current of ~ 160 mA, thus I estimated the "equivalent resistance" is around 55 Ohms for the fan. (I don't know if this concept is correct).