Here is the design of my interface:
My project will be housed in one single enclosure, making it easy to mount and also more robust than if it were in several pieces. The 6 black buttons correspond to each string, and pressing one of these buttons activates the corresponding string. The power switch will be on the bottom so that is easily accessible but could not accidentally get switched off during play. The power indicator, however, will be located on the top so that the user can easily verify that the device is turned on. The beauty of this interface is its simplicity. Given the small amount of buttons/switches and the natural correspondence between the buttons and the strings, it would be very hard to misuse of misunderstand this interface.
3 similar devices:
While there is no current product that is exactly the same as mine which electromagnetically vibrates guitar strings and gives independent control over each string, there are other products that use similar technology.
The first and most obvious is the eBow, which is a device developed in the 70’s. It is a handheld string exciter that uses the same electromagnetic technology to vibrate one string when the device is held over said string. This is an interesting product but it has several disadvantages. The most significant of these is that you have to actually hold the eBow and therefore you can’t really play the guitar normally while using the eBow, or even transition easily between the two. Another major drawback is that you can only bow one string at a time, a problem my project seeks to answer.
The second similar product is made by TC Electronics and is called the Aeon. The Aeon is pretty much exactly the same as the eBow, except it was just recently released. The Aeon is more optimized than the eBow, allowing the user to hold it farther from the strings by maximizing the electromagnetic power of the inductors. The Aeon also has a much sleeker and generally better design – it looks better and is easier to hold and use. However, the same drawbacks that apply to the eBow apply to the Aeon as well – you have to hold this device and it can only play one string at a time.
The third similar product is called the Sustainer, made by Fernandes. The Sustainer is sort of the inverse of the eBow: you don’t have to hold it, but you have no control over how the sustain is used. This is because the Sustainer mounts in the guitar in place of the pickups, then creates a feedback loop from the output jack back to a master driver pickup that vibrates the strings. Therefore, all strings get vibrated when the device is activated, so while you can sustain more than one string at a time, you have no real control over which string is resonating unless you mute the unwanted strings manually. The biggest disadvantage of the Sustainer is that you have to uninstall your pickups and replace them with the Sustainer pickups which is highly inconvenient. My design mounts directly onto the guitar and does not require any internal modifications.
Given the simplistic nature of my product, my manual will be a single, small card describing the basic function and uses of my product. It will also include information such as battery life and mounting procedure and general care. I could potentially include some tips for use, but I have not decided yet.
My enclosure will be 3D printed and I have already begun test printing and verifying my design. Here is a screenshot of the 3D model from Fusion. I haven’t decided on buttons yet, so there are no button holes.
Work done in Weeks 6 – 7:
As stated in the previous calendar, these weeks were all about power optimization: maximizing the output from the driver pickup to achieve the loudest string vibration possible. At first I thought this was a matter of circuit optimization, but it proved to be much more complicated. Luckily I had sufficient output from my original proof of concept against which I could compare my the output of my developed version. What I observed in testing my pickups (version 1) was that their output was significantly lower than the output of the single-string pickups I made for the proof of concept. This perplexed me, as the resistance of both sets of pickups were exactly the same. The main difference between the two was that the proof of concept pickups consisted of coils wrapped around an iron core with a magnet placed on the bottom. After researching the topic of inductance, I found that stronger magnets lead to a stronger magnetic field. So, for my pickups, I decided I would get cylindrical magnets and wrap those with coil, so that I could fit the largest magnets possible in my pickups. This did work, however as previously mentioned the output was too low. So, I decided to do some more research. While I confirmed my previous findings that a bigger magnet means more inductance, I found a new piece of information that I had missed before: by wrapping the coil around an iron core and then magnetizing the bottom of the core, the magnetic field becomes highly directional. Therefore, when the pickup is pointed at the string, the signal is greater than when just the magnet is wound.
So, after learning this, I decided to make a new set of pickups with coil-wrapped iron cores which I could apply a magnet to. To do this, I first had to find some iron cores that were the right size for my design (5mm diameter, 5mm height). I was unable to find any from a manufacturer, but luckily I had some old guitar pickups laying around. These pickups use iron pins for the same purpose I need them, and they happened to be the right diameter. However, they were way too long. To fix this, I cut each pin down with a hacksaw so that they were roughly 5mm tall each. I did this for 12 pins. With my 12 pins, I created two new pickups, 6 pins each, coiled so that they had the same resistance as my proof of concept pickups. This whole process took a very long time, and after I finally got everything fabricated again – surprise! The new pickups had the same low output as the first set.
This was infuriating, but I kept pushing ahead and went back to the drawing board. At this point I felt really stuck. The resistance and general construction of my pickups were now the same as the proof of concept pickups and yet their performance was still worse. I thought hard for several days and read everything I could find on induction. After a significant amount of critical thought, the elusive but simple thought occurred to me: I used a different (heavier) gauge wire to wrap my proof of concept pickups because space was not an issue at the time. Heavier gauge wire means less resistance per length, so when the two “versions” of pickups were the same resistance, the proof of concept pickups had more wire turns on them, and more turns means more inductance. Now I had a real problem: the wire gauge on my PC (proof of concept) pickups (32 gauge) allowed for the right amount of resistance and enough turns for sufficient inductance but it was too thick to fit inside my enclosure. Conversely, the wire on my mk2 pickups (42 gauge) was small enough to fit in the enclosure but too resistive to get enough turns and maintain the correct resistance. The solution: a new wire gauge in the middle!
To figure out the right wire gauge, I first determined how resistive the gauges I already had were in terms of turns. I calculated the following: each turn with the 32 gauge wire has .01 ohms of resistance, and each turn with the 42 gauge wire has .12 ohms of resistance. This is a huge difference. So settling in the middle, I ordered 34 gauge and 36 gauge wire to be tested.
Once the new wire arrived, I made a few more single test pickups, and ultimately decided on the 36 gauge wire. Each turn of the 36 gauge wire has roughly .03 ohms of resistance, so I can get roughly 250 turns while maintaining the needed 8 ohms of resistance. I made yet another test pickups to verify this, and it worked great.
Finally, I made another set of pickups with the new wire, and after all of this they are now working great. The output is strong and consistent. Problems all solved, on to further interface design!
Week 8: Determine proper power conditioning, optimal battery, best hardware (buttons, charger, etc).
Week 9: Design PCB and order!
Week 10: Finish enclosure design, assemble final product!
Week 11: Extra week in case for unpredicted problems
Week 12: Finish manual, give to friends for testing