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BUILDING THE MINI-MIDI USBMIDI CONVERTER PROJECT

By John Burdick

Mini-midi assemble board
The assembled Mini-MIDI USBMIDI to CV Converter.

The MINI-MIDI USBMIDI to CV converter will convert USBMIDI messages received via USB to two control voltage outputs and produce accompanying gate voltage pulses. When powered by the USB host, the control voltage range is 0 Volts to slightly less than 5 Volts and is scaled at 1V/Octave. If the output circuit is powered by an external power supply, the board can supply a voltage range of 0 Volts to 10 Volts with 1V/Octave scaling. 


The board can be used as a first surface mount project, for surface mount soldering practice or as a key fob for that extra set of garage keys. The information below includes assembly instructions for the board and links to obtain programmed microprocessor chips or a complete parts kit.

STEP 1

Assemble the passive components (resistors and capacitors) first. Use a fairly fine soldering iron tip for this work and place the part using a pair of tweezers. Click on the image below to see an enlarged view. 

Locations of resistors and capacitors for the MINI-MIDI project.

STEP 2

Now assemble the semiconductor components. Start with the four transistors. Be sure to put them in the correct locations. The two PNP (3906) transistors go in the top two positions and the NPN (3904) go in the bottom two positions. Next, assemble the IC packages. Be sure to align pin 1 on each package with the corresponding pins on the board. The board pin 1 is below the notch in the the silk screen package symbol on the board. The voltage regulator (U4) pin 1 is marked by a small dot.

Assemble the semiconductor components being careful to align them as shown.

STEP 3

Now assemble the through hole components. Bend the leads on the LED at a 90 degree angle leaving a tenth of an inch spacing between them. Be sure to insert the cathode of the LED in the hole marked by a square pad. The cathode lead of the LED is marked with a flat spot on the side of the plastic package and by the cathode lead being somewhat shorter than the anode lead. 

Assemble the through hole parts. Be sure to orient the cathode lead, shorter of the two, so that it goes into the square pad.

STEP FOUR

This completes the assembly of the module. Take some time to inspect the board to be sure that there are no solder bridges between pins and that all the pins have been soldered to their appropriate pads. Look for cold solder joints. A cold joint is usually indicated by the solder on the connection not having a shiny, reflective texture. If you see a connection that doesn't look correct, just reheat it slightly and it should improve. It might be necessary to put some solder flux on the joint or a small amount of additional solder. To remove solder bridges, solder wick is a useful tool. 


After inspecting and completing any repairs, it is time for a preliminary functionality test. To do this, plug a MINI B USB cable into the board connector and plug the other end into your computer. The board should be detected and identified in the "Connected Devices" list. The board uses generic device drivers so no additional software should be necessary. When the board is correctly detected and identified, the LED should be illuminated and will periodically flash off and then back on again to indicate that the USB connection has been established. 

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