Originally Posted by Tempest

Still not exactly sure what you need. Why do you need 40 LEDs and do you simply want them all to be ON? Do they need to all be conected in series? or in parallel?

Here's what I remember.. LEDs are basically diodes and diodes usually require 0.7Vs and LEDs needed about 10 milliamps to turn on.

So if you have a 3V battery (or 2 1.5V in series), you take 3V - 0.7V = 2.3V / 0.010 amps = 230 ohms.. Thus that's where the 240 Ohm resistors come from (I don't think you can get 230 so you'd use 240).

Are you only powering the LEDs with this? Cause you could just use a single 1.5V battery and 80ohm resistors to do the job.

Like most things, there's multiple solutions to a problem, it all depends on what you're trying to do. That would determine whether you'd connect them all in parallel, series or some combination of that. And that configuration will dictate how many batteries you need and what resistors etc.

For example. You could connect all the LEDs in series with a 240 ohm resistor, but that would require a (40 * 0.7V) + (240 * 0.01) = 28 + 2.4V = 30.4V power supply.

If you connected them all in parallel, it would require 0.01 * 40 = 0.4 amps and only a 1.5V battery and an 80 ohm resistor, but that 0.4amps will drain a single battery very quickly.