>I built the dimmer you designed, mentioned in the RV-List, some time ago.
>It worked! I played around with an idea I had that would make the
>instrument light level vary with ambient light intensity. By using a
>photoresitor (RS part), I got the light to dim when the ambient light was
>high, ie, daylight. This was the opposite of what I was looking for, so I
>wired a 2N2222 transistor across the pot, controlled by the photoresistor.
>This worked fairly well, dimming the instrument light as the ambient light
>decreased. The pot still operated to trim the light level. I suppose with
>more tinkering, I could install a trim pot to vary the response rate of the
>photoresitor.
Good for you! Autodimming has been with us for awhile. The
Cessna 400 series radios and some of the earlier autopilots
produced some of industry's first whacks at the problem.
I think it was fairly successful . . . you will probably have
to tinker with the resistors associated with the photoresistor
to set min-max ratios . . . but you can do this on the bench
using a wall-dimmer on an overhead bulb to simulate approaching
darkness.
Turn down ambient lighting down all the way and let your eyes
dark adapt for about 5 min . . . bring lighting up until you
can just read the panel with no additional lighting . . . this
is the light level where panel lighting wants to be "max" . . .
then dim room lighting to full dark and adjust the "min" to
the right level. . . . doing this on the bench (or in the
shop sitting in the cockpit) will get you in the ballpark
making it unlikely that further fiddling will be needed later.
>This is all just benchtop tinkering....It hasn't been installed in any
>cockpit, so the variables of cockpit brightness, photoresistor location,
>etc., would have to be worked out. I don't know if you could get ideal
>automatic instrument light dimming, but I'm forwarding you this as food for
>thought.
Of course radios have photo-resistors right on their
front panels . . . a bit of a pain in the whatsit since
your hand shades the photodetector when you reach for
controls . . . just when you most want to see what's going
on, the lights on the device dim down! Your notion of
finding a suitable photo detector location NOT on the
panel is a sound one.
Your efforts are a good example of ways amateur builders
can provide bells and whistles that spam can drivers
can only wish for. . . Let us know how it works out.
Better yet, write up an article and share the knowledge.
Bob . . .
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