Simple 555 synth idea

[Yoa01]

Anyway, so back at the topic at hand (this is spiraling towards "help the noob", which is good but totally not following the thread subject), let's start with the 555 oscillator. How would I draw it out so that I can change frequency and pulse width via a pot (or other knob)? Was I close?

 

[CDRIVE]

Ah. Thanks.

Heh, I just read the Readme after installing and it says to not use it for educational purposes. Oh well...

Alrighty, I'll look into that. Thank you!

That's because it's the free version of Tina. Many options are disabled. They're not saying you can't use it to teach yourself. You can't be a school and use this without being hit with a law suite.

Chris

 

[CDRIVE]

Hi Yoa01,

You're not crazy! As an electrician in Australia we use this method when we draw circuit diagrams.
Happy drawing!

That's not the issue. The jump over method was once common and I will understand it in either form. The problem arises when a conductor crosses another (that's connected) but there's no dot to indicate same. It leaves the viewer wondering if the designer forgot to jump over it or forgot the connection dot. Even back in the day when I used to jump conductors I still always denoted a connection with a dot because it leaves no question about my intentions.

Chris

 

[Yoa01]

So basically, dot = connection, no dot = not connected, right?

 

[CDRIVE]

So basically, dot = connection, no dot = not connected, right?

Roger that!

Chris

 

[CDRIVE]

How would I draw it out so that I can change frequency and pulse width via a pot (or other knob)? Was I close?

As Dave did it in schematic CCT. 3 but keep in mind that you can't change the freq or pulse width without effecting one or the other. At least not this way. In other words, the 200K pots, R1 & R2 are not independent of each other.

Chris

 

[Yoa01]

Hm. I really need them separate, though I suppose them changing at the same time would make some interesting effects.

 

[CDRIVE]

I'll give this some thought but I'm currently wrapped up in a program I'm writing for myself. I never seem to finish any of my own projects because I get sidetracked by other peoples projects. I mean to finish this one.

Meanwhile, I'm sure others have some thoughts in this regard.

Chris

 

[Yoa01]

I'm currently comparing other's circuits. I have some for just pulse width, others for just pitch. I'll see if there's a seemingly safe way to incorporate the two.

 

[CDRIVE]

I'm currently comparing other's circuits. I have some for just pulse width, others for just pitch. I'll see if there's a seemingly safe way to incorporate the two.

For sure, you can't be the first person that has encountered this issue, so there must be a solution. If changing freq in steps is acceptable then switching in different values of C will change the freq while maintaining duty cycle. The problem arises when the duty cycle is changed. The freq will be too. Ganged pots inversely wired might solve this. When the duty cycle pot val is reduced the freq pot val is increased. Admittedly, I've not given this a lot of thought and I could be missing something. My gut tells me there's an easier way but like I said, I'm currently distracted.

Chris

 

[Yoa01]

You stay distracted, it's cool. I think I'm onto something using two 300k pots and a varicap, but I'm currently looking at a possibly easier way to do it.

 

[CDRIVE]

You stay distracted, it's cool. I think I'm onto something using two 300k pots and a varicap, but I'm currently looking at a possibly easier way to do it.

Keep in mind that Varicaps are manufactured in the pF range, but then I'm sometimes guilty of not staying current. This might, but I doubt it, be one of those times.

Chris

Edit: Even if you could find a VC in the range you need you would run into issues because the cap sees DC during it's charge and discharge cycle. This will make biasing a VC a PITA.

Chris

 

[Yoa01]

After much research and a family visit, I have decided that, more than likely, changing pulse width and frequency separately would be a total PITA.

The best I could do is use a DIP-switchable diode/resistor array, but I'd rather be able to do a full sweep.

However, I found something even better: I found a frequency variable 555 oscillator that can output square, approximated saw and sine, and triangle waves. All it needs is a 10uF cap and an opamp. I'll draw it out and upload it tomorrow.

 

[CDRIVE]

Why not just post the link?

Chris

 

[Yoa01]

Multiple links, and the way I find stuff I generally just grab pics of schematics and run with it. But, here are the two images I want to combine:


I know I originally wanted just square, but if this is all possible and as simple as it seems, then I'll certainly take that!

 

[CDRIVE]

The bottom circuit uses a virtual ground. Unrelated to that I don't think it's a good choice if you need a spread of frequencies. Speaking of which, what are they?

Chris

 

[Yoa01]

Is a virtual ground necessarily bad, though? Since it can be any voltage, couldn't I just make that the negative?

What are, what, the frequencies I want? Preferably from 16 to 523 HZ (C0 to C5), but anything in the lower audio range (say, max of 1000Hz) is acceptable.

 

[CDRIVE]

Actually, after looking at it again I think I misspoke. What I thought was a virtual gnd appears to be a common bias line for the OpAmps. Please post the link from which you found this.

Chris

 

[Yoa01]

Going through my history...

Here's the first pic, and where I got the idea to use a varicap and get capacitor-driven saw waves: http://www.electronics-tutorials.ws/waveforms/555_oscillator.html (replace second pic's diode with a varicap)
Here's the second pic, where I got the sine and triangle idea: http://forum.allaboutcircuits.com/showthread.php?t=12405

Also, here's where I found how to do pulse-width modulation: http://www.instructables.com/id/Simple-and-dirty-Pulse-Width-Modulation-PWM-Wi/ and http://www.instructables.com/id/Very-simple-PWM-with-555Modulate-every-thing/?ALLSTEPS
You can see why I dismissed the idea of doing PWM and FM with the same 555.

 

[CDRIVE]

Going through my history...

Here's the second pic, where I got the sine and triangle idea: http://forum.allaboutcircuits.com/showthread.php?t=12405

Not surprising that BM of AAC posted that. He's AAC's 555 guru. The ganged pots he shows is exactly what I was going to suggest. I'm going out now but I'll spice this later.

Chris