Hello crew,

Withing the more general scope of making electronic music, I am trying to build this oscillator based on a 4049 chip:

I know it's crap, but it's simple, which is good for learning I have breadboarded it successfully, I'm using a 12V supply and playing with the RC values I can adjust the frequency. Yay!
Now I am trying to add voltage control to the thing. I have found another schematic with an added 1M resistor at pin 3 that says to connect CV there. So I did just that and hooked a pot (10K) wired as a voltage divider there to be able to sweep between 0V and 12V but I only get about a semitone range. This is what it looks like now:

I can increase that up to about an octave and a half by changing the 1M resistor for a 20K (if I remember well) but I'd really like to get at least twice that range. Sadly, with smaller resistor values the oscillator starts to behave quite erratically.
I know that for the thing to be used for music I will end up having to add an expo converter but I feel like I should be able to get the range using a pot first. I've tried to simulate the circuit on the falstad applet to "see" how the electrons move around but the simulation fails when I start playing with the control voltage.

Anyway, so many words to ask how I can increase the range of this thing, if it's at all possible.
Thank you for your time!

 

Now I am trying to add voltage control to the thing.

Do you want voltage control or just the ability to change the frequency?

 

Do you want voltage control or just the ability to change the frequency?

Hey Dennis,
I would like voltage control in order to make a sequencer after that...

 

You can't control the frequency with a circuit like that.
What you need to do is vary the RC time-constant by replacing R with a voltage controlled FET.

 

You can't control the frequency with a circuit like that.
What you need to do is vary the RC time-constant by replacing R with a voltage controlled FET.

Thanks! I'll have a look for some FETs and give it a shot if I find some. I know I have a lot of BJTs but not sure about FETs.

If you can be bothered and you think a mortal like me could understand, would you mind explaining why it wouldn't work as I'm trying to implement it and why it kinda works over a small range?
Edit: What I understand is that the RC network creates a sort of delay between the two inverters so one switches the other but not instantly.

 

By far the easiest way is to use the VCO section of the CD4046 PLL, and leave the phase detector section unused. It is fairly linear specifically if used with a supply of 15 volts, and absolutely simple to use.

EDIT: only a single capacitor and resistor required. If you don’t want a zero frequency with a zero voltage, then a second resistor is required.

 

Thanks! I'll have a look for some FETs and give it a shot if I find some. I know I have a lot of BJTs but not sure about FETs.

If you can be bothered and you think a mortal like me could understand, would you mind explaining why it wouldn't work as I'm trying to implement it and why it kinda works over a small range?
Edit: What I understand is that the RC network creates a sort of delay between the two inverters so one switches the other but not instantly.

The delay it creates depends on the time it takes to charge to a percentage of the power supply, but the pot and 1M resistor don’t change that percentage.
Try adding a resistor between the 100k resistor and the 1M resistor, start with 1M, and see if that makes it work. If not try some different values. It might work towards the middle of the range of the pot but not at the ends.
Unfortunately, when you do get it to work, it won’t be the voltage control law you are looking for, it will be more like a raised cosine curve with low frequencies at minimum and maximum voltage and higher frequencies in the middle.

 

Try this VCO. I tested it on a breadboard.

 

ry this VCO. I tested it on a breadboard.

What was the frequency control range?

 

What was the frequency control range?

With C1 = 10 nF, the range is 80 Hz to 60 kHz, three orders of magnitude.

 

https://www.worldradiohistory.com/UK/Electronics-Today-UK/ETI-Electronic-Circuit-Design-No-1.pdf
see figure 22.
The 4007 has its source contact separated from its substrate for some of its transistors so they can be used as voltage controlled resistors.

 

https://www.worldradiohistory.com/UK/Electronics-Today-UK/ETI-Electronic-Circuit-Design-No-1.pdf
see figure 22.
The 4007 has its source contact separated from its substrate for some of its transistors so they can be used as voltage controlled resistors.

Certainly worth trying.

 

Try this VCO. I tested it on a breadboard.

View attachment 348428

Thanks MrChips! But I wanna learn how to fish, not just have it given to me
This is the path I went down:

I know there's probably loads wrong with this but it kinda works when the pot outputs between 6.3 and 7V which I want to investigate and understand.

I'm really surprised by the fact that the resistor is in parallel with the FET in your circuit. Could you please walk me through how you came up with it?

 

By far the easiest way is to use the VCO section of the CD4046 PLL, and leave the phase detector section unused. It is fairly linear specifically if used with a supply of 15 volts, and absolutely simple to use.

EDIT: only a single capacitor and resistor required. If you don’t want a zero frequency with a zero voltage, then a second resistor is required.

Thank you, and I happen to have one actually, but if I just steer away from my problems I'll never understand the solutions!

 

The 2-inverter oscillator is more difficult to understand than the 3-inverter circuit.

Two inverters in a feedback loop create positive feedback. Hence it is a bistable circuit, i.e. a flip-flop. (Circuit A)

Three inverters in a loop create negative feedback. Hence the frequency of oscillation depends on the propagation delay around the loop. (Circuit B)

We can lengthen the delay by adding R and C to the circuit.