From: Jason Harper <...>
To: Andrew Plotkin <...>
Subject: The Flashy-Light Circuit Challenge
Date: Wed, 29 Nov 2000 21:52:09 -0600

This problem is easily solved by putting a diode (a 1N4001 or similar
will do) across the transformer input.  Assuming that the transformer is
connected between the 555's output pin and ground, connect the cathode
(the end of the diode with the band) to the 555 output.  This works
because the voltage created by the collapsing magnetic field is of
opposite polarity to the voltage that created the field in the first place.

My main concern is whether this would produce enough current to light up
your nightlights.  A 555 is good for about 200 mA I believe, but since
the transformer will need to increase the voltage by a factor of about
22, it will also DECREASE the current by the same factor.  Will the
nightlight run on 9 mA (actually less, due to inefficiencies in the
transformer)?  Watch out for overheating the chip - you're probably
going to end up with a few 555-shaped burns on your fingertips before
this project is done.

I have some doubt that the color changes you're seeing are actually due
to frequency - a difference in voltage between the power line and the
inverters you've used is a more likely explanation.  However, the
circuit may work to adjust the color anyway, due to varying efficiency
of the transformer at different frequencies.

All you should need is a diode as mentioned above, and a transformer
with about a 22:1 voltage ratio.  A 5 volt transformer, used in reverse
(555 connects to secondary winding, 120 V primary winding goes to
nightlight), should do the job.  A 10 volt center-tapped transformer
(leaving one side of the secondary winding unconnected) would work just
as well.  Unfortunately, neither of these is a very common part, you're
much more likely to find 6.3 volt and 12.6 volt center-tapped
transformers which would produce a proportionally LOWER output voltage. 
Using half of the secondary of a 6.3 V CT transformer might be your best
bet - the output would theoretically be about 343 volts, but considering
inefficiencies, and the fact that a 555's output doesn't swing quite all
the way between the extremes of its power supply, the actual results
might be pretty close to what you're asking for.

#include standard warnings about high-voltage circuitry here - you're
playing with potentially deadly stuff.

-------------------------------------

From: Jason Harper <...>
To: Andrew Plotkin <...>
Subject: Re: The Flashy-Light Circuit Challenge
Date: Sun, 3 Dec 2000 20:28:12 -0500

I've just bought one of the oval nightlights to play with myself, and I can
now say that you're going to have a very hard time NOT getting good results
driving them with a 555 and a transformer.  Heck, I was able to get quite
bright flashes of light out of the thing just by momentarily touching a
used AAA battery to the transformer input!

For my testing, I used a 0-2 MHz function generator, which can be thought
of as a precisely adjustable 555 for this purpose, and a couple of power
transformers (about 10V and 14V outputs) I had laying around.  I was able
to get a reasonable amount of light output from about 100 Hz to 10 kHz
(somewhat dim towards both ends of the range) - and this was with only
about 45 volts being produced from the transformer!  The light was greenest
around 1 kHz, and bluest around 5 kHz, so you'll need to shift your target
frequency range upwards in order to get the greatest variation in color. 
The light was brightest when using a square wave output (the function
generator could also produce sine and triangle waves).