Was wandering through my local Jaycar the other day and saw one of these (http://www.jaycar.com.au/productView.asp?ID=AA0274). On the way home I got to thinking (and remember I am fairly new to electronics) "How does this work?"
So As far as I can work out...
Set voltage supply from battery, say 9v
I assume the different mA values are done by providing different resistors to that led. However how can you reliably say what mA is being supplied (as per the written bits on the box) as different led's have different forward voltage drops. (I thought mA was = Excess volt/Resistance)
If they do it right, they have built in several constant current regulators, which is the "advanced way to run a led" ( usually just required for power leds of 300 mA or such )
But there must be a cheaper way than including 10 independent current sources ...
I understand, a blue LED in the "10mA" holes will hava a different current than a red one in the same holes.
Due to the rather big difference between the 9V supply and 1.8 .. 3.4 V Vf of a LED, the error is not that big.
Good enough to compare the relative brightness of two leds of similar color.
Use resistors for each ma, so long as mA is close enough she'll be right, and given the high difference between the forward voltage drop and the power supply (9v battery) the difference in mA will be smaller.