So - it is the Current not the voltage that determines the
brightsness.
Yes. For a LED, the brightness is directly proportional to
the current. (Other kinds of lamps behave differently.)
Voltage can change and not affect the brightness just so long as it
remaines with an operating range.
Am I closer?
Closer, but not quite there. The current depends on the voltage across
the LED.
The other thing I think you're missing is that "absolute" voltage
isn't important, only voltage differences between one place and
another are important. It's kind of like altitude --- climbing
a flight of stairs at street level feels exactly the same as climbing
a flight of stairs at the top of a tall building, as long as you
don't look out the window. What matters is how tall or steep those
stairs are. In other words, the difference in altitude between
the top and the bottom, not the absolute altitude.
People also talk about "voltage drop", which is just a way of talking
about the difference in voltage on each side of a component. If one
terminal is at 5v and the other is at 4v, then you can say that the
voltage (or potential) has dropped by 1v as you move from one terminal
to another.
LEDs have a funny current/voltage relationship. If you put a small
amount of voltage across them, (almost) no current will flow. If you put a
bit more voltage, still no current will flow. If you keep increasing
the voltage, then at some point --- around 2 volts, for a green LED ---
the current will suddenly start to flow, and the LED will light up.
Increase the voltage any more, and a huge amount of current will flow,
and the LED will burn up, pfft!
So you can't just attach the LED directly to a voltage source, because
you're unlikely to get exactly the voltage that will light up the LED
without destroying it. Plus, the voltage (the "forward voltage drop"
of the diode) is different at different temperatures, for different
LEDs, and so on. You need some way to keep the right amount of current
flowing even if you don't know exactly what voltage to apply.
The simplest way to do this is with a resistor. Unlike a diode, a resistor
has a very simple relationship between current and voltage; they're
directly proportional. In other words they follow Ohm's law.
So, imagine this circuit:
[ Battery + ]-----/\/\/\/\/\----->|------[ Battery - ]
(that's a resistor and a diode in the middle there, with a battery
connected to the ends). What do we know about this circuit? Assuming
that it's operating correctly?
- The voltage between [battery +] and [battery -] will be the
battery's voltage. Let's assume we have a 12-volt battery.
- The current through the LED will be 10 mA, because that's how
much current we want. (The right amount depends on the LED ---
bigger LEDs can handle more current.)
- The voltage across the diode is unknown, but it's roughly 2 volts,
depending on the kind of LED.
- The voltage across the resistor is (current times resistance).
So, we can subtract out the "roughly 2 volts" of voltage across the
diode, and that tells us that the remaining voltage across the resistor
will have to be "roughly 10 volts". It's a series circuit, so the
current through all parts is the same: we've decided we want it
to be 10 mA. Ohm's Law tells us how big the resistor must be: 10V/10mA
equals 1 kOhm. Now we know enough to build the circuit and get
10 mA flowing through the LED.
Why does this work? Because now, if the diode's voltage varies by 0.1
volt, or if the battery's voltage varies by 0.1 volt, the current through
the diode doesn't change much. It stays around 10mA (maybe 9.9 mA, maybe
10.1 mA, but that's close enough). Most of the variation is being taken
up by the resistor, which has a very smooth variation of current
with voltage, unlike the LED's very sudden reaction.
Unfortunately, most of the power is being taken up by the
resistor, too. Only about 1/6 of the energy is going to the LED,
and the rest is going to the resistor (and turning into heat). For a
brake light, this isn't much of a problem, but in other cases, it
can be. In those cases, you can use more complicated techniques,
like pulse-width modulation (where you keep the *average* current
correct by adjusting the duration of pulses of current).