Adding Gamepad Joysticks to Shruthi-1

March 15, 2013

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I built my first shruthi last year and the poor little bugger has been poked and prodded since. Not only is it a great synthesizer, it is also a fun playground for a beginner audio noodler like myself.

One of the standard Shruthi mods is to attach controllers to the 4 CV inputs. These can be mapped to multiple parameters in the mod matrix, providing oodles of modulation possibilities.This could be controlled by knobs or touchpads or the like.

The most accessible method is to attach a 10k linear pot to the points on the control board (left of the LCD).
pot

Looking at the pot, the legs should be connected thus:
1: +5V
2: CV (1-4)
3: GND

On the Mutable Instruments forums I read that PS3 gamepad joysticks are basically 2 10K linear pots assigned to X and Y coordinates. The only problem is that the spring can’t be removed without destroying the mechanism. I want the joystick to stay where I put it.

After a lot of searching, it was apparent that few joystick met this criterion, and they tended to be expensive; 45 euro for the Doepfer version, and an extra tenner for the shaft. Bugger that.

Then I came across this excellent blog post detailing how to remove the springs from an Xbox controller.

Not having an Xbox controller to hand, I instead ripped-open an old, broken Logitech PC gamepad I had thankfully kept – along with many other boxes of junk. The hoarder vindicated 😀

EDIT: You can also buy the joystick components separately if you wish. Here’s a good source: Uk-Electronics.

Click pictures for full size:

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Removing the PCBs is easy…

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..and there are our little beauties..

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Noticing that the joysticks were similar to the Xbox ones, I proceeded to follow instructions in removing them from the PCB and then their springs.

The joystick housing need to be carefully de-soldered from the gamepad PCB…in this case there were 15 points attached to the board. Make sure they are all free before prising the whole thing free. Solder wick is essential.

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The spring and button are house in the plastic base. Bend the metal legs to release it. It just pops out.

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With base removed (and thus spring and button)…

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Once the spring is out the mechanism is unsupported and will probably fall apart. But it’s easy to re-assemble – it’s just 2 pieces. The potentiometers can be snapped-off and reattached also. Very elegant design. To keep the shaft in place permanently, I soldered small wire supports like so:

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For testing, I plonked it onto a piece of polystyrene…

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Then I wired-up both sets of pins…

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..and connected them to the Shruthi CV points…

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At first I thought it wasn’t working, however I had wired the wrong CV pads! Switching to the correct ones in the Mod matrix brought a huge smile to my face. The joystick works perfectly, goes where you want it, and STAYS there.

WooHoo!

Initially I kept the wires long for a reason; I wanted to test how sensitive the CV inputs would be to interference. At this length I noticed no adverse effects.

Now, how to house it?

Conveniently, it turns out that all the joystick legs will fit snugly into the honeycomb side panels, but they need affixing. However the pins are close together, so some insulation is in order. I used some heat-shrink tubing. To keep things tidy should I wish to re-wire, I attached all GRN and 5V wires to pins (discarded resistor legs), leaving just single pins to be soldered to the control board.

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Notice that the wiring must be consistent if you want both joysticks to behave in the same way.

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There’s plenty of room for the wires to fit snugly in the case. Note that the additional switch is not needed – it for 2/4 pole filter modes (another easy mod).

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For today, to keep everything steady. I’m using some ‘blu-tack’. I’m waiting for extra components for more mods, so I’ll be taking it apart again soon. When satisfied I will probably glue them on. I also found two tiny covers to put on the shafts. Cute.

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And there you have it: 2 cheap (free) modulation joysticks, without springs, for your Shruthi, or any other purpose you might be dreaming. I know I am….

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