circuit help

[Rob Seastrom]

mo at ccr.org (Mike O'Dell) writes:

> 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.

... per illuminated switch, right?  otherwise you'll have the same
problem you'd have with a single dropping resistor feeding all LEDs.

> 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.

If you don't care about the possible rf consequences (or are willing
to put an appropriate capacitor filter across the output of the
microcontroller to round off the square waves and thus the harmonics),
why not pulse width modulate it (through an adequately sized mosfet,
or a junction transistor if you like)?

An atmega168 (or arduino if you like) is gross overkill for this; you
could use an attiny85.

The bonus here is that you can control brightness with up and down
pushbuttons (maybe sealed reed switches) instead of a potentiometer
and pot the whole control circuit (or at least seal it inside a box
with a big bag of desiccant.  I like the idea of avoiding rheostats in
a marine environment.

-r