circuit help

[Mike O'Dell]

I have a design problem that's been driving me batty
because it seems light there should be a reasonably simple
way to solve it but I can't see it. Sigh.

The Problem:
There are a number of LED-illuminated SPDT ON-OFF-ON switches, 
green for one position, red for the other. The LEDs are fed from
the two switched terminals via a dropping resistor per LED
sized to work switching 12VDC. The switch is designed to 
switch the + side of the circuit between the two positions
and there is a common - connection for the bottom of the LEDs.

Normal use case: +12VDC to the moving terminal, ground the
common LED pin, and take switched 12VDC off the two switched
positions. Easy.

My use case:
Part 1: Similar to the normal case except switching +24VDC.
	can't change the dropping resistor inside the switch
	but can easily add an external dropping resistor to ground.
	Easy.

Part 2: The lighting needs to be *DIMMABLE*. This is for something
	in a boat dash and it *really* needs to dim down to almost
	nothing to preserve night vision. Simple minded-solution is 
	to just add a common rail for the LED ground terminals that goes
	to ground through a variable resistor (big pot or a pass
	transistor).

Now here's the tricky bit:

Part 3: We also need to keep the brightness constant independent of 
	the number of switches in either on position or off because
	we don't want want to turn off a number of them and have the
	rest of them bloom bright and kill night vision.

What has occured to me so far is to use a 3-terminal variable
voltage regulator that can withstand going to +24VDC on the input.

Feed the *input* of the regulator from the LED common rail and 
feed the output to ground through a resistor sized to not exceed
the current rating of the regulator with +24VDC on the input
and set for max output voltage. 

So to adjust brightness: 
Turn up the voltage setpoint on the regulator to make it brighter. 
Turn down the voltage setpoint to make it dimmer.

As LEDs get turned on or off, the effective resistance of the 
various paralled LEDs (and dropping resistors) varies, but the
3-terminal regulator maintains a constant voltage drop between
its input and ground through the load resistor. Therefore the 
LED common rail stays at a constant voltage above ground in spite of how
many LEDs are in parallel at any one moment. 

This gives a constant voltage drop across each LED producing a uniform 
current through each LED and uniform brightness. (Ignoring red-vs-green 
brightness.  One would hope the internal dropping resistors took that 
into account.)


so does this make sense?

	Thanks!

	-mo