You could use varying amounts of diodes in series to each LED. The turn-on might not be as abrupt as you hope, however.
What do you mean with "abrupt"? As in speed in which LEDs switch on or the contrast between the LEDs' on and off state (diffusion).
You probably want something like:
- 2.6V and below: LED not on
- 2.65V: LED half on
- 2.7V and above: LED on
... however instead you might get:
- 2.2V and below: LED not on
- 2.5V: LED half on
- 2.8V and above: LED on
(Numbers are made up)
So with "diode stacks" the diodes act as voltage barriers right? So the first stack would require a voltage of 1.3 for current to go through right?
Yep. The exact vdrop depends on what combo of diodes you use, standard silicon are around 0.6V.
If so, I'm worried about the amount of resistence they add to the circuit which would interfere with other features.
Common diodes passing mA don't add "resistance" as such, instead they mainly just drop voltage. Think of them as voltage regulators, just as how you would make a "zener diode regulator".
EDIT: I like spec's design