ARP 2600 Voltage Processors Part Four dot One--Recreating its Electronic Switch

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Hello again....this time I continue creating clones and work-alikes for a few sub-circuits found in Arp's legendary 2600 semi-modular synthesizer.  

ARP 2600. A classic's classic!

Let's design a workalike for the ARP 2600's "Electronic Switch"....

This circuit will be a bit more complex than ARP VP clones found in recent posts.

Breaking this down--in this post I breadboard a buffer and T flip-flop for use in an upcoming 2600 inspired Electronic Switch Euro Module.

The ARP 2600's "Electronic Switch".

The 2600's switch (a "how to use it" video is here) employs a discrete timer feeding a discrete toggle ("T") flip-flop; see the ARP2600's service manual--here--page 29--to peruse its Electronic Switch sub-circuit.

This is a clever design (ARP was clever!)--I simulted the discrete flip-flop using Falstad; get the sim here--a bit tricky; I had to preload the virtual .1n caps with 5 volts and 1mV respectively, then reset Falstad--discussion about creating discrete flip-flops using Falstad is on Modwiggler's DIY forum: here. 

OK! For the flip-flop section I could have cloned  ARP's design, but that seemed to involve more parts than I thought necessary.

Alternatively I could have used an 8-bit MCU, Arduino, etc., but that would have been too easy:

Why make things easy, ever?

Instead let's use a CMOS JK flip-flop IC....I have too damn many of these in my junk box and have never used a single one in a project.

CD4027--dual JK CMOS flip flop--Falstad sim for T flip-flop is here.

Turning this single IC into a T flip-flop was easy--using the legendary Don Lancaster's "CMOS Cookbook" as a guide (sadly, Don Lancaster died recently--obit is here) I whipped up a CD4027 based T flip-flop on a breadboard:

Here's its schematic:

The momentary switch in the bottom left toggles the flip-flop on data-low, easily modified for data-high:

"IN" is connected to Pin 13 on the CD4027 IC.

When I touched this switch it toggled the CD4027's pins 15 high/14 low to 15 low/14 high, then back. Cool!  

But how do I buffer pin 13 so the CD4027 will work reliably with all the different signals from my bench and modular rig?  

I messed around with different buffer designs; I think I will use the one below--which seemed to work with most of my various triggers, gates, LFO's, VCO's, MakeNoise Arrows Everywhere and so on; the output voltages and trigger durations found on my rig are inconsistent and this levels the field somewhat. 

Breadboarded that too:

Next: how will we switch the audio and CV--use FETs? 

Nope, FETs seem a bit dated, instead, a cool family of parts I found for this: the DGx0x series; an affordable CMOS dual rail analog switch. 

The DG401 looks like a good fit:

 "from the datasheet, I am pretty sure it will work"

Instead of breadboarding a DG401 and an output buffer--I hate breadboarding--I created a DG401 "experimenters' board":

I have laid out too many breadboard-busters and felt impatient, for better or worse I whipped up this one in a couple of hours at most, maybe an hour? and sent off to my ever-patient and endlessly polite sponsor, PCBWAY for fabrication.   

Next time: I will build the experimenter board and tie it to the breadboarded CD4027 T flip-flop/input buffer. 

If it all works, it will then be reimagined into a 4-6 HP Eurorack module to be featured in a future post.  

Fun!

Until next time, flip or flop--better yet, do both! UPDATE: posted...go here.