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Multiple astable 555's

A

Alexander Schoemaker

Hi everybody,

I'm new here, and hav only recently gotten into electronics, so please excuse me if I ask dumb questions. I do really try not to.

Some time ago I built a simple astable 555 synth, but since it doesn't give a very interesting tone, I thought of using something beter: 4 555's.
So, I looked into it a bit, and I found instructions online for converting square waves to sawtooths or sine waves. So, I thought I could drive 4 astable 555's with push button. Then, between the pushbuttons I could use pots, so I could tune each tone. The output from each 555 could go through 2 converter, which can be bypassed with a toggle switch, and then through a passiver mixer and an op amp before going to a speaker. I though I could use 2 speakers, each for 2 555's.

astable555.png

So, did I make some mistakes here? Do any of you see a reason this wouldn't work? Any help would be very much appreciated.
 
davenn

davenn

Moderator
Hi Welcome to EP :)


Alexander Schoemaker said:
So, I thought I could drive 4 astable 555's with push button. Then, between the pushbuttons I could use pots, so I could tune each tone.
Click to expand...


But that isn't going to work is it ;) as adjusting any one trimpot will adjust the resistance, therefore the tone, of the whole chain

before I give an answer, have a think about that and how you would wire them differently :)


Dave
 
A

Alexander Schoemaker

davenn said:
Hi Welcome to EP :)





But that isn't going to work is it ;) as adjusting any one trimpot will adjust the resistance, therefore the tone, of the whole chain

before I give an answer, have a think about that and how you would wire them differently :)


Dave
Click to expand...

Thanks, Davenn.

But, it's supposed to change all the other tones. They add up. So, I first tune the first in line. Then I tune the second one, which adds up to the first one, giving a greater resistence, and therefor a deeper tone. Tuning the third one isn't gonna change the second in line, or am I mistaking? Because, according to my reckoning, pushing the third button in line, gives me the added resistance of the first three resistors, pushing the fourth button gives the added result of the first four. Am I wrong here?
 
davenn

davenn

Moderator
Alexander Schoemaker said:
Thanks, Davenn.

But, it's supposed to change all the other tones. They add up. So, I first tune the first in line. Then I tune the second one, which adds up to the first one, giving a greater resistence, and therefor a deeper tone. Tuning the third one isn't gonna change the second in line, or am I mistaking? Because, according to my reckoning, pushing the third button in line, gives me the added resistance of the first three resistors, pushing the fourth button gives the added result of the first four. Am I wrong here?
Click to expand...

yes you can go that way ...
it would be more common to keep them separate so that the tone of any individual one only relies on the adjusting of a single trimpot :)

that makes it lots easier

Dave
 
A

Alexander Schoemaker

Yeah, I know it's not the easiest approach, but putting the entire circuit to every button in parallel, with bigger resistors for lower notes, has other drawbacks. Besides, I would only have to do this once. I plan on using variable precision resistors, so once the synth is closed up, I'm not planning on tuning it again.
 
KJ6EAD

KJ6EAD

As you've discovered, a simple square wave isn't very musical. You're attempting to create polyphony which will be a significant improvement. This is what the early pioneers of analog music synthesizers did. If you make your extra oscillators run an octave away from your fundamental tone, above or below, I think you'll like the result.
 
A

Alexander Schoemaker

davenn said:
component values change with time ;) maybe you didn't know that




it's your project, I was suggesting a better alternative

if you don't want to follow ... I don't really care, not my problem ;)

remember it was you asking the " dumb questions" ;)
Click to expand...

Haha, thanks.
And I do appreciate the suggestion.

But, my biggest concern is: Would it work to simulaneously drive 4 555s from each single pushed button this way.

As for the keyboard, no, I didn't know components change value over time. I have seen other people who work with this keyboard layout, though. Besides, being used to tuning accordions (up to 10 metal reeds for each note) I don't think retuning once in a while would be my biggest concern.
 
A

Alexander Schoemaker

KJ6EAD said:
As you've discovered, a simple square wave isn't very musical. You're attempting to create polyphony which will be a significant improvement. This is what the early pioneers of analog music synthesizers did. If you make your extra oscillators run an octave away from your fundamental tone, above or below, I think you'll like the result.
Click to expand...

Thanks for the reply.
And yeah, that's the basic idea I had here.
I'm playing with the idea of putting extra capacitors between the 2 and one pin, with a toggle switch, in parallel, so I can do just that: Make the oscillator drop an octave. Also, note the switchable rows of capacitor/resistor combinations in the upper part: The woggle switches there should function as a bypass, and when not bypassed, the first two should convert the squares to sawtooths, the second pair shouold convert sawtooth to sine. Though I see already that this isn't the cleverest design.... I could just put a selector switch there...
 
AnalogKid

AnalogKid

Alexander Schoemaker said:
But, my biggest concern is: Would it work to simulaneously drive 4 555s from each single pushed button this way. .
Click to expand...

Maybe, but not well.

Your single string of pots is connected to four other resistors and four timing capacitors, none of which are adjustable, or will track with temperature, or are close enough in their relative values not to be noticeably out of tune. This will introduce low frequency beat modulation into the output as the four 555 outputs transition at slightly different times.

Sticking to a series string of tuning pots rather than a parallel array means that there is no such thing as tweaking one tone without having to touch all of them, but that's your call. Separate from that, you will have much better tonality control if you have only one primary oscillator, followed by however many shaping circuits you want. The classic function generator circuits start with a square wave oscillator, shape that to a triangle wave, and shape that to a sine wave.

Why do you have a 10K resistor in series with each speaker? For an 8 ohm speaker that is a 62 dB attenuator.

ak
 
Last edited:
davenn

davenn

Moderator
AnalogKid said:
Sticking to a series string of tuning pots rather than a parallel array means that there is no such thing as tweaking one tone without having to touch all of them, but that's your call.
Click to expand...

yup, which is what I said before ... it's really just not a good idea
 
A

Alexander Schoemaker

AnalogKid said:
Maybe, but not well.

Your single string of pots is connected to four other resistors and four timing capacitors, none of which are adjustable, or will track with temperature, or are close enough in their relative values not to be noticeably out of tune. This will introduce low frequency beat modulation into the output as the four 555 outputs transition at slightly different times.

Sticking to a series string of tuning pots rather than a parallel array means that there is no such thing as tweaking one tone without having to touch all of them, but that's your call. Separate from that, you will have much better tonality control if you have only one primary oscillator, followed by however many shaping circuits you want. The classic function generator circuits start with a square wave oscillator, shape that to a triangle wave, and shape that to a sine wave.

Why do you have a 10K resistor in series with each speaker? For an 8 ohm speaker that is a 62 dB attenuator.

ak
Click to expand...


I don't know if I completely understood the first part of your post, but thinking about it, I think I did discover a problem with this. Though I don't know if it's what you meant. By connecting the four 555's to one string of pots, I have made them one circuit, and so the frequency of the first 555 will influence the second, and so on... Hmm... I think I need to find a way to keep thesse separate while still switching4 circuits at a time. I did the thing with a single 555 already, so, yeah it would be easier, but I realy wanna take it a step up, and not do the same thing again.

As for the keyboard, yeah, I know. I discussed it with davenn already, and I may be stubborn, but I'm sticking to it. I used it with the single 555 too, and it worked, and still works.

The 10 K resistor is there for impedance: When looking up a passive mixer, this is what I found as the way to mix different audio signals.
 
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