Posted on August 5, 2024August 12, 2024 by adminw

Turning Input Signals into MIDI with Arduino

My old Alesis DM Pro has been great for the testing of the drum and trigger designs, but does not have nearly enough general purpose inputs for a large percussion instrument (several of the inputs can only be used for cymbals or hi-hats). I did consider a small stack of relatively inexpensive commercial units, the Ddrum DDTI being a strong contender. This would still need an outlay of ~$1000 and would require the merging of multiple MIDI streams externally or inside the computer. In keeping with the DIY aesthetic of my 3D printed drums and triggers, and the fact that I don’t have $1000 to spend on trigger interfaces, I have decided to use an Arduino for the trigger module.

The Arduino has a fixed number of input pins that can be used to measure the outside world. The Mega that I have been using is unusual in having a luxurious 16 inputs available (5 is more common). Unfortunately that is still not enough and the Mega has another problem that is the real deal-breaker – it cannot act as a USB-HID (HID: Human Interface Device) peripheral. USB-HID peripherals such as keyboards, mice, and game controllers are designed for human interaction with computers. They use a standardized protocol that allows for plug-and-play functionality, meaning the device can be recognized and used by the computer without needing additional drivers in most cases.

MIDI from Non-HID Arduinos

The Mega has a USB port that it uses for loading programs and communicating with your computer via a USB-serial connection. This USB-serial can be turned into MIDI by a program called ‘Hairless MIDI‘. The image on the right shows a Bluetooth connected Arduino powered instrument (my nichrome strung Arduino violin), connected to the MacOS MIDI bus. For the Tonnetz drums project, each octave has an independent MIDI controller. I need to some way of merging the MIDI streams from the drum octaves, which is not possible with Hairless (only one copy of Hairless can be running and each copy only accepts one serial input).

The Leonardo shown as a MIDI device in Ableton. The MIDI name presented by the controller can be altered, but only when program is loaded into the controller board. Instructions for changing the MIDI name can be found here.

I would also like to connect the drums to other MIDI devices, such as iPads, where Hairless MIDI is not available. I decided to retire my Mega in favour of a slower, less capable, Arduino with a special feature – the Leonardo. The Leonardo can become a USB-HID device, one that MacOS (and others) will accept as a USB MIDI interface, just like plugging in a MIDI keyboard. I decided to split the drumset processing in to two octaves with a Leonardo for each. This design will make it easier to use a cut-down 1 octave system when space is tight, make the wiring more compact by putting each controller with its own octave, and reduce the processing load on each Arduino (in comparison with handling the whole array in the Mega using a multiplexer).

The Leonardo has another trick that makes it very suitable for handling an octave of drums. It appears, from the silkscreened pin descriptors on top that is has 6 analog input channels; A0 – A5. If you turn the Arduino over and read the back, digital pins 4, 6, 8, 9, 10, and 12 double as analog inputs A6, A8, A9, A10, A11, and A7 (in that order). 12 inputs, just what I needed. My own Leonardo is a cheap clone from Jaycar without the underside labels, so I have to look at pictures of genuine ones to see the extra pin assignments.

Electrical treatment

The Buffer

We have seen in an earlier post, that piezos can generate voltages high enough to potentially damage Arduinos. This is less likely when the triggers are mounted in the drums (on measurement, voltages are considerably lower), but it is still prudent to protect the central controller for the system. To present a better signal to the Arduino, I decided to buffer all of inputs with inexpensive quad opamps (in this case, the LM324 operational amplifier). These amplifiers are set up for ‘unity’ gain – they simply present to the output what they see at the input, with three important changes. Firstly, the output impedance is much lower (better for the Arduino for technical reasons) and the output voltage cannot exceed 1.5V less than the power supply for the amplifier chip itself. By supplying the opamp with 7.5 Volts, the signal can never exceed 5V. Lastly, in the configuration used below, the opamp cannot amplify a negative signal, protecting the Arduino from ‘ringing’, a negative voltage sent by the piezo as it recovers from the transient of the initial strike. When testing candidates for trigger elements, I have seen ringing voltages approach -3V.

On the right is the buffer layout I hand-wired on prototyping board. Manual wiring like this can be time consuming and error prone but is useful for prototyping as it requires minimal materials and, in comparison to ordering a custom printed circuit board, it is far easier to fix small mistakes.

Power routing is on the left: Positive is red. Ground is passed through the module to the output connectors. Signal routing is on the right: Blue is input, cyan is output.

The Whole Controller Module

This is not the way I would recommend this system be implemented now that the design has been tested and shown to work. A sensible approach would be to develop a single PCB to hold the buffers and the other input conditioning, saving hours of wiring work. It is, however, an implementation that can be attempted for very low cost, without specialty PCB designing software.

Two one-octave controllers using the Leonardo

A more advanced designer might also use a bare microcontroller instead of an Arduinio, saving more time and money. Unfortunately I have neither time nor money to spare. I have to keep the system modular and, where possible, made from parts I already have available. The Arduino, buffers or input board from this project might be re-used in several more projects and so must remain discrete, modifiable, and accessible.

Two DC-DC voltage converters (the blue modules) power the project. The large converter with the digital display supplies power to the opamp module – the display allows for rapid adjustment on-the-fly without a multimeter. This is desirable because the opamp voltage effectively sets the upper voltage limit, and hence the maximum MIDI velocity, that can be processed by the controller. The smaller module can power the Arduino. This is intended to allow the controllers to operate without a computer attached, sending MIDI via a 5-pin jack mounted on a MIDI shield. The Leonardos are currently powered via the USB connection that sends MIDI to the computer, so these DC-DC modules are not wired in. The required voltages are regulated down from a 9V plug pack.

An Arduino MIDI shield allows the controller to operate without a computer

The 3D printing file for the controller base and the MIDI transmission software for the Leonardo are available from the project download page.

Posted on July 23, 2024July 25, 2024 by adminw

Assembling the Tonnetz E-drum Array

The current form of the e-drum array is two adjacent octave groups, requiring 4 rows of drums. The array has a pronounced curve around the performer, requiring each row of drums to have increased space between units.

Four rows of drums require 3 different clamp styles (A, B, and C, shown on the left) to accommodate the expanding width between the drums. In order to make the array neater and more stable, we can also remove unused mount points at the edges of the array to make edge clamps, such as the clamps D, E and J. After assembling the array full array for the first time I felt that the top row of drums in each octave needed more support (they were held at only one point) so I created fill-in clamps H and I to add rigidity.

The clamp set

Assembly Plan

Each of the clamp models available on this site has an identification letter somewhere on the part that relates to the assembly diagram above. To assemble the full 24 drum array you will need: 3 A clamps, 4 B clamps, 4 C clamps, 1 D clamp, 1 E clamp, 3 F clamps, 1 G clamp and 1 j clamp.

Putting the Drums Together

When the array begins to take shape it can become difficult to access all of the clamps and nuts, so these instructions are intended to be very general and simply illustrate the required steps. You will need to find your own strategies to complete the whole array.

Prepare the rim

Push the 75mm M6 bolts through the rim, ensuring that the hex heads are embedded in the matching rim sockets. Refer to the layout map above and put a nut on every bolt that will need to carry a clamp. This is my ‘C1’ drum, so every side is attached to another drum.

Inset the mesh skin and place the clamps over the top of the nuts.

Insert the shell and fit nuts to all of the bolts. Mesh skins do not require a lot of tension – it is possible at this stage to reach playing tension by compressing the rim and shell with your hand while tightening each nut. The drum trigger is shown mounted in this photo, but they can be slipped into place though the bottom of the drum after assembly as well. This also allows for easy maintenance of the triggers if any become faulty.

Push the clamps firmly against the bottom rim of the shell and secure them by finger-tightening the loose nuts installed in the first step.

You can now tighten the nut from above the clamp (as shown) or from the other side of the lower rim, which may be easier to reach as the array grows and drums are surrounded by their neighbors. Tightening from below the drum with slightly raise the skin tension on that lug. Mesh heads do not need to be tuned, so small variations like this are not detrimental to the drum performance.

Mounting of the array.

All of the clamps have holes designed to accept the top of a small cymbal stand. The setup below uses four cheap stands to support the array, or two to support each octave. This allows the array to be spit into two groups if the whole array is too large. I also prefer to break the array into octaves for storage and transport. You are free to use as many cymbal stands as you wish, but each octave should be independently supported.

Also shown below is a 10″ drum assembled from 3D printed segments.

A ‘C’ clamp attached to a cymbal stand. The ‘C’ row has larger holes that fit my Tama cymbal stands. All other brands that I own work with the smaller holes. It is not necessary to use a cymbal topper, the weight of the array is sufficient to prevent it from moving.

Connecting the drums to the controller will be covered in the post detailing the electrical and software design.

Posted on July 3, 2024August 12, 2024 by adminw

MAX/MSP and M4L tools for Patchworld and Virtuoso VR

I do appreciate that both of these VR instrument platforms have given some thought to allowing performance data to leave their VR environments, but the implementations are currently rather restrictive and incomplete, especially if you use an Apple computer. I have made or modified some tools that work for my simple needs and might be generally useful. I expect these tools to be rapidly superseded by official tools with greater capabilities. At least, I hope that will be the case.

Patchworld

Patchworld has a suite of internal objects designed to interface with Ableton Live, collected under the ‘Ableton toolkit’ category in the object menu. A range of sliders, toggles, knobs and strange disquieting stretchy interface blobs can be wired to an 8 channel bridge device. This device can send a range of either 0-1 or 1-127 on any of 4 sets of 8 channels. Surprisingly, there is no facility for sending notes or note velocity values, so I made one to use with my drum array.

Information is received by a MAX patch available from maxforlive.com. Although versions 1.0 and 1.5 are available I have yet to find any difference between them, even after opening them in editing mode. The embedded javascript object that is supposed to automatically link the bridge to a running copy of Patchworld has never worked and appears to be missing the code. The bridge will function if you manually enter the IP addresses.

I have addressed the lack of note information by simply sending the MIDI note number and corresponding velocity over two channel pairs previously used for MIDI controllers; 5+6 and 7+8. The remaining 4 channels are still available for MIDI CC. Each bridge can be set one of 4 channels, each with 8 lanes, so the loss of 4 to note values should not be much of an imposition. I originally used only two channels for note transmission but the response time was not fast enough to handle the data generated from two handed playing.

The original PatchWorld OSC Bridge version 1.5, by maceq687.

My modified version, available from the project download page.

Using the modified Patchworld OSC Bridge

The new sections I have added are:

Input Detect. This button flashes when receiving note information through channel 5 or 7.
Note Test. Pressing this button will send a copy of the last note received at a velocity of 64. If no notes have been received this button will send note 64 for channel 5 and 66 for channel 7. It can be difficult to use a VR instrument to send notes when mapping MIDI or tuning parameters inside Ableton, even with good pass-through cameras such as those on the Quest 3. My Quest is always very quick to enter sleep mode when lifted away from the face, making it difficult to ‘peep’ at the screen without having to wait for the headset to wake each time. Use these buttons instead of juggling your headset.
Test CC in is similar – rather than trying to keep continuous gestural controls engaged in the headset while working inside Ableton, these dials can be used to send the same CC numbers for testing instruments, assignments, etc.

Virtuoso VR

There is a already an official companion application (and a VST3 plugin) that allows Virtuoso to send note and controller information to your computer. Neither of these work on my Mac. Although source code is available, the pre-made executables are Windows platform only and I cannot use them. To address this lack I have made both standalone MAX applications and M4L objects that can connect to a running copy of Virtuoso. They currently only support the harp and marimba instruments, with hand rotation sensing for the marimba (the harp does not send this information for some reason). The standalone application can send MIDI information to whatever port and channel you select. M4L objects have no concept of port or channel (all information is channel 1 no matter what port number you send), so I will, in a future version, allow channel number to be used as a coarse MIDI controller for parameters the do not require fast updating – it may be useful for selecting timbre or physical properties for Collision. This is a work in progress and will be expanded to cover all of the Virtuoso instruments. For instrument builders or tinkerers who have to deal with real-world information flows, Live’s built-in MIDI mapping is still, in 2024, infuriatingly unusable, with no way of directly entering mapping information. To make mapping possible I have taken the MAP buttons from the Patchworld bridge.

Using the Standalone Application and M4L tool

The layout of the two interfaces is nearly identical but some controls have different functions as described below.

Each instrument has a circular input detector. This will flash when MIDI notes are received for that instrument.
Each instrument has toggles to accept Note Off messages. Note off is not always needed, especially for percussion sounds where it can cause problems by abnormally terminating sounds. Most continuous-note instruments, such as synthesisers, will need to receive note-off messages or the sounds will never stop.
Each instrument also has an ‘Accept MIDI Channel’ toggle. In the stand alone application this allows the channel selection tools of the virtual instruments to modify the MIDI transmission channel of the relevant instrument output. M4L has no MIDI channel information, so this will be repurposed into another MIDI controller receiver.
The left and right tilt channel indicators, the red arcs, show input data and can also be manipulated with the mouse to send controller information for testing purposes. The percentage boxes underneath can be used to set the lower and upper bounds of the controller mapping transmitted to mapped Live targets.
‘Send local IP’ sends the computer’s IP address to Virtuoso VR via the network.

Posted on May 21, 2024August 12, 2024 by adminw

Troubles with Triggers

Everything was going so well. I had worked out a fast, repeatable way to churn out triggers for my 6″, 8″ and 10″ printed drums. All I had to do now was crank out about 30 of them for my massive drum array. Then I made three triggers in a row that didn’t work. Some others only worked intermittently, when struck directly on the cone, or returned very low voltage. Something was wrong – something new that only showed up when I started the production line. My very first triggers were still working with no issues apart from a bit of variance in sensitivity that could be dialed away in the drum brain. So what went wrong? I had some hypotheses:

Bad piezos. They are the cheapest of the cheap .. but the distribution of failures does not seem to support this. My triggers were working, then most of them were not. That suggests a problem with my manufacturing process. Also, I would be shocked to find that the expensive and the cheap piezos were actually coming from different factories, not just identical units priced for their destination market.

The sensors are flexing too much, damaging the piezo material. I have never been able to experience a real trigger from a commercial product because I am too poor to waste money buying one – everything I’m doing is experimentation and guesswork. I don’t know exactly what stiffness of foam is used above and below the sensor and it is possible that the hard foam cones are transmitting too much force to the disc or not supporting them properly underneath. I do have some early sensors I made with soft foam that are working very well, but they are fitted to 10” drums where I used the soft foam to compensate for the large travel of the mesh ‘skin’. I switched to using hard foam because the soft foam was impossible to shape with power tools and did not respond to a hot wire cutter.

The adhesive on the foam sheeting is too strong and is damaging the sensors. It is impossible to open up any of my failed sensor packages – the adhesive tears the packaged piezo off the baseplate, leading me to think that the adhesive might be doing the same during assembly or when in use. Commercial products use 3M adhesive sheets that seem just as powerful, but if they are attached to softer foam this might not be a problem. I have also seen instructional videos where the foam cones on Roland drums were changed and it is possible to remove those without destroying the underlying piezo. This one seems plausible.

I purchase piezo elements with leads already soldered on. The solder joints are sufficient, but have huge solder balls. It is possible that the strong adhesive is attaching itself to the piezo around the solder balls when I press the stacks together, then damaging the attachment point when I let go (or when hit during normal operation). Official and third-party replacement cones have little channels cut in the cone to accommodate the lead attachment points but I have not been very consistent with my hand-cutting of the relief channels in my own cones (foam with strong adhesive attached is hard to cut reliably with anything. Even scalpels are fouled by the adhesive).

These effects are linked by the physical properties of the system and may affect the outcome collectively or independently.

Now I can try some experiments to see if I can pinpoint what is going wrong. I can’t do any forensics on the assembled triggers as they cannot be taken apart without destroying them. There are, however, a few things I can try at the design phase to sort out what the problem is.

Trigger Assembly and Testing

A range of new triggers to test materials and assembly strategies

Make the foam cone from softer materials.

This could help with:

Flex or shock damage by reducing the forces on the piezo transmitted from impacts on the drum head.
Possible adhesive damage – the soft foam is not held well by adhesives so it is unlikely to damage the piezo by pulling it apart.
Possible damage to the solder attachments or surrounding material – the soft foam will not exert a high degree of force around the sensitive areas.

2V peak is not a lot to work with, but it does work

This sensor works – I’m not surprised, I’ve built them before, but I have not directly compared them to sensors made with hard foam. I was not expecting the voltage peak to be this low. This may be from the reduced travel of the 6″ drum skin in comparison to the 10″ I have been using with these softer cones. It may be worth trying to source another foam that has a hardness somewhere between the two I am already using. This foam also has a lot of trouble staying attached to the adhesive on the top of the piezo. In the 10″ shell these soft cones eventually detach from the tape and ‘walk’ out of the drum as they are hit. This is not the design I need.

Reduce the flexing of the piezo

This could help with:

Possible damage from the cone adhesive when the piezo moves
Damage to the piezo from flexing with impacts

I have designed a new sensor base that replaces the bottom 6mm of foam (the layer directly under the sensor) with solid plastic. The piezo will mount instead to a length of 2mm think double sided foam tape. I expect that this will drastically increase cross-talk by reducing the isolation from neighbouring pad vibrations, but that can be addressed in software.

A healthy 6V peak and an almost flat tail in comparison to other traces on this page

This sensor package is working well, delivering good voltages when installed in a 6″ drum. Curiously, although it supplies an almost identical peak voltage to the normal sensor with a foam base, the oscilloscope trace shows a very flat recovery from the drum hit with almost no bounce or ringing. A possible interpretation is that sensors made with a foam lower disc store and return energy, allowing the piezo to flex (flexing also returns voltage from a disc piezo). This is worth looking into further, but with 24 drums in one array, total crosstalk and transmitted noise from the mounting brackets could become a major issue.

Assemble the cone upside down

This could help with:

Possible damage to the solder attachments or surrounding material from the powerful adhesive on the bottom of the foam sheet.

The top cone is made of three layers of foam sheeting. By assembling it upside down (with the adhesive layer on top) I can attach the bottom of the cone using the much softer and weaker Bear tape (with a hole cut out for the solder joints and leads). As a bonus I can use the adhesive on the top layer to add some material that protects the tip of the cone that rubs on the mesh drum skin. I made a sensor like this accidentally when I was working out the manufacturing process and it has been working flawlessly on an 8” drum.

A healthy ~6V peak, but look at that ringing afterwards

This is working just as well as my normal designs. The vibration after the initial impact is interesting but falls well below the threshold of hit detection.

The piezo in this design is sandwiched between two layers of thin, soft and slightly stretchy double-sided tape with a generous cut out for the sloppy factory soldered leads. As usual, the cone base has a channel cut for the leads though this may not be necessary when the cone has no adhesive. I’ll build some more in this design and see what works.

A Note from the Future

Several months after writing this whole page I started experiencing problems with the triggers again, and made a new discovery – the lower adhesive of the Bear tape was necessary for the survival of the piezo (and does allow for the replacement of the foam cones without destroying them), but in the photos above I am using far too much tape on top of the sensor. The tape should only cover the central disc, not the whole unit. I had to perform remedial surgery on all 24 triggers, as shown below. I also added a 330kΩ resistor from the positive of the piezo to the Cliff sockets. This forms a voltage divider with the 1MΩ input resistor that effectively lowers the input by 1-2 Volts, a much more comfortable range for the buffer circuit that follows.

Side mounted sensors

I recently found an old Roland pad in a music shop, a PDX6 or 8, and saw that the sensor in this model was mounted at the rim of the drum, with a flat-topped cylinder of foam contacting the skin. My current practice of mounting a cone shaped sensor in the middle of the drum should guarantee an even response around the drum head but if any hits land on the cone tip, they blast though regular playing at 100% velocity. A side sensor might fix that problem and side-step some of the issues described above that might originate in mechanical stress. A drum head deforms the most in the centre so a side mounted pickup should see much lighter impacts, less flex and less travel overall. I designed some side-mounting fittings and put together test rigs in soft and hard material. These sensors used a pack of smaller 25mm piezos from an earlier project.

The soft cylinder of foam returned very low voltages

The hard cylinder was more usable. Even if, for some reason, it returned negative voltage

I was expecting lower voltages from the side mounted sensors. They were smaller in diameter (25mm vs 37mm) and the side mounting would transmit a lot less of the stick energy to the piezo. I was not expecting numbers this small. The cylinder of soft material was useless, my hardest hits were just touching 0.5V. The harder cylinder was more promising, giving me ~1V, inexplicably negative. The wires were correctly installed on the piezo and correctly connected to the Cliff jack. This piezo just wanted to be different.

From watching a repair video for the PDX8, I can see that the side mounted piezo element in that pad is much closer to the skin than in my far deeper, more drum-like design, with a much shorter cylinder of foam. I also noticed that the replacement foam does not have a channel for the solder attachments, so they do not seem to be concerned that the adhesive will damage the top of the piezo.

The biggest problem I found with side-mounted sensors was a wildly varying sensitivity across the drum skin, with the side opposite to the sensor unplayable. These side sensors appear to need either a larger drum diameter (for more energy) or a much flatter foam column to get more of the impact energy to the piezo.

What About Tension?

That did get me thinking about the role that the drum head tension is playing. For this test I used the under-performing soft cone, to see if I could make it more usable. Tighter heads move less than loose heads, so would loosening the drum heads give my sensors more energy to use?

It does. But not a significant amount considering other variables, such as cone material, can have a much larger impact. In a not-very-scientific test I barely finger-tightened a double thickness mesh skin, measured some strong hits, then tightened the tensioning nuts by a half turn for each test afterwards. It only took two steps to get to the maximum tautness I would want to play with. As can be seen above, the loose skin returned ~2.5V, then just under 2V at medium and ~1.5V at maximum tension. The medium tautness is the loosest skin I would want to actually play, so while these are interesting results, they are not enough to make the soft cone a better sensor than the firm cone for supplying ~5V of velocity sensitivity to an Arduino input or even an Arduino set to 3.3V of input range. I would rather slightly exceed the input ceiling than lose sensitivity to light hits.

Single or Double Ply Heads?

I started this project with high quality Remo Silentstroke single-ply heads. They have not been available in my area for months and I have continued the project with some very quality-challenged Chinese drop-shipped double ply heads. I was curious about the loose 2nd layer of mesh – was it interfering with the operation of the sensors? I ‘converted’ some of these cheap skins into single ply by simply cutting the bottom ply out. No change in voltage readings was observed in the soft or hard foam sensors.

Conclusions

After all of these experiments, what really stands out to me is that all of these sensors worked. That does not give me any new information to help diagnose what went wrong with my last batch, but this shootout leaves me feeling more confident that I am on the right track with my firm, centrally mounted cones on a 37mm piezo and that they will work if I am careful and switch to the upside down cone design for the rest of the drum array.

Posted on May 9, 2024August 12, 2024 by adminw

Arduinos and Piezo Triggers

It isn’t hard to make a DIY e-drum system that ‘sorta’ works, but there is a much longer road to making one that is good to play. The Alesis DM Pro that I have using is the latest of a series of Alesis products that I have used for processing my home-made drum pads. I’ve also owned a DM10, DM5 and a DM4. My very first E-drum unit was an ancient Roland PM16. All of them gave a far superior ‘feel’ to just sticking peizos straight into the Arduino analogue inputs. This direct approach has problems detecting some triggers, strange velocity response and crosstalk (one input setting off other adjacent drums) even when only one drum is plugged in.

We are using roughly the same hardware components, my old Tama Techstar pads were just piezos glued to plywood, so what is going on and how do these commercial products make it work so well? To get a more predictable, stable drum we need to improve our handling of the sensors. Look at the oscilloscope captures below, particularly at the Vmax(1) at the bottom of the images.

Loose 37mm piezo struck with a fingernail sending 28.4v

Piezos can damage your Arduino. A piezo, struck hard, can send (depending on the model) 10, 20 or even 30v down a line that would really rather be seeing about 5v. We need to limit the voltage to safe levels.
Piezos are the wrong impedance for easy sensing. The Arduino wants to measure from a low output impedance (~10KΩ) but it is currently seeing about 1MΩ from the input resistor. We can fix this by adding an active buffer (or putting a lower value resistor across the input, but this will reduce sensitivity).
Commercial products have gain controls (usually electronic, but the PM16 had tiny potentiometers), velocity curve selection and crosstalk detection. These are software solutions and shall be addressed later.

For the sake of simplicity and getting a system up and running quickly, we can use a unity buffer to protect the Arduino and condition the signal.

A quick 3D printed test bed containing an Arduino Mega, 8 channel multiplexer (not used yet), breadboard and eight Cliff jacks.

Opamp Buffering

A unity amplifier is one of simplest ways to use an opamp – you just connect the output of the chip to the negative input, as shown below. Any voltage coming in will be mirrored to the output.

The LM324 is a quad opamp that can operate at low voltages. Output cannot go higher than V_cc – 1.5V (1.5v lower than the voltage you are using to power the opamp), but that should still be enough for testing. If we run it from the Arduino’s 5v out line we will still have 3.5v of room to play. If everything works out an independent power supply can be arranged at a higher voltage to allow a full 5v output.

The oscilloscope screen pictured on the right shows the voltage from the drum hitting 3.6v and flattening out for the duration of the hit. This behaviour can be used to our advantage.

The 8-inch drum input buffered by am opamp powered by the Arduino’s 5v line. The spike tops out at 3.6v from ~13v input.

A simple unity buffer. Just hook the output into the negative input.

Is this good enough already?

Experimentation with an 8″ drum showed that a lot of expressive playing happens below 3.5v with the hard voltage limit acting like an audio compressor – hard hits are effectively flattened out to similar velocity values, smoothing out runs and fast rudiments. The Arduino Mega also has the ability to use a range of reference voltages for measuring input on the analogue lines. We can set the Mega to measure against 4.3v (the closest preset available) instead of 5v or supply 3.5v to the input reference pin to get a full velocity range from our sensor. We could also just use a 3.5v Arduino like the ESP32 that expects a maximum of 3.5v input, but in that case we would have to find a 5v source from outside the Arduino to run the opamp. We could also just adjust the velocity range in software but we are still losing precision and my work on the sampling violin showed that after you account for a reasonable noise floor that loss of precision really does matter. The biggest problem I found was sitting down at a drumkit is a very different experience from hitting a drum in front of an oscilloscope; what I thought was a reasonable velocity range in lab testing was nowhere near the velocity achieved in real playing. All of my hits were flattening out on the 3.5v rail, giving a very artificial drum-machine like feel to performance. This is not good enough, it feels lazy to leave it half-baked like this, and I’m going to need a lot more drum inputs than even the Mega can give me so we might as well do the inputs properly while working out the multiplexers than can give me 8 inputs per analogue channel.

Posted on April 4, 2024August 12, 2024 by adminw

3D Printed E-Drum and sensor system

This is a build document for a large array of E-Drums I am making to support a VR hyper-instrument project. All of the 3D printed objects, Arduino source code and instrument files will be made available through this website with appropriate copyleft licencing, mostly dependent on the most permissive possible licence of the source materials I am using. The hyper-instrument itself will be covered later in a separate series of posts after the construction of the physical instruments has been completed.

One piece shell and one piece rim printed in ESUN ABS+. The skin is a Remo Silentstroke 6″. Tension Bolts are 75 or 80mm M6. The shell is modified from a set published by RyoKosaka on Thingiverse under the **Creative Commons Attribution-ShareAlike** licence. In accordance with that licence my modifications and my sensor mount design are licenced similarly.

Get a good printer.

After several years of mostly solid performance, I decided to upgrade my 3D printer. My old refurbished Wanhao Duplicator (Sold as Cocoon through Aldi) had seen me through the construction of the Ambsonic dome, but was unable to reliably print ABS or run overnight. Temperature variations would always detach print jobs from the build plate no matter how much glue stick or hairspray I applied. It was also far too slow for the big, dense prints I needed to create drum shells. I needed a cheap, fast, fully enclosed printer.

The Creality K1 has mixed reviews and comes with many warnings from the cognoscenti of Reddit about poor build quality and recurring print issues. I considered myself experienced enough to fix whatever went wrong and bought one at a good discount. It has jammed only once in several months of hard use. It is fully enclosed and reaches temperatures of up to 50°C internally with heat from the build plate alone. This is not always desirable, so leaving the door open when printing PLA in hot weather is a must.

Printing the drums

I found several sets of designs for drum shells on Thingiverse (a site with a huge selection of free-to-use ready-made 3D print designs) and quickly settled on this collection of shells deconstructed for printing on 20cm print beds. The shells and rims are printed in sections and held together with M3 screws. The K1 is just big enough to print the 6″ shells in one piece, taking 6.5 hours in fine mode. The 6″ rim also prints in one piece, taking 2 hours. Shells printed using both ABS and PLA have held tension with skins installed for around a week with no signs of structural problems. Larger rims, such as the 8″ and 10″ sets have distorted (shown below) but are working well enough and are not losing tension.

Printing a one piece shell in rainbow silk PLA from ESUN.

Making the sensors

E-drums use a type of pressure sensor called a piezo. These are usually sold as flat discs that may have wires already attached. In the type of sensor I am using, force from the drum stick impacting the mesh head is transmitted to the piezo through a cone of soft material. The piezo is sandwiched between the cone and another disc of the same material that insulates the piezo from the frame of the drum. Multi-sensor drums that can detect playing on the rim often mount another peizo directly to the drum frame.

Completed cone sensor for the 6″ Drum. The cone is made from 3 layers of 12mm self-adhesive EVA foam sheet.

The mounting plates for the internal sensors included with the downloaded files made no sense to me, so I quickly designed a new friction-fit sensor sled that allowed for another jack to be installed underneath the drum, to be used for tight arrangements where the sides of the shell might not be accessible. The sensor is patterned after the Roland cone sensors used on their V-drum products. Although ready-made cones are available from aftermarket manufacturers, they are easy to make from materials available from Bunnings, for a fraction of the price

I experimented with several types of foam when developing the cones, including hybrid cones that used softer material for the top half. It didn’t really matter – every foam I tried gave good results, but a cone with a pointed tip such as the one shown above will need to be constructed from a firm material. Having a small point is especially desirable for drums with a centrally mounted sensor as it makes the foam cone harder to hit directly with the drum stick (which usually results in a very, very loud hit that stands out from regular playing). The sensor sled is a tight friction fit inside the drum but is secured with 20mm M3 bolts to keep it in place when plugging leads into the integrated Cliff jack.

Making the cones

I use a 3D printed cone shaping jig and a reciprocating scroll saw to shape the cones. Firmer foams, such as this EVA, can be shaped with a powered wheel sander. Soft foams have a tendency to grab blades and sanding discs/belts and can be dangerous to work with if you are using high-speed equipment, so I would recommend shaping those materials by hand with a scalpel. The jig is designed to replicate the 66° slope of the original Roland cones.

Using an ABS 3D printed replica of a Roland cone as a reference, cut four circles of 12mm EVA sheet.

Assembling the sensor sled.

Stick the remaining disc of EVA to the sled

The sensor sled mounted to the 6″ shell. The Cliff jacks are simply wired with the red wire from the piezo to the tip of the inserted 6.5mm jack.

The tip of the cone should ~2-3mm proud of the height of the shell, allowing it to make good contact with the mesh drumhead. The cone described above comes out at exactly the same height. Take care when tightening the drum head as high tension may cause the sensor to stop working. If a satisfactory tension cannot be reached without stressing the cone, trim a very small amount from the top, taking care to leave a flat, smooth surface.

Bill of Materials

Part	Materials		Cost
6″ Drum shell	ABS or PLA filament	55m/165g	$4.62
6″ Drum rim	ABS or PLA filament	22.3m /67g	$1.86
Sensor sled	ABS or PLA filament	3m/9g	$0.18
37mm Piezo	In a pack of 25 from Ebay	1	$0.30
6.5mm Cliff jack	Bulk order of 30 from Element14	2	$3.70
37mm disc of EVA foam	Cut from 400 x 500 x 12mm Adhesive Rubber Mat	4	$0.80
Hookup wire	Anything lying around	To taste	??
Double sided tape	Something thin – not foam	To taste	??

STL Models for Printing

The models used above, and others discussed in other parts of the blog, are available from the project download page.

Posted on September 27, 2022May 9, 2024 by adminw

Making an instrument from a scan of my own teeth.

My dentist recently mapped my mouth with a 3D scanner. I asked if I could have a copy of the file, promising to create a bespoke instrument from the scan.

The information arrived as an STL file. The scan had a good level of detail, but some problems needed to be addressed before I could turn this into a printable model. It has been many, many years since I last used Blender, so I decided to use this project as a re-introduction. I looked for ‘Blendercam‘, the modified version I used to use for creating cutting files for my CNC mills, but there were no valid downloads that would run on my Mac. Vanilla Blender it is then, with Ultimaker Cura as the 3D printing processor.

The raw model was inside out – the inner surface of the model was on the outside, preventing me from turning it into a printable object. I used Autodesk’s Meshmixer to clean up the edges of the mesh and ‘flip’ the faces so the outside was properly outside. I am aware that Blender has similar tools but using Blender is very similar to using Avid’s Pro Tools – it is filled with a seemingly random mix of useful and esoteric functionality that is not navigable until you have spent a few weeks unraveling the interfaces and learning the hotkeys. It is often quicker to use another, simpler tool that is focused on the task you want to achieve. Meshmixer can also turn a surface, like my scan, into an solid object. It does a good job but I was unhappy with the loss of detail in the final model. I imported the fixed mesh back into Blender and manually extruded the scan into an object.

Rough scan of teeth. — The original state of the scan mesh in the raw STL file.

Meshmixer: The pink faces are inside out – they show the ‘outside’ of the object.

Meshmixer’s ‘Make Solid’ command does a good job, but will take away some detail.

In Blender I used the circle select tool to separate the teeth into objects. I have a very old 3D Connexion Space Navigator that is still supported, even in Monterey on an M1 Mini. Flying around the object with the left hand and controlling selection with the right makes Blender so much easier to use than just a mouse/keypad/keyboard combination. After isolating the teeth I used ‘Fill’ to create new faces, filling the open mesh holes in the teeth and in the gums. Blender’s sculpting tools filled the faces with a dynamic mesh that I could push and pull into shapes that I felt comfortable printing. I added rods to the teeth and subtracted them from the gum object – they will allow me to mount the teeth and run wires into them from underneath the gums.

Removing the teeth from the gums in blender.

Using Blender’s sculpt tools to fill in holes in the gums after removal of the teeth.

Posts for mounting teeth and routing wires through the gums.

Test print after turning the mouth scan into an object.

Posted on July 8, 2020August 26, 2022 by adminw

Nichrome wire as a controller input for Max Msp

The bow hair has been replaced with strands of nichrome wire. When the wires are connected by contact with a violin string or another piece of metal they form a conductive loop. The aim is to reliably measure the length of that loop and deduce the position of the bow. The electronics packages also contain gyros and bluetooth modules.

Recently I attempted to create a violin controller that used lengths of nichrome wire to replace the horsehair in the bow. The theory was that I could use the predicable resistance of the nichrome to detect where the bow was contacting a conductive element on the violin bridge. It turned out to be a little more complicated than I anticipated.

28 gauge (0.3mm) nichrome wire (Jaycar). 13.77 Ω/m resistance.

What is nichrome?

Nichrome in an alloy of nickel, chromium and (often) iron. It has high resistivity and is used to construct heating elements. DIY types would know it as the hot wire in a home-made foam cutter. The resistance increases linearly with length – a property I thought would make it ideal for constructing my own sensor.

Turning nicrome into a length sensor

Arduinos, and their clones, make measuring resistances very simple. The usual method is to construct a voltage divider using 5v or 3.3v sourced from the Arduino (or a power supply) and a known resistance (usually something in the order of a few kΩ). There are two reasons why this method will not work with the length of nichrome I am using in my project:

The usual way to measure a resistance with an Arduino. Ideally, the known and unknown resistances need to be similar, but that is not possible with my project due to current concerns.

The sensitivity and voltage range of the Arduino

The change in resistance over the length of nichrome I am using is small – only ~16Ω, and I need to measure that small resistance with the highest accuracy I can get.

Using the voltage divider equation, a divider using, for example, a 1k resistor as the known value will give me a tiny range of 5V to 4.92V on my 5V powered Arduino Pro.

The standard Arduino only has a 10 bit resolution for measuring that input voltage – that’s a 0 – 1023 range, of which I can use ~80 with that tiny change in voltage. To make matters worse, in the real world you sacrifice some of that range to ensure that the values you are measuring are all relevant and all captured. A system set up to use all of the input range has no room for values to slip, components to change, or for just the random behavior that seems to creep into home-built hardware. It is possible, however, to tell the Arduino to use a different measure for comparing voltages.

Input voltages are usually measured against a reference voltage that matches the voltage used to power the Arduino. This is convenient, as the Arduino is commonly the voltage source for sensing attached switches and potentiometers. It is possible to set a different reference voltage inside the Arduino, using AnalogReference(). This approach has some limitations, not the least of which is the lack of uniformity across the range of Arduinos and clones. This project began on an Arduino Mega clone, was installed on genuine Arduino Pro units for the final product and was moved to a Duinotech Nano for one of the interfaces. The second round of hardware has spent some time installed on 3.3v Duinotech ESP32s and a 3.3v Arduino Nano 33 BLE Sense (the sense has an optional 12 bit resolution but uses 10 bits for compatibility).

Lets’s look at the available reference voltages for the family of boards that I have used for the first round. Setting anaglogReference(INTERNAL) gives different results on different boards, some values are only available on Mega boards and the standard value differs between 5v and 3.3v versions of the same boards.

Arduino AVR style internal reference voltages:
5V (5V power supply) or 3.3V (3.3V power supply)
INTERNAL: 1.1 volts on the ATmega168 or ATmega328P and 2.56 volts on the ATmega32U4 and ATmega8 (not available on the Arduino Mega)
INTERNAL1V1: a built-in 1.1V reference (Arduino Mega only)
INTERNAL2V56: a built-in 2.56V reference (Arduino Mega only)

So why not maximise the voltage available for measurement by using a known resistance similar in value to my length of nichrome? Because that would be very dangerous for my Arduino. The nichrome in the bow forms a loop when the two strands are bridged by a metal contact, so the total resistance would be < ~14Ω when in use (you won’t usually use the very ends of the bow, so you won’t see the full 16Ω). Even if we double that and use a 25Ω resistor, with a 5v source that is is ~200mA (1W @ 5v!). I’ve looked through the specifications for a few models of Arduino, and they have all had a 40mA maximum for IO pins. So how do we do it safely?

Safe sensing

Connecting a constant current source to the Arduino.

We can measure low resistances safely using an external constant current source. R = V / I, so with a known voltage and current we can measure resistance without presenting a risk to the Arduino. The LM317 regulator shown in the diagram provides a known 104mA. We can round this down to 100mA (0.01A) and see that R = 10V. My 14Ω usable resistance range is now a manageable voltage range of 0 – 1.4V of 1.6V total range measured against 5V. That immediately improves the 5V Arduinos measurement resolution to ~287 of ~328. The hardware will be re re-implemented on newer 3.3V modules which improves the Arduinos performance to ~434 of ~496. The 3.3V Nano 33 BLE SENSE has an optional 12 bit mode (0 – 4095) that improves the bow reading accuracy again to ~1736 of ~1984.

Making it Better

There are some easy ways to make the hardware better – the most obvious being altering the current source, but these modules are already built and I’d like them to be useful for other low resistance tasks, so they’ll stay at 100mA for now. There is, however, a simple way of drastically improving the apparent resolution of a hardware sampling system without touching the hardware at all:

Next installment: oversampling

LM317 modules in action:

Bow electronics package. The LM317 is visible on the bottom right.

Another electronics package for the body of the violin, to sense a nichrome replacement for one of the violin strings. The LM317 and 12Ω resistor are visible on the middle right.

Posted on February 7, 2017August 22, 2022 by adminw

Making keys for the uniform keyboard

It was immediately obvious that painting over the top of re-shaped keys was just not going to be good enough. The existing keyboard I had been shown was functional but looked like a mess. These keyboards would probably see heavy use by students so I needed something very durable that also felt natural. It’s hard enough learning a new way of playing a keyed instrument without the distraction of paint coming off under your fingers.

I decided to re-cast the tops of the raised keys (previously the sharps and flats) in solid white resin. This increases the mass of the key a little, but that is not a drawback. The more expensive Korgs that use this keybed have weights installed to give them a better feel and even out the stronger spring response of the shorter keys. The extra weight of my solid tops is improving the keyboard!

The epoxy only takes 10 minutes to harden and has a pleasant glossy finish that matches the existing keys well.

Posted on December 12, 2016August 22, 2022 by adminw

Bilinear Uniform Chromatic Keyboard

This is an interesting project. I’ve just been commissioned to alter a Korg Triton LE keyboard to the bilinear uniform chromatic keyboard standard. It fairly obvious to anyone who has studied keyed instruments that the traditional keyboard is exactly the sort of mess that you end up with when you extend an interface far past what is was originally meant to do. We can’t undo the unholy dog’s breakfast that is tempered tuning, but at least we can address the crazy lopsided way we approach the piano keyboard. If you are interested in how it works, there is a great explanation here.

Black keys now alternate with white keys, so more are needed.

All of the previously ‘white’ keys need to be trimmed to a symmetrical shape and widened. There are more ‘black’ keys than before, so I’ll have to order some in as parts. Luckily the Triton was a very popularkeyboard, so parts are easy to find. Shaping and re-casting the keys will the tricky part.The Triton is very easy to take apart and the screws are large and mostly restricted to two sizes, making eventual reassembly much easier. After lifting off the bottom plate the whole keybed comes out as one unit. The keys clip in and out of the bed easily. I’ve arranged the ‘black’ keys where I need them, but five more need to be ordered to fill the gap.

That’s the easy part. I’ve measured the keys and laid out my replacement white key in Solvespace. Now it’s time to make a mold and start cutting them up.

Row 1
Row 2
Row 3
Row 4