This subfolder contains the basic electrical circuit details used in building the teensyExpression Box.
Physically, the most demanding task was to build the switch array, with 25 little circuit boards, and 25 LED's to connect together in a grid. Each LED is connected into a series by 3 wires ... in two directions ... ground, 5V, and data in / data out from the first, to the last LED. A single 3 pin connector connects all 25 LED's to the teensy (motherboard) and power supply.
The switch harness, overlayed on top of LED serial connections, consists of the 25 buttons connected (soldered) together into a 5x5 grid that presents itself as two 5 pin connectors to the motherboard.
The motherboard is a 70x50 mm prototype board that has a socket (two rows of female header strips) into which a teensy 3.6 is plugged, and which has connectors (female header strips) for the LEDs, switches, rotary controls, serial port, TFT LCD display. The host USB port connector is on the teensy 3.6 board itself.
Virtually every pin on the teensy was used in this exersize, at the sacrifice of future expandability, but the gain of simplicity in the circuit design. Previous design iterations had used IO multiplexer chips, for example, to communicate with the swtich array, via 4 pins, instead of 10, but in the end I just decided to let the teensy handle all 10 pins from the switch array in favor of a simpler electronic circuit.
The TFT/touchscreen is connected to the motherboard through two 8 pin connectors.
Besides those three main components, there are separate subassemblies and/or circuit boards for the four rotary countrollers, the USB port and amp-meter, the serial port connector, and, finally, the back panel with it's four expression pedal 1/4" stereo jack inputs.
There is a general overall sybolic schematic for this project. You can click on the image below to see the PDF file.
Each switch array circuit board contains a swtich and a ws2812b addressable LED. The LEDs are wired back together with three wires between each one, in a series, starting from the top left position, row-wise, back and forth, so the last LED is at the bottom right.
I cut the 25 ws2812 LED's from an inexpensive light strip that can be had for as little $2, with shipping, on ebay.
The strip I used has 30 LEDs, with 30mm spacing, and with capacitors on about every 8th LED. So one small strip was sufficient to make the box.
The switch circuit boards were cut to 20x25 mm from commonly available 20x80mm prototype boards that cost about 60 cents each on ebay. I bought them as part of a pack which included some other sizes for something like $8. With care 3 of switch-boards can be cut from each prototype board, and, since 25 are needed, you'd need 9 of the prototype boards to reproduce this project.
click on images above to see them in ebay
I had already decided to make the buttons have a 45 mm spacing, a little larger, and with a little more space than the drum pads on an akai MPD218 (typical midi drumpad device). The LED strips have a 30 mm spacing ... and so, after some consternation ... I opted for the demanding task of cutting the strip apart and then soldering it back together.
I tried to uniformely distribute the LED capacitors throughout the harness.
The switches are soft silicon with momentary contacts. They seemed to provide enough spring back to work, and they work fine in practice. I found them available on ebay for $1.85 for 20 (so you'd need two packs, but have 15 left over at the end).
click on image above to see in ebay
To complete the circuit boards I put a drop of super glue on the back of each LED piece, held it on the circuit board, and bent the connectors around to the back, trying to keep it as close as possible (centered) to the switch, which occupies a 2x3 footprint on the board.
The switches are wired together in an array, with columns connnected to one pin of each switch and rows connected the other pin of each switch. Rows start at the top and go down, and columns start at the left and go to the right. These wires end in two connectors which are called OUT (columns) and IN (rows) which plug directly into the motherboard. If you get em backwards, you merely have to change the IN and OUT defines in the source code.
The circuitry on the motherboard is pretty simple. Most of it was connected by using female headers with long leads and bending the leads and connecting direclty to another header.
Here are two diagrams that show, respectively, the wiring for the back, and the front of the circuit board. The solid lines are made either by bending leads and soldering them, or by running beads of solder over multiple holes in the prototype board. The lighter blue lines are wires that are soldered from one point to another.
above, motherboard back (as seen from the front)
above, motherboard front
The Touchscreen is connected directly to the motherboard by two 8 pin dupont female to male cables.
The pinouts are detailed in previous diagrams.
I didn't take any pictures, or keep any detailed design description of the Rotary Encoder board I threw together. So I would have to open the box up and disassmble it to present it properly.
TODO: Open up the friggin box and take pictures of the rotary controler board.
I am not using the momentary switches on the rotary encoders. They were kind of an afterthought, and I had no more pins available on the teensy by that point in the hack/implementation.
As I remember, I did not put any resistors or capacitors on it, I just wired the ground to the ground pin for the momentary switch (don't even think it'sda used), the 3.3V to the center pin on the rotary part of the things (the side with three pins), and the pairs of connectors (IN and OUT) to the left, and right pins on that side of the things.
If you get it wrong you merely have to change the definitions of the IN and OUT pins in the source code.
During earlier prototyping, and in general, I was curious about the power consumption of the device, and so had taken to using an inexpensive USB power meter to monitor the power usage.
These power meters are available on Ebay for about $7.
click on the image to go to ebay
I merely desoldered the USB connectors, and connected the ground and 5V from the main input USB typeB port to it, and ran it back to the micro-usb connnector that connects to the teensy.
I was very pleased with these little micro-usb connectors that I also got on ebay, at 10 for $5.79 (about 60 cents each)
And I needed the female TypeB connector as well (probably around $1 on ebay).
What made this step considerably more fun, and easy, was the purchase of some prewired headers and sockets for 2.54mm circuit boards. I got 10 of em for under $4 on ebay, and used to two in this project. I will definitely use these more in the future, as I hate dupont connectors and trying to build them from scratch!
The circuit board is approximately 24x38 mm, and the typeB USB connector projects about 3mm through the back of the box.
But it's really simple. Basically it connects the typeB female usb port ground and 5V to an outgoing pair on the connector that goes to the power meter, and the incoming pair that comes back and is connected to the ground and 5V pins on the outgoing (to the teensy) micro-usb connector.
The D- and D+ pins from the typeB connector are wired directly to the output connector, and I added a jumper so that, if I want, I can remove the USB meter and jumper the 5V and ground directly from the input typeB socket to the output micro-usb connector.
Sorry about the lack of detail here! I am writing this after I finished the hardware aspects of the project, but I think you can figure out how it goes from this, and other standard information available on the web about USB connectors.
The serial port connector is merely a 1/8" stereo jack connected to a 3 pin dupont connector. The tip of the connector is the RX into the teensy, the ring is the TX from the the teensy, and the sleeve is the ground.
The female, gold plated, 1/8 stero jack was about $1.50 on ebay.
The host port USB connector board is another 24x20 mm piece of one of the 20x80 prototype boards, with a micro USB female connector at one end, and a 4 pin dupont connector at the other, that connects to a 4 pin header soldered onto the teensy 3.6 board.
The female micro USB connectors are about 30 cents each, when purchased in lots of 10 on ebay.
That leaves the back panel and the four 1/4" expression jacks.
I attached these to the 3D printed back and soldered the wires on in place so-as to get appropriate lengths of wire with a minimum of fuss. They are wired with ground going to the sleeve of each connector, 3.3V going to the ring, and the return "sense" pin wired to the tip. The jacks were probably about $1 each on ebay.
After I put it together, I ran into some problems with this part of the build. Notably, that when you would plug a 1/4" jack into the the things, it would momentarily short the 3.3V to ground, yikes, rebooting the system.
It is not a good idea to short the 3.3V output from the voltage regulator on the teensy directly to ground!
Yet I think the connectors are actually designed to avoid doing that by the spacing of the tip and ring parts of the male vs the contact points of the female jacks.
The final solution was that I put a 1K resistor on the motherboard between the 3.3V output from the teensy and the common 3.3V connectors (rings) on the connectors. Although that works, by limiting the current that can flow in the event of a short circuit, it should not have been necessary and I am still thinking about how to do it correctly (next time!).
So, as a curiousity, after the fact, I have tallied up the following costs for the electronics part of the build. Clearly the most expensive items is the teensy 3.6, which I got WITH pins from at: prjc.com. They are currently $33.25. Some of the other costs are just estimates, but I think its pretty close.
| item | price |
|---|---|
| Teensy 3.6 pins | $33.25 |
| USB Power Meter | $7.00 |
| 3.5" Touch Screen | $5.00 |
| 30 x LED ws2812b strip | $2.00 |
| 25 x push buttons | $2.50 |
| 4 x Rotary Encoder | $4.00 |
| 4 x 1/4" female jacks | $5.00 |
| 1/8" female jack | $1.25 |
| female micro USB connector | $0.50 |
| mother prototype board | $2.50 |
| rotary encoder prototype board | $2.50 |
| 10 x small prototype boards | $10.00 (max) |
| Wired connectors and jacks | $1.00 |
| 5 x 40 pin female headers | $2.50 |
| Wired Dupont Connectors | $4.00 (max) |
| Other Wire | $1.00 |
| Solder, etc | $1.50 |
| Total | approx $85.50 |
It was my goal to try to keep the cost of the project at, about, or under $100. If I figure I used about 1kg of plastic in 3D printing it, at $20/kg, then, I came close. This box could theoretically be built by anyone with a 3D printer for about $110.
Though I doubt anyone will take my exact design and replicate it, apart from wanting the box itself, I wanted to prove that it could be made on a budget, if one is willing to put the effort into it.
Also note that you could eliminate stuff (like the rotary encoders and the usb power meter) in order to cut the costs a bit. I don't actually envision using the rotary controllers that much as it is a floor based foot pedal.
For me it is replacing a $270 Softstep II, a $100 akai MPD218, and a $170 audio-front expression-to-usb-midi convertor, or nearly $600 of gear ... and three separate devices ... with one consolidated usb midi device, so in addition to cost factors, for me it also reduces the complexity of my rig, and the setup and teardown times thereof.
It also provides me with much more flexibility to do what I want, and have not even yet envisioned, going forward. More on that in the software section of the project.
I have not seen any comparable hardware that sells for less than $200. In fact, a box similar to this would probably retail in the $400-$500 price range, and some go for as much as $1500 to $2000. I couldn't find what I wanted in the way of a foot controller for running the FTP in conjunction with SampleTank, Tonestack, and Quantiloop, on Audiobus, on an iPad, so I built this.
Of course, it took several weeks of 3D design work, prototyping, and nearly 2 full 40 hour weeks for me just to consctruct the final prototype. And I'll probably be working on the software for months, or mabye years, more. Nonetheless, it might be something that a typically impoverished musician could put together.
So, with that, I conclude the "hardware" portion of our show!
Thanks,
Patrick