[sdiy] 555 vco (electronotes)

[TheMysticSatin]

After running into 555 power supply drain in a sequencer I was working on I
opted to use what I've found labelled as a C555 ( actually I'll be using a
C556 to make a dual vco) Is that the same as the ICM chip or a reasonable
equal? It IS a CMOS chip.
----------
>From: Scott Gravenhorst <music.maker at gte.net>
>To: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] 555 vco (electronotes)
>Date: Fri, Nov 9, 2001, 7:03 PM
>

>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
>
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