[sdiy] 555 vco (electronotes)

[TheMysticSatin]

Okay, that makes more sense.  I can see the type of connection you're
talking about.  Here the pins 2 & 6 of the 555 are hooked up to the
collector of the transistor, also pin 7 is hooked up to the collector bu
through a 220 ohms resistor (these are the pin #'s for the normal 555 in the
schematic, they're different for the c556 but I've made adjustments so
that's pin 2-trigger pin 6-threshold and pin 7-discharge). But the timer is
powered her between GND and +15v.  So should the resistor go to ground or
should it go to supply?  Hey by the way if anyone happens to have a scanned
copy of this schematic they could throw up it would be a great help!
----------
>From: Scott Gravenhorst <music.maker at gte.net>
>To: "TheMysticSatin" <elmystico at earthlink.net>
>Subject: Re: [sdiy] 555 vco (electronotes)
>Date: Sat, Nov 10, 2001, 10:15 AM
>

>I probably wrote that poorly.
>
>Again, I can't see the schematic, so this is rather general.
>
>The way these things work is that a current source (or sink)
>is used to supply the charge current to the timing cap.  This 
>is done with a transistor.  The FatMan VCO is an excellent example 
>of this because it uses a 555.  In the FatMan, the 555 is powered
>from ground (Vcc or pin 8) and -12volts (Vee or pin 1).  The 
>pin numbers may be different for the dual timer part.  Charging 
>current  is supplied to the cap by an NPN transistor, collector 
>to the cap, emitter to ground (actually "virtual ground", but it 
>is still a zero volts reference and ground would be used with the 
>resistor).  So current starts from ground, flows thru the 
>transistor and into the cap and to -12v.  In this example, you
>would remove the collector of the transistor from the cap and
>place a resistor from ground (real ground), like 10K, to the 
>point where the collector had been connected.  This should cause 
>the oscillator run at a fixed frequency.  What that frequency is
>and whether it will be audible depends on the cap.  But, even
>if not audible, you should be able to see the sawtooth waveform
>at cap with an o'scope.  If the 555 works this way and not with 
>the current controlling transistor connected, then the transistor 
>is (for whatever reason) not supplying current as it should.  This 
>would tell you to trouble shoot the current source, not the 555.
>If it does NOT oscillate with the resistor, then something is wrong
>with the way the basic 555 is wired.
>
>"TheMysticSatin" <elmystico at earthlink.net> wrote:
>>Hold on let me get you straight, just add a resistor along the CV input line
>>so there would be my standard 100k input impedence resistor and then  a 10k
>>in series with that before it?  Or am I misunderstanding?
>>----------
>>>From: Scott Gravenhorst <music.maker at gte.net>
>>>To: "TheMysticSatin" <elmystico at earthlink.net>
>>>Subject: Re: [sdiy] 555 vco (electronotes)
>>>Date: Sat, Nov 10, 2001, 7:18 AM
>>>
>>
>>>Hmm.  I don't have EN, so I can't look at it, but this
>>>ought to be a fairly straightforward circuit.  Other than
>>>the CV input, there should be very little different from
>>>a plain 'ol multivibrator circuit.  If you can, try to
>>>replace the CV input current source with a resistor, I'd
>>>guess to start around 10K.  That will tell you if your
>>>problem is in the CV input or in the basic sawtooth 
>>>oscillator.
>>>
>>>"TheMysticSatin" <elmystico at earthlink.net> wrote:
>>>>Yeah I was wondering about syncing problems from using 2 timers on the same
>>>>chip.  Figure only way to find out is to try it (also I only have the dual
>>>>timers around right now), I can't get the thing to do anything right now all
>>>>I get is a -5v at the square outs and a -3 at the saw outs so the syncing
>>>>problem will have to wait till I have something to sync (or swim)...
>>>>----------
>>>>>From: Scott Gravenhorst <music.maker at gte.net>
>>>>>To: "TheMysticSatin" <elmystico at earthlink.net>
>>>>>Subject: Re: [sdiy] 555 vco (electronotes)
>>>>>Date: Fri, Nov 9, 2001, 10:33 PM
>>>>>
>>>>
>>>>>My guess is that it's a second source for the same
>>>>>part.  To be sure, get the data sheet PDFs for both
>>>>>parts and compare.  You might also want to get the
>>>>>data sheet for the bipolar part and compare it to
>>>>>the part you want to use.  It's *probably* the same,
>>>>>but only the data sheet can clear that up.
>>>>>
>>>>>Also, I'm not real sure how well the two timers on
>>>>>the dual will be isolated.  You may wind up with
>>>>>soft synch problems anyway due to coupling within
>>>>>the IC.  (via internal power connections).  I've
>>>>>never used the dual, so I'm no authority on that.
>>>>>
>>>>>"TheMysticSatin" <elmystico at earthlink.net> wrote:
>>>>>>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
>>>>>>>
>>>
>>>=========================================================  
>>>- 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
>>>
>>>
>
>=========================================================  
>- 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
>
>