[sdiy] 555 vco (electronotes)

[Scott Gravenhorst]

I would not use a bipolar 555.  I would look at using
an ICM7555 which is a CMOS 555 clone, pin compatible.
I've replaced 555s with ICM7555s in 4 fatman VCOs.
Work perfectly, no change in component values and there
was no change in the sound.  (The stock FatMan comes with
bipolar 555 timers.)

The main reason to use the ICM7555 instead is the fact that
a bipolar 555 allows both the upper and lower TTL totem pole
transistors to be on at the same time for an instant while the
output is transitioning from low to high or high to low.  This
causes a high current path between the supply and ground pins.
This brief high current pulse WILL appear on your power supply
rails and it WILL cause problems with other things on that
supply.  Most notably, the VCOs in the FatMan will "soft synch"
when you try to get them close together in pitch.  Simply 
replacing them with the ICM7555 makes it much harder to get
the VCOs to lock together.  This is because the ICM7555 does
not do this.  (It's declared so in the data sheet).

AFAIK, timing values for the ICM7555 and the 555 will be the
same.  I am not sure about the differences in output (pin 3)
drive current, but in a VCO, pin 3 is not usually used to 
drive anything of significant current requirement.  I use pin
3 to clock a suboctave, which needs only a very tiny current to 
do, well within the spec for either part.

"TheMysticSatin" <elmystico at earthlink.net> wrote:
>Has anyone out there built the 555 based "Dirt Cheap Utility" VCO from
>electronotes #67 and can give me any troubleshooting pointers, especially if
>there are common problems or critical areas that might need fine tuning as
>far as values.  This is my first VCO... what else can I say?  Any general
>555 timer VCO tips would be appreciated as well.
>Thanks in advance
>Gavin

=========================================================  
- Government: The other religion.
- The media's credibility should always be questioned.
- Lambs who lie down with lions are lunch.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html