Re: ATM tiny multi-position switches

["Fred Maxwell - No spam e-mail accepted." <fmaxwell@erols.com>]

Gerald Pearson wrote:
> 
> I want to make some variable-intensity LED flashlights with intensity
> controlled by switching various fixed resistors into the circuit.
> These would contain two (2) AA penlight batteries and 12 or more LED's.

Whoa!  Slow down there.  There are high brightness LEDs that provide far
more light than you ever need for use in an astronomy flashlight.  You
don't need 12 or more of them.  Just use one of them and send a square
wave of varying duty cycle (i.e. 100% = Full brightness, 0% = 0
brightness).  Using this approach, you could use a potentiometer with
on/off switch (like the volume control on old table radios).  

Alternatively, you could get a mini Mag Lite and put the LED in it. 
Using the focus, you can vary the surface brightness (i.e. wide angle =
dim illumination, narrow angle = bright illumination).  No electronics. 
Simple solution.

> In order to make these fairly compact and _also_ have enough brightness
> levels so that brightness steps are usefully small, I need at least 6
> switch-combination settings (plus "off") with at most 2 switches, and
> the switches should be fairly small.  I'd like to find switches which
> are not as tall as the things which I've been able to find recently.

This is sounding like a painful gadget to operate in the dark.  Imagine
holding a star chart while trying to remember which combination of
switch positions will yield a slightly brighter beam.  "Lets see, switch
one is in the middle position of three and switch two is in the top
position of three.  We need to move switch two to the bottom and switch
three to the top, but if we do in in the wrong order, instant night
blindness!"

{snip}

>         2.  miniature DPDT_center-off toggle switches.  Used with a
> SPDT_center-off switch, this gives 3 x 2 = 6 brightness positions.
> The bodies of the switches are small enough, but the long switch paddles
> would project well over 1/2" above the surface of the flashlight.

So, why not cut them off?

Seriously, though, I think you would be a lot better off with a single
control device.  Remember, you have to operate this thing by feel, in
the dark.

Also, remember that you are using red LEDs because they have the least
tendency to affect night vision, so 6 brightness levels may be overkill.
But, if you are determined to build it this way, perhaps you could
construct one with three SPST pushbutton switches.  Wire the left one to
4 LEDs, the middle one to 2 LEDs, and the right one to 1 LED.  You have
a binary flashlight with 7 levels of brightness + off that operates as
follows:

0 = off, 1 = on

000 = off
001 = dimmest (level 1)
010 = level 2
011 = level 3
100 = level 4
101 = level 5
110 = level 6
111 = brightest (level 7)

Each level number is equivalent to the number of LEDs turned on.  You
will, of course, probably find design of a suitable reflector (to
provide even illumination and to eliminate dead spots) challenging.

Regards,
  Fred Maxwell