I’m interested in modifying some existing open source designs and ultimately designing a few of my own - but I’m not quite sure how to get started. Following a build guide is one thing, designing a circuit from scratch is another. Maybe some of the volt wizards on the board can share some of their tips and tricks?
For instance - I’d like to modify this Elby Octave switcher to add a control voltage input. Is there a standard way of adding CV control to a pot?
And, more generally, is there a good circuit design cheat sheet or reference guide that anybody can recommend? I’d also be interested in finding any simulation tools so that I can test out a circuit in software before moving it over to the breadboard, is such things exist.
What do you want to control exactly?
To replace S101, you can use an analog switch (CD4051 for example) or a quantizer/DAC.
To replace a resistor, you can use a vactrol, or a JFET, or an LM13700, depending on your application.
I mainly use LTSpice to simulate my circuits, but I think that most people here use circuitlab. There is also Falstad, which can be useful to understand electronics.
The goal of adding CV would be able to change the octaves with a signal from an envelope generator or a square wave. If you like electronica, an artist called Clark uses this technique (or something like it) to wonderful effect.
It can’t just be adding the control voltage to the output signal, since it should only ever output the input plus a multiple of 1.
In my mind, this meant that it would be adding an input before the U101A, but maybe that’s wrong?
U101A is set up as a voltage follower, to make sure that the signal from S101 will stay the same, independent of the load.
U101B is in inverting adder configuration, summing the output of U101A and P102.
If you just want to add another voltage, then add a branch (with a resistor) on the pin 6 of U101.
But if you just do that, you will not add exactly one octave with the CV input, but also everything in between. If you want to add exactly one octave, or parts of octave (the correct word is semi-tone, right?), then you need a quantizer.
You’re not talking about adding CV to a pot in this instance — both the pots in that circuit are trimmers, one to fine tune the rotary switch steps to 1V and one to fine tune the offset to zero. Adding CV to one of those pots would just give you a way to mistune your octaves with control voltage.
Anyway, the simplest way to add CV to a pot is to put a vactrol either in series or in parallel with it. It’s not all that good a way to add CV. For one thing vactrols are slow, it takes a while (tens or hundreds of milliseconds) for the LDR to return to its dark resistance. Also you have limited choices as to what resistance range and current response they’ll provide and it varies from one piece to another. Certainly if you want for instance 1V/octave response a vactrol’s not going to do that. Finally, it’s not that simple to use them if you want to use them right: just putting the CV across the LED (and a series resistor) kind of works, but you’re using the CV to run the LED and that’s not good practice. A CV should be used only as a voltage reference. So the right way is to use a transistor or op amp as an LED driver controlled by the CV.
There are books out there, a few are mentioned here and here. I’ve tended to the more disorganized approach of reading explanations of particular circuits and googling particular topics until it starts coming together (and a lot hasn’t, yet). As for simulation tools there are lots of them, the one I’ve been using is circuitlab.com. It’s of limited usefulness if you don’t pay for “pro” access or whatever they call it, which as I recall is about $24 a year. Which I pay because I find it worth that.
Thanks for the resources. Just realized I’ve actually got that Make book on the shelf, need to dust that off…
Back to this particular issue - is there such thing as a voltage controlled rotary switch? It seems like adding a quantizer to this circuit is overkill. Maybe I could send all of the +n voltage lines to a chip, and then have the output of that chosen by a CV input, summed with voltage controlled by a pot? Do switch-logic chips exist?
I’m a software developer by trade, which I think actually hinders my thinking about circuits - I keep trying to make everything into digital control flow!
Adding a quantizer to this circuit would be overkill, but using a quantizer instead of this circuit might be a good option. After all, a quantizer module has uses besides just this.
I have a dual quantizer whose “esoteric scales” set includes all equal divisions of the octave up through 24 notes — which includes 1 note per octave, which means it puts out octave control voltages from a continuous input control voltage. There’s a knob but it just adds an offset (CV2, normalized to 5V) to the output CV which isn’t what you want for manual control but you could fake that by using both units: plug the CV into one quantizer, turn off quantization, and plug its output into the other quantizer, set up for octaves. That would just give you a manual offset of the incoming CV. Or another option would be to modify the Arduino code to add CV2 to CV1 before quantization instead of after. One limitation is that it only handles 0-5V CV, so the range is five octaves instead of the nine of the Elby circuit.
I was also thinking along the lines of using an ADC for quantization to control a CD4051 type analog switch, but then I realized it might actually be simpler, especially for a software person, to use a microcontroller such as the 8-pin ATTINY with a few resistors and capacitors and one opamp.
The microcontroller reads the input voltage with its ADC then converts it back into the appropriate output voltage using PWM.
The ATTINY can only output 0 to 5V so the output voltage is multiplied (and buffered) with an opamp to get the 0 to 8V desired range.
Just noticed the power supply is ±10V in the original circuit. That also works with the circuit in the sketch above, just need to make sure the opamp can drive 8V from that supply, apparently the TL082 can or the original circuit wouldn’t work.
Probably should also add a Schottky protection diode from the ATTINY analog input to +5V.
OK, I thought this would be a quick note and I went and wrote a novel again.
