Saturday, December 29, 2012

4017-Based Sequencer

Hi all 14,000+ of you that have read this blog in 10 different countries! (That's just way cool to me)

So I came up with yet another useful yet super-simple synthesizer module: a full-width (that's 84 hp) 10-step sequencer!
Ok, so it's a little... odd, so let me explain the parts:

-First you see the Xanu logo with the description below it. :)
-Below that is the clock section. The knob controls the internal oscillator's speed, and that can be voltage-controlled via the 'Speed In' input. Or, you can insert your own clock signal via the 'Clock In'. This is slightly unusual, but I think having the option is useful.
-Then you get to the 10 steps. Each has a CV control (+9V to 0V), LED step indicator, and individual CV and Gate outputs. You'll also note the three-way switch. This can turn on or off each step, or can set that step to reset the sequence (so you can literally have a 1-step sequence if you want, or you can switch the 9th step to reset and have a musically normal 8-step sequencer)
-After that you have Hold and Reset inputs. Hold will hold a step indefinitely given a high gate signal. Reset will reset the sequence to step 1. Either can be controlled with anything that can create a high gate signal, even a +9V CV signal. Each also has an attenuated input, should you need it.
- Then you have 4 5th Outs. Each of these creates a high or low gate signal every 5 steps. Can be used with the Reset and Hold inputs for neat effects.
-Master CV and Gate are just like the individual CV and Gate, but each step outputs to it. This way you can, say, patch the master CV to an oscillator and sequence the pitch.
-I'm also thinking about adding a clock output, in case you want this sequencer's clock to control something else. It wouldn't be hard, at least!

You may ask why I did what I did here, and it's mainly for maximum patchability. It's totally possible you'd never use parts of it, but I'm sure you can see the usefulness of each part (assuming you know how most sequencers work nowadays)

And, since I'm a nice DIY synth person, here's the schematic:
This schematic isn't entirely accurate, mainly because my version of LTSpice doesn't have some of the parts. Given the description you can build it, though. You'll have to pay attention to your exact 4017 and its outputs, otherwise this schematic will be a bit wonky for you. Oh, and of course, you'll want to connect your 3-way switch's 3rd setting to your reset pin if you want that. You could also just patch a step's gate output to the reset input and use a normal on/off switch.
I actually learned something useful with this project (as I usually do): the 555 timer's speed is controlled not only by the resistor, but the capacitor between pins 1 and 2. The shunting cap is only for precision and can be just about any size (at least, between 1uF and 10nF). Using a 1uF cap, like in this circuit, you can get a frequency range of .5Hz to 1500Hz. With my normal oscillator (which will probably be changed now) used 10nF caps, and bottomed out in the hundreds Hz range and went out of the audio range. That's kinda useless given that it's the main source of sound in a synthesizer.
Also, given this new knowledge, I think I know how to create an LFO module: just use the clock circuit from this, use my oscillator's waveshaping section, add voltage control, and bam! Instant LFO module. Though I do wish I knew how Pittsburgh Modular got such an insane frequency range with their Oscillator...

Oh, and for those of you looking for pics of my modular, well, it doesn't exist. I don't have all the parts or skills to actually build these as modules. Given a number of pots, jacks, and the ability to work sheet metal, then I'll start building them.


  1. Could you help me wire it as a DPDT? As in have each step be step on, step off, and reset to step 1?

    1. DPDT wouldn't make much sense in that case. You'd want to use an SP3T switch for on-off-reset.