Point-to-point Voltage Controlled Oscillator

Hi!

You've found a project where we take one really cheap microchip, a CD4069(nice), and stick some parts to it, and get a very useful pitch-tracking voltage controlled oscillator! The version we will build only has a saw or ramp waveform, which is one of the best waveforms to use for analog synthesizers. It's tempting to try to get a sine wave or triangle wave or PWM-able square wave, and you can add on to this circuit and get those. But that would be a different project.

You won't need a PCB or stripboard or perfboard or any kind of board, just the components and the chip and a couple potentiometers and a healthy dose of patience and hand-eye coordination. If you're more comfortable with some kind of board, there are probably projects you would like better. If you're here for the deadbug revolution, read on!

This project is based on this VCO by René Schmitz, slightly modified, so huge thanks to him for the design and the excellent schematic. This project doesn't use the thermal resistors and ignores the PWM-capable square wave section. If you want those features, you can add them! This project does have a more stable signal output, though.

Here's what you'll need!

• 1 CD4069 (or CD4049) microchip

• 2 100K potentiometers (values between 10K and 1M will work)

• 1 680R resistor

• 2 10K resistors

• 2 22K resistors

• 1 1.5K resistor

• 3 100K resistors

• 1 1M resistor

• 1 1.8M resistor (anything from 1M to 2.2M will work)

• 1 1K multiturn variable resistor, trimmer

• 100nF ceramic disc capacitor

• 2.2nF film capacitor (other values should be fine, between 1nF and say 10nF?)

• 1uF electrolytic capacitor

• 2 1N4148 diodes

• 1 NPN transistor 2N3906 (other NPN transistors will work but beware the pinout!!!)

• 1 PNP transistor 2N3904 (other PNP transistors will work but bewaaareee the piiinoooouttt!!!)

• 1 tin can with the lid cut off using a "No Sharp Edges!!!!!" type can opener

• Various Wires and Stuff

Here’s the only chip we need for this project! It’s a CD4069, a hex inverter. That means it’s got six “gates” that take the voltage put into one pin and invert it going out the other. If you supply this chip with 12V and ground, and put more than 6V into the inverter’s input, it will flip the output LOW (0 volts). Put less than 6V into the inverter’s input, and it will flip the output HIGH (12V). In the real world, the chip can’t flip either way instantly, and if you use a resistor between the output and the input, you can make a little inverting amplifier! These are the interesting properties of this chip, which we will take advantage of to create our VCO!

The pins in all ICs are numbered starting at the pin to the left of the notch on one end of the chip. They’re numbered going around the chip counter-clockwise, so the top left pin is pin 1, and on this chip, the top right pin is pin 14. The reason the pins are numbered that way is because when electronics were all round glass tubes, there would be pin 1, and the bottom of the tube would be numbered clockwise around the circle.

In this step we're going to mangle the pins like this: pins 1, 2, 8, 11, and 13 all get the skinny bits cut off. You don’t need to cut them that way, but it’ll make things easier later on.

Pins 3, 5, and 7 get bent under the chip.

Pins 4 and 6 get ripped right off, we do not need those pins for this project!

Pins 9 and 10 get the skinny parts bent towards each other.

We’ll solder these together later on.

Pin 14 gets mangled until it’s pointing forward like a weird yoga pose.

Turn that chip upside down! Confirm that all the pins look like they do in this picture, and hurl the 100nF capacitor into the circuit like this.

The capacitor connects to pin 14, closely, then the other leg slips under pins 3, 5, and 7. Pin 14 will be the + power pin, and pin 7 connects to ground. Pins 3 and 5 are also connected to ground to keep them from freaking out (they’re inputs) and we can use them as convenient places to connect other parts that need to be grounded.

Let's do this to a pair of 10K resistors.

Then, let's solder them to pin 2 of the CD4069 like that.

The other ends of the 10K resistors get connected to pin 11 and pin 13.

Now, eagle-eyed Instructabreaders will notice that this chip is suspiciously different from the one I was using earlier. You see, I messed up the other build, and managed to fix it, but it was ugly, so I used this CD4069, which is from a different manufacturer.

Whoah, look! The first picture shows the 22K resistor between pins 8 and 11.

The next picture shows the 22K resistor connected to pins 12 and 13. It will be easier to solder the straight resistor leg first to pin 12, then bend the resistor leg over to touch pin 13, and hit it with the soldering iron.

What in the world? What is this part? It’s a diode. The black side of the diode goes to pin 1, the not-black-striped side connects to pin 8. Make the leads niiiiice and straight, and look very carefully to make sure no metal is touching anything else made of metal. Except for the bits you soldered together. Those are obviously touching.

The body of this kind of diode is made of glass, so it can touch metal bits and nothing bad will happen.

Here’s another diode! And a 680 ohm resistor. Solder them together like that.

And ignore that 680 ohm resistor doing the douchey flagpole muscle showoff pose. What a jerk.

What we’ve done here is take a 2.2nF capacitor (film type, but honestly any type will probably be fine) and soldered it to the non-black-stripe side of the diode-resistor thing.

That little assembly goes like that. The free leg of the capacitor goes to pin 1, the resistor and diode leg goes to pin 2.

Oh, remember how I had to use a different chip? This is the mistake I made, I soldered one of the 10K resistors from step 3 to pin 1. That is wrong. It’s a mistake. I messed up and had to re-do those steps (with that different style 4069 chip!) for those pictures.

Your build will have the twisted ends of those two resistors connected to pin 2. That is correct. Don’t panic.

Look at that wrongly placed 10K resistor and JUDGE ME.

Grab an NPN transistor next. Any normal NPN transistor will do, but they don’t necessarily share pinouts, so maybe just stick with the 2N3904. 2N2222 transistors will work just as well (and they have a way cooler name, all those twos!) but the BC547 has the pins the other way around. If you’re in a hurry and all you’ve got are the BCs, I’ll leave it up to you to figure out how to bend the pins.

Here’s where the 2N3904 goes. The bent pin closest to the camera is the leg with the arrow on it in schematics, the “not pointing in” arrow that the acronym NPN stands for (it doesn’t stand for Not Pointing iN). So the arrow leg goes to ground. Remember the pins we bent under the chip and connected to the ground side of the ceramic disc capacitor? That’s why we connect the leg to pin 3, not because it’s pin 3, but because it’s ground.

I have avoided making puerile jokes about that middle leg so far, and will continue to avoid making puerile jokes.

Transistors come in two flavors, NPN and PNP. NPNs are slightly more common generally because... something about they can pass more current so are more useful to control higher current draw devices like motors or whatever. But the main difference is in the way they turn on. NPN transistors allow current to pass when you provide voltage to their base. PNP transistors allow current to pass when you provide a path to ground (or a more-negative-voltage) to their base. You can tell a transistor is PNP in schematics because the arrow is Pointing iN (Please).

The 2N3906 transistor is a PNP transistor. Say hello.

Anyway, you don’t have to bend the pins of your 2N3906 to get it in this project, not yet, at least. You just slap the flat face of the transistor against the flat face of the other transistor (a tiny drop of superglue here will make things a bit easier) and solder the middle pin of the first transistor to the pin closest to the camera of the second transistor. Having these two parts touching each other is actually important. They help the VCO stay in tune even as the temperature changes.

More on “temperature” and “in tune” later on. But for now…

Here’s some trimmed transistor legs. Both the long middle leg of the first transistor and the side leg of the second transistor gets cut short. We can cut them off right where they’re soldered together. The middle leg of the second transistor is trimmed like that, and the other side leg of that transistor gets bent down out of the way.

Later on, that other side leg will be connected to negative voltage. It’s the only part of the VCO electronics to be connected to the negative power rail (besides the pitch-setting potentiometers).

There's, uh, two views of it. You can see that I didn't glue the transistors together, but if you've got the superglue handy, you may as well!

Look! A blue trimmer! With the number 102 on the top!!! I haven’t talked about capacitor and resistor naming conventions yet, so get ready to download some knowledge into your brain. The first two digits are the value, the third digit is how many zeroes to slap on the end. So 102 means the resistor is 10, the 2 means there’s two zeroes on the end. 1000! One thousand ohms.

Capacitors follow the same convention, except the unit isn’t ohms, it’s picofarads. The 222 capacitor in previous steps is 2200 picofarads, which is 2.2 nanofarads (and 0.022 microfarads).

Right. Grab the leg nearest to the adjustment screw and bend it out. Take the middle leg and bend it in the same direction. Cool, we’re done with that.

Here’s where the trimmer goes. We’re going to connect the two bent-together pins to ground, and pin number 5 is a convenient place to do that.

There's two views of the same thing.

Snatch a 1.5K resistor from where you keep your 1.5K resistors and solder one end of it to the unbent leg of the trimmer, and the other leg to the middle leg of the second transistor. That point right there, where the 1.5K resistor connects to the middle leg of the transistor, is where the control voltage will enter the circuit. A more positive voltage here will make the oscillator oscillate more quickly! Magic!!!

Grab a 1M (one megaohm) resistor and hurl it into your circuit here. One leg goes to pin number 14 of the 4069 chip (this is where the + power will be connected) and the other leg goes to where the middle leg of the first transistor and the side leg of the second transistor are soldered together.

The reason we waited until now to add this part is that since the 1.5K resistor goes from the transistor to the trimmer, the transistor is going to be held in place when we melt the previously made solder joint. An important technique in building circuits like this is to make sure parts stay put if you need to re-solder any joints.

Look out! It's a giant potentiometer! Covered in old solder and paint!

Potentiometers all have the same pinouts, so if yours looks different from this it’s be okay, as long as you wire it the same as this project. You can even use different values, from 10K to 1M, and this circuit will work almost exactly the same.

So anyway, rummage around in your electronics trash bin (or whatever) and find a potentiometer you’re not otherwise using. I like to bend my potentiometer legs like that, since I can cram more knobs in my faceplates that way. In this project where we’re connecting the circuit directly to the potentiometer legs, so having them bent like this helps.

Okay! I think of potentiometers as having a “high” side and a “low” side. When you use a potentiometer to attenuate a signal, you connect one leg to the signal and one leg to ground. Then the middle leg will be the dividing point between the full-strength signal and the full-strength ground. The middle leg is connected to the wiper, which wipes along a resistive track when you twist the knob.

Picture the wiper moving with the knob, with it twisted all the way clockwise (volume up!) the wiper will bump into the end of the resistive track which is connected to the leg on the left side of this picture.

Twist it the other way, and the wiper will bump against the other leg! So in my way of thinking, the left leg in this picture is the “high” side and the other one is “low”.

AAAAAaaaaanyway, pin 14 of the 4069 gets soldered to the “high” side of the potentiometer. The unconnected-and-bent-down pin of the second transistor reaches and reaches as far as it can and we’ll connect it to the “low” side of the potentiometer. The middle leg of the potentiometer connects to the CV entry point of the circuit (the transistor middle leg and 1.5K resistor we discussed earlier) through a resistor…….

Here’s where that resistor should go. It’s also a good picture to show how that side leg of the second transistor gets bent all around to reach the “low” side of the potentiometer. Okay, what resistor value should you use there? Let’s talk about that!

This VCO can go from subsonic to ultrasonic, so you’ll need a coarse pitch knob and a fine pitch knob to take advantage of all that range AND be able to get an exact pitch.

A 100K resistor from the wiper to the CV entry point will get you all that range, but the knob will be super sensitive.

A 1.8M resistor will let you have finer control of the pitch (in my experience, about two octaves) but the VCO won’t be able to get to the very-low or very-high limits of its potential range without another potentiometer as the coarse pitch.

So we should settle on two potentiometers, one with a 100K resistor to the CV entry point. That one will be the coarse pitch control. Then we’ll have a second potentiometer with a higher value resistor, something between 1M and 2.2M is best. That’ll be our fine pitch control!

But we'll deal with that second potentiometer in a bit. First we'll deal with the output side of this circuit.

Electrolytic capacitors are polarized, which means one leg has to be connected to a higher voltage than the other. One of the legs will always be marked with a stripe, usually with little minus signs in it. The other leg from the marked leg needs to get connected to where the signal will come out of this VCO, which is pin 12.

The reason we need a capacitor here is that this oscillator puts out a signal between its rails, which are connected to +V and ground. That kind of signal is “biased”, meaning the average voltage of the signal is not neutral (ground) level, it’s all positive voltage. We shouldn’t have positive biased voltage going out of this module — we’re not trying to power anything.

This capacitor will “fill up” (saturate) with the bias voltage, block it, and only let the oscillations in voltage get through. There does need to be one more part of this bit of the circuit: a resistor connected to whatever new voltage you want the oscillating signal to center around. Wow look!!! There’s a ground physically very close to that minus leg of the capacitor how awesome! We'll use that ground in our next step.

Here’s where the resistor to ground goes. Pin 8 of the chip is one of the pins which is connected to ground. Pin 8 is the most important one… but all those pins are held to the same ground level because of how we built the circuit way back in Step 2.

Other resistor values will change how the waveform of this VCO looks and sounds. A smaller value like 4.7K will let the capacitor saturate more quickly since more current would be passing through it, making the saw wave have peaks and curved slopes toward ground. Higher resistor values will be okay, but if this circuit is powered up with anything connected to it, the positive-biased voltage will get through for a longer amount of time. This will make a “THUMP”, which you will have heard if you’ve turned on many amplifiers that have parts of their circuitry set up like this.

Hey hey look what time it is! Time to connect the power wires!

Our positive voltage (+12, +15 or +9V will all work just fine) goes to the “high” leg of the potentiometer. Our negative voltage (the same voltages but negative will all work super great, they don’t even HAVE to be symmetrical but they basically always are) goes to the “low” leg of the potentiometer.

Make super-ultra sure you don’t accidentally let any of these joints touch anything they’re not supposed to. Stuff can burn up with the currents these wires will be carrying.

Now at this point, we’ve got a functioning VCO! Gaze upon this picture and behold the slightly overdriven saw wave!!!! It isn’t perfect, but that little hump in the top won’t be audible to mere mortals.

We're almost there. Just these two resistors need to be added, another potentiometer, and putting the project in an enclosure is all we've got left.

You can do it!!!

Remember the 100K resistor connected to the middle leg of the potentiometer? The pot wiper? Step 19? You remember? Great! That resistor and the potentiometer will set the initial frequency for the oscillator. But we need to influence the circuit with outside voltage, that's like the whole deal with CV stuff. So this new 100K resistor will connect to a jack to the outside world.

"What?" you ask, "is the 1.8M resistor for?" I'll tell you: it's a fine pitch adjust. The coarse pitch knob will take the oscillator from LFO frequencies to ultrasonic, so if you want to tune your VCO to any particular frequency, something less twitchy will be necessary.

The twisted-together-bits of those two resistors gets connected to the CV input point. It’s been a while since we messed with the pair of transistors hanging off the side of our project, but the CV point is the side leg of the transistor that also had a 1.5K resistor* going to the trimmer and that 100K resistor going to the middle leg of the potentiometer. That spot.

Connect the pair of resistors there. We’re all done with that spot unless you decide to add more CV inputs, which you totally could. Add a couple more 100K resistors here and connect them to jacks to inject exponential FM, vibrato, more complex sequences… go crazy!

*Ahem..... uhh.... in this picture, you can see a tan resistor....... ignore that, nothing to see here... I accidentally used a 510 ohm resistor where the 1.5K resistor was supposed to go so I added that tan 1K resistor in series. Yes, I make mistakes frequently, and mistakes are surprisingly easy to troubleshoot and repair when you can see exactly where every component goes.

...or if you get very lucky, you'll have a brand new one you can use! Like this one! It's so clean and shiny!

Pristine...

This is going to be the fine pitch control. The power leads going in to your project get hooked to the two ends of the potentiometer just like this. Positive voltage goes to the “high” side, negative to the “low” side.

The middle leg of the potentiometer gets a little wire soldered to it.

And the other end of that wire goes to the 1.8M resistor we added in step 25. The unconnected 100K resistor can be curled over to help us keep track of it for later.

If you're still with me, we have built the VCO! It's a bit useless just hanging out like this, waiting for somebody to put a copy of Titus Groan or a dirty cast iron pan on it (if I had a nickel...), so we'll need to load it into an enclosure.

I use tin cans for enclosures. If you use a "leaves no sharp edges!!!" type of can opener, cans make very useful enclosures with a lids sturdy enough to take some abuse, but soft enough to make holes in without power tools. I have a whole video on the subject right here.

I also use RCA jacks which are so easy to work with. The closest part in the first picture is the back side of an RCA jack. This is where the CV will come in from the outside.

This VCO is small enough to not need any other support besides the connections it has to the potentiometer. Once we get the potentiometer nice and tight, we should look very carefully at all the leads and bare wire in the circuit, using a small screwdriver to pry any parts away from places they shouldn't touch.

The wire on the left is the CV connection, going from the jack to the 100K resistor, the one with the curled over end.

The wire on the right goes from the spot where the 1uF capacitor and the 100K resistor meet. It’s pretty hard to see from this angle, but I don’t have a better picture.

And there we have it! A pitch-tracking saw-wave VCO made for less than $2.00 in parts!

But the real value is in the friends we made along the way.

Pitch-tracking VCOs are amazing, because you can set a pair of them (or more) to play in a harmony, and then feed both of them the same voltage, and as they go up or down the frequency spectrum, they will remain in harmony with each other.

But analog electronics like this need to be calibrated. There's many resources out there to help you learn how to do this, but I'll try to explain it here as well.

First, devise a way to safely power this module while its guts are easily accessible. Hopefully you’ve already powered it up and confirmed that it works. Make sure your trimmer screwdriver can reach the trimmer well — for my build I had to carefully bend the trimmer up a bit. Turn on the power to this module (and your synth), and connect the output to speakers somehow. If you don’t trust your ears to set octaves properly, connect an oscilloscope to the output as well, or have a guitar tuner listening to the pitch the VCO is making.

Once stuff is connected and making noise, let it sit for a few minutes to allow the circuitry to reach a stable temperature.

Connect a 1v/octave voltage source to the CV input of the circuit. Play octaves and notice that middle C is not exactly one octave below high C!!! With the VCO playing a higher octave, turn the trimmer. If the pitch of that note goes down, that means the range between the higher note and the lower note will have gotten smaller. Adjust the trimmer back and forth until you dial it in so that “Note” is the same note but one octave down from “one octave up from Note.”

If you don’t have a 1V/octave voltage source, you can just leave it tuned however, but if you want two or three (or MOAR!!!) of these to stay in tune with each other using the same CV levels from your synth (think a chord sequence moving up and down the scale), here’s what you do. Tune a pair of these to the exact same note with a CV connected to the pair. Change that CV and adjust one of the VCO trimmers to stay in tune. Then turn it back down (it will no longer be in tune at the first CV level) and adjust again. Rinse repeat rinse repeat rinse and repeat until finally you get a pair of VCOs that have the same response to CV!!!

Fancy expensive VCOs will have high-frequency compensation, temperature compensating resistors, linear FM, triangle, pulse, and sine waveforms…… some of the resources out there will probably mention these, and obsessive types will most certainly be concerned with pitch-accuracy up to 20KHz and down to 20Hz, but for my purposes, this is a fantastic little workaday VCO, and the price is very, very right.