[EricVanWyk]

I agree with MrForbes - you have your LEDs "stacked too high" given your power rail. Reducing the stack height (the number in series) and increasing resistance to compensate will make it less sensitive.

For your students, I would draw ideal I-V (current vs voltage) curves for various stack heights. Pretend the LEDs are magic perfect diodes with a fixed voltage drop, and pretend that the amount of light that comes out is perfectly linked to current. Each stack height will intercept the current axis at a different voltage equal to the diode drop times the number of diodes - mark these points on the graph first. Then pick your desired operating point - at 12Volts, I want e.g 10 milliamps - and draw that point on the graph. Then connect each intercept to that operating point with a straight line. The slope of this line is the resistance that you are selecting. Now, visually "wiggle" the input voltage and see what happens to the operating current for each stack height. The taller stacks will be more sensitive than shorter stacks, but shorter stacks will waste more of their power in the load resistance.

I'd give it even odds that this will encourage a student to push the stack on to a regulated power supply away from the unregulated battery voltage. Awesome. Pick the DSC's 5V or 6V rail, and encourage them to do the math to see which option is more efficient. ((unless you are drawing plural amps, in which case use the 6V so you don't brown out the more important 5v)