[sdiy] 555 VCO (was : <no subject>)

[Scott Gravenhorst]

media at mail1.nai.net wrote:
>First off, it's good to include a Subject: field.
>
>At 5:27 PM +0100 04/27/01, Seb Carr wrote:
>>
>>Hi, i've been silent on the list for a bit, just reading and learning, and
>>feel ready to start on something.  While sometime i rckon i'll probably
>>build an oakley modular, i don't want to straight away as i'm a novice, so
>>i'd like to start more simply.
>
>Actually, building kits, where the circuit is already designed for you, is
>easier than building something from scratch.  Not only do you not have to
>design anything, you don't have to etch a PCB or juggle around parts on a
>"vectorboard".
>
>>    Using the idea "stick to what you know" i was considering making an VCO
>>from using a 555, as the 555 circuits i know are very simple and i've read
>>that the 555 is very stable.
>>so does this mean that its tuning shouldn't drift too much?
>
>Afaik, it's reasonably stable, but not VCO stable.

I'd disagree.  I use a FatMan which is 555 based.  I can
set my master pitch in tune with my Wavestation, and it
would stay in tune for 5 hours of rehearsal.  Also, many
times, I would use it again the next week, power it on and
lo and behold, it was in tune without having to touch it.

BTW, The FatMan is a kit, and a very good one at that,
though not expo.  And although expo is more popular,
linear is no less musical, easier to construct and less
expensive.

>
>However, the big problem is that its frequency is not easily controlled by
>an external voltage.  Pin 5 is labelled "control voltage" but, afaik, from
>looking at the data sheet it is for adjusting the threshold voltages of the
>internal comparators.  I'm taking a wild guess that if the output was other
>than a square that would control its frequency.

Yes, you cannot use it as a VCO if you expect to use pin 5 to
control it.  PAiA used a standard sort of current source to 
feed the integration cap, and would control the current source
with the pitch CV.  The schematics are posted on www.paia.com.
It's musically linear over the 5 octaves it responds to.

To make a decent VCO with a 555, you do need some other parts;
like an opamp and a couple of transistors.  

I'm not sure, but I think it's pretty hard to find a good 
"VCO chip" these days.  The AD654 would be good if I could just 
get my hands on one, but these things and others like it, have 
gone the way of the dodo bird.  You really have to resolve 
yourself to putting one together with other parts such as opamps,
OTAs, transistors etc.  Many people on this list have posted
schematics for a good variety of VCO designs or various complexity
and performance.

>Check out these websites:
>
>http://www3.ncsu.edu/ECE480/images/aa-555-01.gif
>http://www.andy-clarkson.co.uk/555/
>

[snip]

>>    I was also wondering if anyone has any simplke but functional vco
>>schematics for the 555.
>>    Sorry to add another PSU question, but all the 555 circuits i know run
>>happily from a 9v battery but everything on this list appears to be a 12v
>>>or 15v bipolar. Why is a bipolar supply such an advantage?
>
>You can use negative voltages.

PAiA does this to make the current source work right.  The whole
555 part of the circuit is powered with -V and ground.  It needs
a +V supply because it uses OpAmps and OTAs.

========================================================  
  If children are unable to read by the 4th grade, 
  then one must ask:  What is being taught?

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: www.teklab.com/~chordman
-- NonFatMan: members.xoom.com/_XMCM/chordman/index.html