I'm working through "Make: Electronics" (german translation; I'm German so please bear with me if I'm using the wrong terminology). An excellent book so far. However, I wonder why the specific resistor values where chosen in experiment 10. It looks like this:
R1 = resistor 180 Ohm R2 = resistor 10 Kiloohm R3 = resistor 680 Ohm S = switch Q = NPN transistor 2N2222 D = LED
If I understand the experiment correctly, as soon as I hit the switch and enough current makes it to the base of the transistor, a current is flowing through R1 to the transistor's collector, out of it's emitter, through R3 and the LED. I assume that the current flowing to the base can then be ignored, thus the resistance that matters for the LED should be just R1+R3, correct?
But how would I calculate which values to use for R1 and R3? It looks like the book assumes the LED to require 2.5V and 20mA, so R1+R3 should "use up" 9.5V. Since we now have 9,5V and 20mA, I'd have calculated R = U/I = 475 Ohm, but R1+R3 = 860 Ohm. So it seems I'm missing something, but what?
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