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VCO Design / 555 oscillator

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hamishg

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I'm doing a university project on how sound is generated [in relation to modular synthesizers/analogue circuits]. So i've looked into the different modules of an analogue synth, and of course the Voltage Controlled Oscillator is one of the main focal points.

Firstly i must say I'm a novice when it comes to electronics...

I went out and got a solderless breadboard, a 555 timer IC, and some resistors and capacitors. I've got the following tester circuit going-

**broken link removed**

I hooked up a little 8ohm speaker to the output and can hear a continuous 'wail'. When i touch both resistors R1 and R2, this 'wail' goes up considerably in pitch. This tells me something about the relationship between the value of these resistors and the sound that is produced...

I want to implement a potentiometer instead of 2 separate resistors [R1 and R2] to have some sort of control over the frequency/pitch of the sound.

The desired outcome of my project would be to have a [controllable] sound generating circuit [with a frequency bandwidth that would be realistic for a synthesiser - i.e. within the 20Hz-20kHz range]. I have a signal, but I'd like to maximise my control over the sound of it - by integrating additional simple oscillator circuits [as a synth would have more than 1 oscillator], and/or simple filter/amplifier circuits.

I was wondering whether anybody could kindly point me in the right direction?! As the majority of *synth* VCO circuit designs I've come across are too complicated!
 
Synth designs are complicated because they have to be in order to work - a 555 is a useless VCO - as you've found out. A better option (although still not great) would be a function generator chip such as the 8038 or XR2006.

Bear in mind you don't really need 20Hz-20KHz, that's far wider than even a piano.
 
The sound that you get depends on several things. Of course the fundamental frequency is important. The harmonic content is also important. Harmonics are integer multiples of the fundamental frequency. You get them with any waveform other than a pure sine wave. The amplitude of each harmonic is different when you compare, for example, a square wave to a triangle wave. Each shape of waveform has its own unique sound as a result of these different harmonic levels. So, if you want to play with synthesizing sound, it is very useful to be able to control the shape of the waveform. Nigel's suggestion of using a function generator chip like the ICL8038 is a good one because the one chip delivers at least three different carefully controlled waveforms.

You can generate unique combinations of harmonics by distorting a sine wave in different ways. You might see various sound generator designs that use unique ways of distorting a sine wave. You can do that too, using various op amp circuits.
 
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