Universal Synthesizer Panel – Part 2

This is the second part in a series of posts describing how I’ve approached building a “universal synthesizer panel” to house a range of microcontroller synthesis projects.

• In the first part, I detailed the general idea and overall design of the panel.
• In this part I will build the IO panel.
• In part 3 I will build the panel IO board.
• In part 4 I will build a microcontroller board for an Arduino Nano.
• In part 5 I will walk through the final assembly and test code for the whole thing.
• In part 6 I will port the Arduino Multi-pot Mozzi FM Synthesis to the panel and show it in action.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

Panel Design

A number of cardboard mock-ups were used to explore a range of options.

As you can see, the first was trying out a full size 162x250mm panel, and the first and second were using 5-pin MIDI DIN sockets.  Eventually I settled on a half-sized panel and a single MIDI TRS socket, as shown in the third photo, thinking I can jumper it between MIDI IN or OUT as required.  The MIDI is the bottom right socket with the MIDI LED just above it and the audio out just above that.

The last photo on the right is the third trial panel shown from the back.

Panel Circuitry

There are two elements of the circuit (as described previously) that seemed best built on the panel itself: the power switch and associated LED; and the stereo panning audio output circuit.

The power circuit will receive a feed from the IO PCB housing the USB power socket and pass back the output of the switch as a 5V signal.

Note that as the final board may allow for both 5V and 3.3V microcontrollers, I don’t wire up the 5V signal from the power switch to anything on the panel directly, but feed it back to the IO board.  The one exception is the I2C display which always requires 5V regardless of the logical signal levels used elsewhere.  This will be explained more clearly in a future post.

The complete audio output circuit is as follows.

The first part of this (up to the second capacitor) is a simple low-pass filter and will be built into the IO board.  The four resistors will be soldered to the panning-control potentiometer.

Here is an annotated photo of the pack of the mock-up panel showing where most of the connections will have to be made.

Making the Panels

I’m not going to go through a “how to” for the mechanics of building panels.  There are a number of tutorials out there and it depends on the materials you are using, the tools you have available, and the style of finish you are aiming for.

Having said that, here are a few recommendations, if you don’t know where to start.

• Be very careful, slow and accurate, marking your panel.  “Measure twice, cut once” really are the words to live by here.  Also be wary of the size and width of your markings themselves.
• Always clamp the panel down prior to drilling or cutting.  You really don’t want a piece of aluminium with possibly sharp edges suddenly working free and start to spin round at high speed if it gets caught on your drill bit.
• I started with a small (3mm) drill bit to make the initial holes, then used a larger drill bit to get to the right size.
• A drill size gauge is really useful.  You can put your pots, switches, and so on in it and find the exact size of drill bit required.
• I kept the protective film on the aluminium, marked it on the film side, and drilled starting from that side too.  It meant when it came to filing off the holes, I was “scraping” the rear side, and kept the front nice and tidy and free from scratches.
• Once the protective film is off, finish the panel as required.  I used a wire brush to buff it up for a “brushed aluminium” style finish.
• Be careful of slipping spanners or pliers when tightening or holding nuts for components as they will scratch your finished panel on its “good” side.  A socket from a socket set works well here.
• Always take the appropriate precautions and use suitable protection when using power tools.
• Don’t forget the mounting holes for the PCBs and the panel to the rack.

Assembly

Here are some photos of the assembly of my panel.

As a general rule it is always worth mounting the components before soldering them.  This means you can cut wires to accurate lengths; you are not worried about components turning when tightening them up; and different components might need pushing through from the front or the back depending on their design anyway – any pushed through from the front have to be mounted prior to soldering.

The dodgy drilling of the power switch is down to the fact I didn’t have a 10mm drill bit and was too impatient to wait for one to arrive in the post (I have one now though).  I used a large washer to hide it, and actually I quite like the finish and may use that for all modules in the future.

The 4-way header socket for the I2C display is hot-glue-gunned into place from the back, as are the LEDs.

Once the components were added I started on the interconnects, starting on the GND connections first, including the two cathodes (short legs) of the LEDs.

The power LED anode (long leg) is connected to the “switched side” of the power switch via a 2kΩ resistor.

Here is a close-up of the panning resistor and power switch part of the circuit at this point.  Note that the two “sides” of the resistor network connect up to the two outer connectors of the potentiometer.  The centre “wiper” pin of the pot will be connected to GND.  The “top” of the resistor network will connect to the audio output of the IO board.  The two remaining single leads will hook up to the left and right channels of the audio out socket.  I oriented my 3.5mm socket to ensure that the left channel and the “hard left” of the potentiometer lined up.  Once it was all connected up, I used a blob of hot glue to stick the cables to the pot.

Panel Connectors

I built some custom “Dupont header” style connectors.  A little while back I invested in a crimp tool and a range of sizes of “header shells” and it has been so worth it for this.  There are many videos and tutorials showing you how to use the tool for neat and robust connections each time.

I added the wires for the connections last of all and built them in the following order:

• Make up the Dupont leads as female headers each with wire to spare.
• Insert the leads into the appropriate sized headers.
• Temporarily “mount” a PCB to the panel (you could use the real one if you’ve built that already, or a dummy one if you haven’t) and use that to gauge where to cut the wires.
• Cut and solder the wires to the panel.

Here are some photos of the final link-up connectors.  Notice that I used a short length of shielded audio cable for the audio connection.  Everything else was standard “hook-up” wire.

 

The MIDI Connector

The MIDI connector requires three wires:

• TRS “ring” which connects to the equivalent of “pin 4” – the current source – for a MIDI Din socket.
• TRS “tip” which connects to the equivalent of “pin 5” – the current sink – for a MIDI Din socket.
• TRS “sleeve” which connects to the equivalent of “pin 2” – the ground – but only for MIDI out.

But which microcontroller pins this is connected to depends on whether the socket is being used as a MIDI IN or OUT port.  The design for the IO board includes the MIDI circuitry for both and terminates in a header with the following pinout (this will be discussed in detail in the next part of these posts).

The left-most pin is the connection for the MIDI indicator LED.

I opted to use a 6×2 way header block with the pins connected up so that when the top row is used, it is connected as a MIDI IN circuit, and when the bottom row is used, it is connected as a MIDI OUT circuit.  The LED is connected in both cases.

Hopefully you can see this in the following photo, with the connector plugged into the IO board.  Notice the following:

• The pink (ring) and green (tip) wires go to different places on the two rows.
• The black wire (GND) only goes to the bottom row.
• The purple wire goes to the same pin on both top and bottom rows.

Different sockets may have tip/ring wired up differently, but for me tip was on the right of this photo.  Always check orientations with a multi-meter before wiring up.

Closing Thoughts

I am not particularly mechanically minded – I just don’t have the patience and skill for a good finish.  But this is not too bad for me – its about as accurate as I can get it with a hand-drill and seems relatively robust in use so far.

In the next part I’ll look at the design and build of the IO board itself.

Kevin