MIDI Accordion

This page is about my La Tosca accordion, originally manufactured in the early 1930s.  It was given to me by a family friend in the 1970’s when I was a teenager – it had some wheezy notes, some of the reeds had actually fallen out, and the bellows leaked pretty badly – so back then I used hot glue and duct tape to try to fix it – absolutely not the right way to do it!  I have taken the accordion out of the closet and played it from time to time but it became ever more frustrating as it aged even more, so finally in early 2015 I decided it was time to fix it right.  The first picture below is right after I finished the renovation and modernization.  The second picture was taken some time later after I had decided to modify the front panel electronics to add two more switches for bank select and drum modes.  Also shown is a block diagram of the electronics that I designed and added.  Check out videos of me playing the accordion here.   

Rewaxing and Rebuilding the Reed Blocks

I ordered bellows tape, reed wax, valves, bellows gasket, new straps, and other hardware from various accordion suppliers on the internet.  Titebond “No Run No Drip” glue is a great PVA glue for attaching the bellows tape.  Some wax “spoons” were fashioned similar to pictures found online and a cheap crock pot is the perfect way to heat the wax.  Removing all the reeds, cleaning off all the old wax, and keeping everything in order is tricky. There are 4 ranks of 41 treble notes, and 5 ranks of 12 bass notes in this accordion. That’s 224 separate reeds. Each reed has a “blow” and a “suck” side, and all except the highest reeds have a little leather flap, called a valve, on each side. This accordion has a total of 412 valves. The valves originally were leather, but I decided to replace mine with modern mylar plastic valves which don’t dry out like leather and are softer for a more responsive action.  Getting the technique for waxing the reeds took some practice, but eventually I was able to get the hang of it. Here are some pictures of the unassembled bass ranks and the reassembled treble ranks. 

Treble Sensors

Now the really fun part – adding key sensing and MIDI electronics!  Accordions have a treble side (where the keys are) and a bass side (where the buttons are) so I designed electronics for both sides with a cable connecting them through the bellows.  The 41 treble key sensors are Hall Effect sensors.  A magnet is attached to each of the 41 key valves and the Hall sensors are arranged in such a way that pressing a key brings the magnet close to the sensor to cause it to trigger.  The first picture below is the unmodified treble section.  The next photo is the circuit board mounting blocks and magnets attached to each valve.  Next is the treble sensor board with the Hall sensors, and then the treble sensor board installed on the mounts.  The schematic shows the 41 AH337 Hall Effect sensors along with pullup resistors on the outputs. When a magnet is brought close to the sensor, the output goes to ground.

Circuit Boards

The circuit boards are one-sided PC boards that are etched using the PNP-Blue toner transfer method.  Once the boards are etched, they are drilled and a legending layer is added to the top side to aid in placing the components and jumpers.  Once the circuit boards were assembled, an extension frame was added to the treble section and the grille was modified to make more room for the added sensors and circuitry.  The processor and treble decoder boards are mounted to the extension frame and to these boards the ribbon cables from the switch panel and hall sensor strip are connected.  The grille was originally silver colored, but was terribly scratched and tarnished, so I painted it black and replaced the cloth with a new piece of fabric.  The large switch/LED bezel is a piece of poplar fabricated using my CNC machine.  There is a voltmeter to monitor the battery status and a bank select LED digit on the side.  The bezel for this was printed on my 3D printer.

The processor schematic features a PIC16F887 which is the brains of the accordion. It scans the Hall sensors and generates the serial MIDI output data. The RGB control panel LEDs are driven by U2, U3, and U4. The nine switches are connected in a 3 by 3 matrix which is driven from just 6 PIC pins.  The treble decoder schematic shows the 74151 8-input multiplexers which are used in scanning the Hall sensors in groups of 8. There are 41 treble keys, so 5 multiplexers handle 40 keys, and the last key has its own direct input to the processor.   The control panel schematic shows the nine RGB LEDs and switches used in the minimalist user interface. The Red, Green, and Blue elements of each LED have independent current limiting resistors to help balance the individual color’s brightness. The switch matrix includes a diode on each “out” line to allow detection of multiple switch presses if needed.  The bank display schematic was an add-on to the original build to display the selected sound bank digit.

Bass Sensors

The left-hand section of an accordion has 12 rods that run the length of the instrument for the “bass note” valves and 12 more rods for the “chord” valves. The matrix of pushrods and pins select which of the rods will rotate and thus which of the reeds will sound for every given button press.  To add sensors to the bass section, it was necessary to add a lever and a magnet to every one of these 12 bass note and 12 chord rods.  6 chord levers were fashioned from brass rod stock and soldered to the left side and 6 were added to the right side of the accordion. The 12 bass levers all were added to the bottom of the bass section in a single row.  This particular accordion is not designed to keep the bass note rods from moving when a chord rod of the same note moves, so it was necessary to add 12 springs to the chord levers to prevent inadvertent bass note triggers. I found some soft springs at True Value Hardware that were just right. With the added springs everything works as it should.

The bass decoder schematic shows the 74151 8-input multiplexers which are used to scan the Hall sensors in groups of 8. As mentioned above, there are 12 bass notes plus 12 chord notes, for a total of 24 inputs, so 3 multiplexers are needed for the AH337 Hall sensors.  Also, a Freescale MPXV7002 +/- 2kPa (+/- 0.3psi) pressure transducer is used to sense the accordion bellows movement in both directions (“blow” and “suck”). The pressure sensor input is used for MIDI volume – when the bellows are moved more forcefully, the volume gets louder.  

Radio Interface

I wanted to avoid having a cord between the interface box and the accordion so there’s an XBee wireless link to send MIDI data from the accordion to the interface box which is connected to the synthesizer module.  The self-contained power supply in the accordion is seven 2/3A size NiMH batteries. The radio interface has a power regulation circuit to provide the 5V and 3.3V needed by the various components from the battery pack. The batteries last 5 to 6 hours when fully charged, and a small LED voltmeter indicates the status of power remaining. An external charger can easily be connected to refresh the batteries.  The radio circuit board and batteries are mounted inside the bellows section – there’s just enough room for the board to fit next to the bass reed blocks at the top of the accordion.  A 10-conductor ribbon cable connects the two halves of the accordion together through the bellows. The cable is of course long enough to unfold and stretch as the bellows is extended while playing.

Receiver Interface Box

The receiver interface box contains an XBee receiver and all the interfaces to connect to a standard 5-pin MIDI jack, which provides the data to the MIDI synthesizer module.  The foot control and foot switch shown on the schematic are only usable when the interface is set to “wired” and a connecting cable is used to the accordion.  Power supplies for both the receiver (3.3V and 5V) and a synthesizer module (12V) are included in the box.      


The original case I had for the accordion was a disaster – cracked, chipped, ripped, and dirty. It was time to make a new one.  The new case is made from 1/2″ baltic birch and 1/4″ luan plywood, covered with textured sheet vinyl. The latches and case corners are from Rockler.  The inside is blue felt.

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