Re: help with 555 schmitt trigger resetting
- From: "John O'Flaherty" <quiasmox@xxxxxxxxx>
- Date: 24 Jul 2006 08:19:03 -0700
zander wrote:
Thanks john, I'll try that.
The TIP42's also drive a LED with 1k resistor, could the 10v on this
resistor when the lights are on be used to somehow reset the trigger?
No, because unless you can delay that somehow, if the circuit can reset
itself it will do so instantly. I'm not sure what output effect you are
looking for- just a light on when there's an audio input? If so, you
could detect the audio with an envelope detector (just a couple of
caps, a diode, a resistor) and a comparator to compare the detected
audio with some level. Then that could control the light. If the
circuit is supposed to make the light flutter or something, you could
use the envelope detector to gate the output of your existing circuit
so it would turn off when there was no input.
--
john
The output effect is, the lights flash in time with the loadness of
your voice.
lowering R2 to 100k didn't work well, the lights did always go out but
the effect was lost.
I'd like to try a envelope detector to trigger the 555 so it turns off
when there's no input.
Being a novice I searched online for info on envelope detectors and
found this link,
http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/RadCom/part9/page2.html
but finding the values for C or R,looks complicated.
Would using an IN4148 diode, 100n cap and a 10k resistor be ok to try?
Does the diode connect to the existing C1?
The complication only arises if you are trying to recover a good audio
waveform from modulated RF. For your purpose, you just need a long
enough time constant (R*C) so you get a steady output when there's
audio. Say you want it to respond in about 1/2 second. Then R*C should
equal about 1/2. Starting with your 10k resistor, divide 1/2 by that,
and you get .5/10000 = .00005, which is 50 uF. That's kind of big, so
crank the 10k up to 1 Mohm, and use a .5uF cap. Then you'd have to use
a comparator with input resistance a lot higher than 1 Mohm to not load
it. Ayway, unless you're playing with line voltage, anything you want
to try will be ok.
Thinking about it, there's probably a simpler way than using an
envelope detector and comparator. You could use another 555, triggering
off the same signal, but use a one-shot that retriggers as long as it
has an input, and doesn't reset until about 1/2 second after the last
input. Then you can gate the inputs together so turning the light on
requires both 555s to be triggered. There's probably a suitable circuit
on that page you referred to. The new 555's output could hook to the
reset pin on the first one, but the way you've got it hooked up, it
might keep the lights on rather than off, so you'ld just have to
rearrange the circuit so it worked right. Good luck.
--
john
.
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- From: zander
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- From: John O'Flaherty
- Re: help with 555 schmitt trigger resetting
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