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

[media at mail1.nai.net]

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.

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.

Check out these websites:

http://www3.ncsu.edu/ECE480/images/aa-555-01.gif
http://www.andy-clarkson.co.uk/555/

The first shows an animation of the 555 used as an oscillator.  There are
also several "555 calculators" that will run on a PC that are free
downloads.

Regardless, a VCO usually contains an exponential converter to use 1V/oct
control voltages.  You can look at the exponential converter and the
oscillator itself as two separate parts.  Building a stable exponential
converter is the bugbear of analogue synthesis.  In order to be stable it
requires some sort of temperature compensation.  It also requires a stable
current source and a very well-regulated supply voltage.  If you search the
archives of this list, you will thousands of posts discussing exponential
converters, matched transistors, current sources, tempcos, etc.

>How do you get different waveforms?

Most VCO circuits output a single waveform at their core (usually a saw or
triangle) and use various waveshapers to get the other outputs.

>    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.

>Are the bipolar regualted supplies from maplin ( i believe they sell a
>>quite cheap hobby one) any use or do i have to build one?

I don't know what is available in the UK, but in the U.S., it's less
expensive to buy a 723-based bipolar supply from Power One or Stancor than
to build one yourself.  It's also much less work!!  Of course, there is
nothing wrong with building your own power supply if you want to learn how
to build power supplies.

>If building should a use a normal or toroid transformer?

Almost all of the power supplies I have ever seen use "normal" transformers.