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<DIV><FONT face=Arial size=2>One way to get a variable length pulse is to patch
an envelope into a comparator. The envelope should have the shortest possible
attack, unless you want a delay. (Hint, hint! Trigger delays are cool,
too!) The comparator output will be your variable length pulse, either
"off" or "on."</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The envelope can come from an ADSR, AD, AR, or a
simple one shot monostable. A multivibrator is one way (?) to make a one shot,
but it s</FONT><FONT face=Arial size=2>eems to me that you still need a
comparator (made with an op amp or something like an LM311).</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Another way to do this is with a circuit that does
something like this:</FONT></DIV>
<DIV><FONT face=Arial size=2> Main ingredients: flip-flop,
capacitor, a pot, and a comparator.</FONT></DIV>
<DIV><FONT face=Arial size=2> When triggered, discharge the
timing cap and SET the flip-flop.</FONT></DIV>
<DIV><FONT face=Arial size=2> While the flip-flop is SET, send
a HIGH signal to the output of the module -- that's your output
pulse.</FONT></DIV>
<DIV><FONT face=Arial size=2> Use that same HIGH signal to
charge a cap through a variable resistance (the pot).</FONT></DIV>
<DIV><FONT face=Arial size=2> When the cap reaches the
comparator's threshold, the flip-flop is RESET and the pulse goes
off.</FONT></DIV>
<DIV><FONT face=Arial size=2>It's similar to part of a sawtooth core oscillator.
Discharging the timing cap is like SYNCing the oscillator.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>By the way, that chip (74VHC123) can't take 12V or
15V power. There must be a CMOS chip that'll do what you want. (If only I could
name one... ;-)</FONT></DIV>
<DIV>--<BR>john <--(still working on the Soddy voting stuff)</DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT><FONT face=Arial
size=2></FONT><BR> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=moog@qwest.net href="mailto:moog@qwest.net">James R. Coplin</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=synth-diy@dropmix.xs4all.nl
href="mailto:synth-diy@dropmix.xs4all.nl">'Synth-DIY'</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Wednesday, February 02, 2005 2:46
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [sdiy] Multivibrator IC
questions...</DIV>
<DIV><FONT face=Arial size=2></FONT><FONT face=Arial size=2></FONT><BR></DIV>
<DIV class=Section1>
<P class=MsoNormal><SPAN class=GramE><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">I’m</SPAN></FONT></SPAN><FONT
face=Arial size=2><SPAN style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"> looking
into making a circuit that takes a trigger or gate as input and outputs a
modified pulse length. <SPAN style="mso-spacerun: yes"> </SPAN>I was
looking at some <SPAN class=SpellE>multivibrator</SPAN> ICs
like:<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><A
title=http://www.fairchildsemi.com/ds/74/74VHC123A.pdf
href="http://www.fairchildsemi.com/ds/74/74VHC123A.pdf">http://www.fairchildsemi.com/ds/74/74VHC123A.pdf</A><o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">My problem is that I often seem to
be stuck with pulses, triggers, clocks etc. that I need to transform into
shorter or longer bits for control of envelopes, sequencers, etc. <SPAN
style="mso-spacerun: yes"> </SPAN>The <SPAN
class=SpellE>multivibrator</SPAN> seemed the easiest approach to have a user
assignable gate length based on a triggered input. <SPAN
style="mso-spacerun: yes"> </SPAN>The pulse time in seconds is
PW=Rx*<SPAN class=SpellE>Cx</SPAN> where Rx is ohms and <SPAN
class=SpellE>Cx</SPAN> is farads. <SPAN
style="mso-spacerun: yes"> </SPAN>There is no limit on <SPAN
class=SpellE>Cx</SPAN> but Rx is limited to 1k.<SPAN
style="mso-spacerun: yes"> </SPAN>So I thought by using a 1k push-pull
pot, you could select two different caps and have a short and long time
control. <o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">So, there are a <SPAN
class=SpellE>kazillion</SPAN> of these chips out there. <SPAN
style="mso-spacerun: yes"> </SPAN>Is there really any difference or would
the above one work out fine? <SPAN style="mso-spacerun: yes"> </SPAN>They
come two to a chip so for a couple of ICs I would get 4 independent processing
<SPAN class=SpellE>thingees</SPAN>. <SPAN
style="mso-spacerun: yes"> </SPAN>Are there other approaches I should be
considering, pitfalls etc.? Thanks!<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">James R.
Coplin</SPAN></FONT><SPAN style="mso-no-proof: yes"><o:p></o:p></SPAN></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">***************</SPAN></FONT><SPAN
style="mso-no-proof: yes"><o:p></o:p></SPAN></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">If anyone asks
of my whereabouts,</SPAN></FONT><SPAN
style="mso-no-proof: yes"><o:p></o:p></SPAN></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">simply tell
them i've gone out the window</SPAN></FONT><SPAN
style="mso-no-proof: yes"><o:p></o:p></SPAN></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">for a spot of
tea and am not</SPAN></FONT><SPAN
style="mso-no-proof: yes"><o:p></o:p></SPAN></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">expected back
any time soon.</SPAN></FONT><SPAN
style="mso-no-proof: yes"><o:p></o:p></SPAN></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-no-proof: yes">***************</SPAN></FONT><o:p></o:p></P>
<P class=MsoNormal><FONT face="Times New Roman" size=3><SPAN
style="FONT-SIZE: 12pt"><o:p> </o:p></SPAN></FONT></P></DIV></BLOCKQUOTE></BODY></HTML>