[sdiy] 555 vco (electronotes)
TheMysticSatin
elmystico at earthlink.net
Sun Nov 11 18:41:53 CET 2001
okay, no dice on getting an outout by applying a resistor across the E-C
connection, at least I don't think so. I'm working without a scope here but
there's no audible output and a meter reading at the outputs shows a -5v dc
at the pulse outs and a -2v dc at the saw outs. The weird thing is that
both the vco's show the exact same voltages at the outputs. I tried
swapping in a much larger cap (.01) for the charging cap on the timer to
slow down the oscillation to something that would either be audible or show
on the meter but the readings and audio output are the same, maybe a way
bigger cap would do the trick, I'll try that when I get a chance. Here's a
weird quirk, both the outputs have a large -voltage applied to them before
they out, the pulse out gets -15v applied to it thru a resistor and the saw
out goes thru the +input of an op amp, the -input of which is getting +15v
thru a resistor. ??!!??
----------
>From: Scott Gravenhorst <music.maker at gte.net>
>To: "TheMysticSatin" <elmystico at earthlink.net>
>Subject: Re: [sdiy] 555 vco (electronotes)
>Date: Sun, Nov 11, 2001, 9:14 AM
>
>Ok, it looks like the transistor that charges the cap (emitter
>collector connection) can be replaced by a resistor. The other
>transistors comprise the expo converter.
>
>As far as 'stable' goes, I can't complain at all about the 555
>VCOs in my FatMan. I can tune it and then turn it off.
>Turn it on a *week* later and it's STILL IN TUNE! And I'm a
>guy who is extremely sensitive to pitch and mistuning. When I
>was in a band a while ago, I would set it up in the
>drummer's garage which was either colder or hotter than at home
>and I rarely had to touch the tuning knob. And then it would
>stay in perfect tune as we played for about 4 hours.
>
>BUT, it's a linear pitch CV. Expos have a more serious problem
>with temperature stability because the conversion method is
>a transistor quirk that is wildly termperature dependant. That's
>probably why the FatMan is linear, that and a good expo converter
>is sort of expensive.
>
>"TheMysticSatin" <elmystico at earthlink.net> wrote:
>>I KNOW!!!
>>Yes, the Emitter of the Xsistor is hooked up to +15. I also neglected to
>>mention that the collector is also hooked up to a .001uF cap which is going
>>to GND. This is the charging cap for the oscillator, right? Then it goes to
>>the + input of an op amp, the -input of which is connected to +15v thru a
>>10k resistor, there's also a 10k feedback resistor to the -input and the
>>output of this op amp is the saw out. The Base of the Xsistor that's
>>feeding this whole thing is hooked up to the base of another xsistor... whoa
>>just noticed these 2 are pnp, and the collector of a npn, the base and
>>collector of the 2nd pnp i mentioned (the first one in the current path) are
>>both also hooked up to the collector of the npn, which in turn has its base
>>to ground, its collector connected to the collector of another npn, the base
>>of which is connected to the op amp managing the CV's and the emitter of
>>which is hooked up to +15v thru a 100k resistor and a 100p cap which is in
>>the feedback loop of another op amp, the output of which is also going to
>>the collectors of both npn's... whew! this probably is a pointless
>>description because it's just too hard to follow... Anyway, I imagine this
>>is not the most stable VCO, it's more of an exercise for me to learn how a
>>VCO works and it's the simplest one I've found
>>----------
>>>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, 5:36 PM
>>>
>>
>>>Crap! for the love of seeing the schematic! It sounds
>>>'odd' to me. That means that the discharge will have
>>>to flow thru that 220 ohm resistor, slowing the discharge
>>>time, which is not usually good. (in a linear [or expo]
>>>system this will cause the VCO to be flat of 'correct'
>>>as pitch increases). However, it should oscillate that
>>>way. The emitter of this transistor is where? It should
>>>be at V+ (or thru a small resistor to V+). In that case,
>>>the substitute charging resistor would go from V+ to the
>>>cap (side that's not grounded).
>>>
>>>"TheMysticSatin" <elmystico at earthlink.net> wrote:
>>>>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
>>>>>
>>>>>
>>>
>>>=========================================================
>>>- 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
>
>
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