I'm honored that Nick Jaffe (aka Just Nick) has made a demo video for the Arcadiator. Thanks Nick!

Check out his YT channel:
www.youtube.com/user/JustNickMusic

6. Octave down, Flip Flops and Latches
Lets have a look at a few different ways to create octave down using CMOS chips.

In all of the examples below something called a flip flop latch will do the frequency dividing. Sometimes it's just embedded into chips with other functions. The flip flop/latch is one of the fundamental logic building blocks. It's used to store data and is how RAM memory in our computers works. One flip flop can store 1 bit of data, which means that it can hold a high or low state until we tell it otherwise. This can be very useful, for example when we want to use a momentary switch to turn something on or off until we press it again. Flip flops can also be used as oscillators, counters and much more.

But now let's focus on frequency dividing. What it does, to put it as simple as possible: Each time the clock (our input signal) goes high, the output changes state. Since the output only changes on the positive transitions and holds that state on the negative transitions we get exactly half the frequency at the output, like this:

Pre-filtering
Making a octave down circuit with CMOS is easy since we are now working with 1's and 0's. Getting good tracking is harder, but there are a few simple measures we can take to get acceptable tracking. The first thing to keep in mind is that we can improve tracking significantly by using the neck pickup of the guitar and the treble rolled off. But we also need to add a lowpass filter after our schmitt trigger since it adds alot of high frequency content. There are many different lowpass filters we can use, varying in complexity, but for this article we'll use a simple passive RC filter.

1. Dual D-Flip Flop (CD4013)
This chips is used in the MXR Blue Box, MadBean Lowrider, The Rocktave and many others.

This chip has two D-flip flops. The D prefix means that it changes output state on positive transitions only, exactly what we need. For two octaves down, we'll just put both flip flops of the chip in series so that the second flip flop will divide the output of the first flip flop.

• Input goes to Clock
• The NOT Q output (Q with a line) goes back to the data input (D) via a 10K resistor
• Set and Reset goes to ground
• Q or NOT Q goes to output. NOT Q is the inverted output
• VCC to V+ and VSS to ground

2. Binary Ripple Counter (CD4024)
This chip is what I used in the Arcadiator. it's very common in DIY synth stuff and it's also used in the Slacktave (by Slacker, the designer of the Echo Base) and the Bit Commander by Earthquaker Devices. It's very simple to use and require no extra external components.

The CD4024 chip is a counter that advances one count every time the clock changes from high to low. The first output is divided by two, and the next output divides the preceding output by two and so on, up to output 7 that has the input frequency divided by 128. This means that each output is one octave lower.

• Input goes to Ø
• Reset goes to ground
• Q1 is the one octave down output
• Q2 is the two octaves down output
• VCC to V+ and VSS to ground

3. NAND Gates (CD40106, CD4093)
Let's look at how it's possible to make a D-flip flop out of NAND gates.

As you can see, it requires many gates and even one 3-input gate, which means we have to use at least two chips. But remember that I mentioned that schmitt triggers can be configured to make NAND and NOR gates?

A schmitt trigger gate can be made into a multiple input NAND or NOR gate using 1N4148 diodes and a resistor, with as many inputs as we want. This is something I picked up from Ray Wilson, he calles it "mickey mouse logic". This makes the CD40106 a very versatile chip, and using this neat trick we can make a flip flop out of a single CD40106 chip.

The same mickey mouse logic can be applied to the 2-input CD4093 schmitt trigger by first connecting both inputs together (which makes a NOT gate).

A good way to test these octave down circuits on the breadboard is to connect a LFO with a LED to the input and another LED at the output. That way we can easily see if it works. I tested the NAND schematic first with a freeware program called Logic Circuit just to make sure it would work, then I breadboarded it. This is the result.

Now onto the fun stuff. :)

But first, there's a couple of important things to watch for when using CMOS.

3. CMOS usage rules

• All inputs must go somewhere. A unused gate must be disabled by connecting it to either +9V or ground, directly or with resistor. The input of a CMOS gate is very high impedance, so a floating input can randomly determine what the circuit will do, making it change state erratically, causing noise and high current draw.
• Inputs that goes off board should have a load resistor connected (1M resistor to ground).
• CMOS chips are sensitive to static. They have built in protection diodes, but sometimes it's not enough. It's a good idea to wear a anti-static wristband when working with CMOS.
• CMOS chips accept a wide range of supply voltages (generally from around +3v to +15v), but it's always a good idea to double check the datasheet just to sure. There are exceptions. We will only be using chips that works well with +9 volts.

We will configure one stage of the inverter by adding negative feedback, using a resistor from the input to the output of the inverter and decoupling capacitors.

• R1: Load resistor
• R2: Sets the gain of the amplifier. Try a 1M trimmer/pot here insted for controlling the gain.
• C1 and C2: Decoupling capacitors. This is always needed at the input and output of a circuit, but it's also nessecery for the input of a linear amplifier to work properly.
• R3: voltage divider trimmer to control the output volume.
• C3: optional lowpass filter cap. Can be anything from 100pf to 47nF. How much treble it cuts depends on the value of R2 aswell. Experiment with different values!

Stage 2. Schmitt Trigger
This stage completes the "CMOS'ifier". It will turn the signal into a square wave, making it compatible with other CMOS chips. This part is inserted before the output capacitor. R3 and R4 sets the upper and lower thresholds for the schmitt trigger. It's best to leave R4 at 1M and experiment with the lower threshold resistor R3. Higher values will make it more gated, but low values can result in oscillation. Here we need to find the sweetspot that depends on how strong input signal we are getting. Increasing the value of C3 (1nF - 22nF) can also help against oscillation, noise and misstriggering. Very low output pickups may sound too gated, in that case use two preamp stages in series before the schmitt trigger to increase the sustain.

If we have a schmitt trigger chip (CD40106 or CD4093) in our design we can just use one gate of that chip insted directly after our preamp stage. In that case a pulldown resistor (100K to ground should always be used at the first schmitt trigger input.


As you may notice, a square wave is pretty much immune to any filtering that comes before it and will sound the same no matter what kind of circuit we use to create it.

In the next part, we'll have a look at doing something a bit more advanced - several few ways to achieve octave down!

I hope this series is interesting. I'm trying to keep it as basic as possible, but it will become more advanced in later parts. Please let me know what you think and if there is anything I can improve. Thanks!

Many of my designs are based around CMOS chips and a few people has asked me if I know any good resource for stompbox designs based on CMOS. I don't really...

So I thought it might be fun to share alittle of my knowledge of the CMOS basics and hopefully you will learn something and will be able to experiment with your own CMOS based circuits.

We will use the breadboard later, but first, lets start with the basics (the boring stuff...)
I won't go into detail on how they work on the insides. For that I really recommend reading the book: The CMOS Cookbook. It's been a huge help and inspiration for me.

1. CMOS LOGIC
The first thing one must understand is that CMOS works with digital logic:
They put out 1 or 0 at their output, depending on the 1's and 0's on their input(s).
These two states are also often called "high or low", "true or false" or "on or off" ect.

I think it's easiest to think of these two states as either V+ or Ground

1 = V+ (high)
0 = Ground (low)

There are exceptions that doesn't follow this logic.

This means that no matter which kind of waveshape we put into a CMOS logic chip, the output will always be a squarewave. There is no in-between, no dynamic range, just 1's and 0's. The output logic changes at about halfway up the power-supply voltage, so to be able to use a CMOS chip with a guitarsignal, the signal needs to be boosted into logic levels, then preferably made into a square wave: digitalizing the signal (in a crude way). This is the reason you don't see a gain control on many of my designs and why it often sounds totally fuzzy. A square wave is as distorted as it can get. This is also the reason why the output of a CMOS based guitar effect have a gated sound with a decay that dies abruptly (as the input signal falls below the schmitt trigger threshold).


More about schmitt triggers later.

Here's a short video of my oscilloscope that displays a guitar signal that is boosted to reach logic levels, then later fed through a CMOS schmitt trigger to make it a square wave so it will play along with other CMOS chips. No audio output is connected in this video, it's for the visuals only, so you can get a idea of how the waveshapes look when it's getting "CMOS'ified".

Later we will breadboard the circuit I used, a very basic square wave fuzz, so you can listen yourself to how it sounds.

But first, lets iron out alittle more of the basics...

Buffer (CD4050)
This chip is rarely used in stompbox applications. But I wanted to use it as an example. As you can see by the Truth Table, if the input is high the output is high, and if the input is low the output is low. So this chip is simply a buffer (mostly used for interfacing to TLL logic).

Inverter (CD4069/CD4049)
This chips, also sometimes called a NOT gate (or hex inverter because it has 6 gates), inverts the input. The output is 180 degress out of phase with the input. This is probably one of my favourite CMOS chips since they also can be configured as linear amplifiers. More about that later.

Notice the small circle at the output on the schematic symbol. This means that the gate is inverting. 

AND GATE (CD4081)
This chip has four AND gates. Each gate has two inputs. With either or both inputs low, the output will be low; with both inputs high, the output will be high.

Both input A and input B needs to be high for the output to be high, hence the name "AND gate".

NAND gate (CD4011)
This is basically the same as a AND gate, except that it inverts the output. It can be useful when doing gated oscillators. I used a NAND schmitt trigger chip (the CD4093) for that application in my Flawed Logic Fuzz. We will breadboard a gated oscillator later on.

NAND stands for "NOT AND". Remember that a NOT gate is an inverter.

OR GATE (CD4071)
This chip has four OR gates. Each gate has two inputs. With either or both inputs high, the output will be high; With both inputs low, the output will be low.

Input A or Input B needs to be high for the output to be high, hence the name "OR gate".

NOR GATE (CD4001)
You can probably guess by now that the NOR gate (NOT OR) is an inverting OR gate, so I will not add the NOR gate schematic symbol and truth table.

XOR GATE (CD4070)
This chip has four Exclusive OR gates. It also has two inputs per gate. It's a very versatile chip, because one of the inputs decides if the gate is inverting or non-inverting by tieing the input to either +V (non-inverting) or ground (inverting). In the Arcadiator I use this feature to make the alternating octaves - more on this later. This chip can also be used as a comparator as identical inputs gives a low output and different inputs gives a high output. So basically, if two different signals is put into the two inputs the output is the sum of the differences of those two (hence the name "exclusive or gate"). This is patrially how my ring mod The Corruptor works.

SCHMITT TRIGGER (CD40106)
Here's another very versatile chip. It's an inverter, but it has something called hysteresis built into the chip which makes it a schmitt trigger. Hysteresis is basically a "noise immunity zone". For example, if we use a 5 volt supply, the input signal will have to reach above 2.9 volts for the output to go low and it will have to drop down to belop 2.3 volts for the output to go high. That's a 0,6 volt hysteresis that will keep out any noise at the input that is below the threshold. The built in hysteresis also lets us configure this chip with diodes and resistors to do other logic funtions: multiple inputs NAND or NOR gates, pulse width modulation and much more.