Universal Synthesizer Panel – Part 3

This is the third 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 part 2 I built the IO panel.
• In this part 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!

The Circuit

Above is the design for the IO board.  This contains the circuitry that is largely common between all microcontrollers and a specified layout of IO pins.

All connectors around the edge of the board will connect “down” to the panel IO directly.  The two rows of “inner” headers will connect “up” to a microcontroller board.

A “blank” microcontroller interface board is shown below.  The idea is that your favourite microcontroller can be added to this blank with the right pins routed to the connections shown.  I’ll talk more about that in the next post though.

Points of note:

• The IO board includes circuitry for a low-pass filter on the audio output side.
• The IO board includes both MIDI IN and MIDI OUT circuitry.
• All MIDI IO, including the MIDI indicator LED, is brought out to a 6-pin connector on the right hand side of the board.  This will connect to the TRS MIDI socket and LED on the panel.  A resistor for the LED has already been included in the IO board, so it is not required on the panel.
• The power links are on the opposite sides of the board.  The microcontroller link is at the bottom, but the connection to the IO panel is at the top.  The actual power feed in is via a USB breakout board in the top right of the diagram.
• The power connectors allow for the following:
  • For the IO panel to include a power switch (and independent power LED) and route the switched output back to the IO board.
  • For the IO board to supply 5V to the microcontroller board, but for the microcontroller board to route its own “VCC” back to the potentiometers.  This allows for a 3.3V microcontroller board to be used if required.
• There are jumpers in the MIDI circuit and low-pass filter on the audio output circuit to allow both to be optimised for either 5V or 3.3V operation.  When the three jumpers are “horizontal” they are in the 5V position.  When off or vertical (in the case of the MIDI output circuit) they are in the 3.3V position.  This is the same approach (and design) I used previously – there are more details here: Universal Microcontroller MIDI Module.

The rest of this post will focus on building the IO board.  The details of the microcontroller board will follow in a later post.

Assembly

Here are some photos of the assembly of the IO board.  The dimensions are such that the microcontroller board fits perfectly on a 4x6cm (14×20 holes) piece of protoboard and the IO board works well on a 5x7cm (18×24 holes) piece.

I’m using female headers on the larger IO board and male headers on the smaller microcontroller board as shown below.

At this point I’m leaving the microcontroller board – I’ll come back to that in the next post.

The starting point for the IO board is the rest of the headers so they can be used as a reference for the rest of the build followed by the USB B breakout.

In the following photos you can see:

• Right-angled headers for the analog IO (on the left).
• Digital IO and I2C (on the top) bridged with solder to the microcontrollers digital and IO headers.  Notice that this does not include the MIDI LED link or the RX/TX links to the microcontroller board – these will come later – so there are 8 right-angled header connections, but they start 1 pin in from the left of the microcontroller header.
• MIDI IO (on the right).The USB B breakout board.
• The low-pass filter circuit: 2x 150Ω resistors, 1x 270Ω resistor, 1x 68nF capacitor, 1x 10uF non-polar capacitor, including the two-pin output header, and a jumper to allow the switching in of the second resistor to support 5V levels for the resistor divider.
• The MIDI IN circuit: H11L1 socket, 1N914 diode, 1kΩ and 220Ω resistors, 100nF capacitor, linked to the top two pins of the MIDI header.
• The MIDI OUT circuit: 2×220Ω, 1×10Ω, 1×33Ω resistors and jumpers to choose between the 5V and 3.3V pairs, connected to three pins of the MIDI header.

In the next pair of photos, the following have been added:

• The 2kΩ resistor and connecting lead for the MIDI LED from the top header left-most pin to the right header bottom pin (purple).
• The six links for the analog IO connections (blue).

The following photos now add:

• The microcontroller’s RX link to the H11L1 for the MIDI IN (green) – note how the connecting wire is extended beneath the board to reach the H11L1.
• The microcontroller’s TX link to the collection of resistors for the MIDI OUT (green).

In the following you can now see:

• The addition of the “power to the IO panel” connector in the top right hand corner of the board.
• The USB breakout 5V linked to the middle pin of the power connector (orange).
• A solder bridge linking the GND of the USB breakout to the rightmost pin of the power connector.
• On the underside, several GND links (black) hooking up the power connector GND to the H11L1, to the analog header and the microcontroller power header, on to the MIDI header.

Finally, in the following photo we can see:

• The 5V connection from the IO panel power header (left-most pin from the top, right-most from the bottom) to the microcontroller power header (red).
• The VCC connection (which could be either 5V or 3.3V depending on the microcontroller used) from the microcontrollers power header to the H11L1, the MIDI OUT resistors, and the analog header (orange).

Closing Thoughts

So far, so good.  There is little more to do at this point, but it is always worth visually inspecting things with a magnifying glass and checking for short circuits with a multimeter, especially across the different power and GND connections.

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

Kevin