On Thu, Mar 10, 2011 at 12:20:31PM -0600, [email protected] wrote:
>
> Ben,
>
> I suggest using LEDs instead of neon. Neons get lazy with time and may
> require some sort of lampholder. LEDs are available with lampholder
> included at Radio Shack. They may even be available in a flashing style.
The reason I specified neons is their incredible tolerance for supply
variations - and in this situation, that's a requirement. This circuit
is extremely simple, but it uses a somewhat unstable side effect - so I
had to get a little tricky with the design. (Not a problem, that; I cut
my teeth on Forrest Mims' writings. That man didn't know how to think
_except_ for sideways. I loved that stuff. :)
Factor #1: The effective voltage between the two (out of sync) AC
sources is unknown and likely to wander between 0 and 110v - but will
definitely cross the 70v boundary at least a part of the time, which is
what it takes to trigger a neon bulb.
Factor #2: A neon bulb has persistence - meaning that, once triggered,
it will stay lit enough of the time that it will look like it's on (or
at least flickering) even if it's actually on only 15% of the time or so.
Factor #3: The effective current may be extremely low - microamps,
possibly, depending on the shift between the two sources.
An LED will not work if any one of those factors obtains. Due to the
unpredictable voltage, it would be impossible to calculate a series
resistor for it; LEDs have essentially zero persistence (which is really
good in other situations, like rapidly-updating displays); last of all,
LEDs require an average current in the tens of *milliamps* range to show
a light. A neon bulb will work even if all three situations occur at the
same time, and a 22k-100k or so resistor should work fine in that
situation.
As to fading, they certainly do - but only when driven hard (at least to
the best of my knowledge), which doesn't apply here. I've seen 40+ year
old neon bulbs still working fine, once you've wiped the dust and the
grime off. :)
Ben
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