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Subject: Audio controlled decoration
Date: Tue, 17 Dec 2002 16:20:02 +0100 (CET)
X-Virus-Scanned: by AMaViS snapshot-20020300
"Steve W.EE" wrote:
> I would like some ideas on how to build a circuit
> to make Christmas tree lights flash to music. I
> want each of 3 or 4 colors to respond to certain
> sound frequencies. For example the white lights
> respond to over 3000Hz and would flash when cymbals
> or similar instruments are played. I tried building
> one of those Ramsey kits but the filters were
> horribly non-selective you could not really tell
> which lights were responding to which frequencies.
> There is more stuff about the Ramsey kit I don't
> like. It has a small 8:500 ohm stepup transformer
> 3 pots, 3 real crude RC filters and 3 SCR's.
> The SCR's of course will only light the bulbs to
> 50 percent brightness. I guess what happens is
> when the filtered audio gets above 0.7 volts the
> SCR triggers. The lights flash rather nervously
> and the effect is not pleasing at all.
The SCR's are a cheap but not very good way to modulate
the lights to the audio. What is happening is that when
the audio signal, on the SCR gate, gets above 0.7 volts
the SCR triggers. If this happens to be on a positive
half cycle (of the powerline) the SCR will turn on till
the zero crossing of course staying off for the entire
negative 1/2 cycle. On the next positive half cycle it
will turn on as soon as the audio again goes above the
threshold of about 0.7V.
The audio frequency is usually a lot more than 60Hz so
if its amplitude is anything above 0.7 it will turn on
pretty much at the beginning of the 1/2 cycle and stay
on the light will be at half brightness.
If the amplitude is near the threshold (where it should
be) the light is going to turn on somewhat randomly during
the positive half cycle so you'll get pulses of varying
brightness somewhat at a 60Hz rate. It looks nasty because
of the interaction between the audio frequency and
the 60Hz. The Ramsey kit design is just a bad circuit.
> I'm not sure whether I need an AGC because the music
> source I'm using is quite compressed. I do want
> something which flashes the lights in a pleasing
What you need is a triac not an SCR so the lights can
come to full brightness. You need some circuit like
a light dimmer but controlled by a small DC voltage
where the brightness varies linearly (or in some
monotonic fashion) with the applied voltage. Perhaps
pseudo-logarithmic so that brightness corresponds to
decibel level or perceived loudness like doubling every
Also need to envelope detect the audio, then filter the
AM-demodulated signal down to a bandwidth of maybe 3 to 5
Hertz something pleasing to look at and won't interact
with the 60Hz. Separately on each channel after the
bandpass filtering of course.
> > Can anyone point me to some circuits which I could
> build in a couple days with common parts. I've got
> a half dozen optos and triacs left over I'd like
> to use them to feed the bulbs with full-wave AC.
Unfortunately I don't know how to build a circuit like
this but at least it's some ideas.
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