Archive for the ‘DIY’ Category

Another custom order, it’s the venerable Commodore PET professional computer – you know it’s a pro because it had 32kB of memory (in 1979, that was quite a bit, you pesky kids).

This is the 2001N model – the first to have a real keyboard and boosted RAM (up from 8KB in the original 2001 model). Available in the Rabbit Engineering online store.

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10 months after the prototype was completed, and three concentrated months of product development, the Model G1 is now available to order. We’ve sold two already!

It’s quite a lot of work taking a prototype to a state where you can sell it – improving the design for better manufacturability, deciding what needs to go into the manual, deciding on packaging, setting the price, marketing – it’s an exhausting list. And I’m sure that lots of interesting hiccups are still waiting around the corner 🙂 Click here to go to the details page, and order one if you like!

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After about 500 hours of printing reliably, my Makerbot Replicator 2 finally barfed. I walked in to a 4 hour print completion that looked like this:

Woah. When you walk in on a print failure like this, it’s important to root-cause the problem, so that you can prevent it in future. Not only did I lose about 4 hours on this job, but also about $5 in PLA. I looked over the printer first to check for the usual problems – extruder head movement restricted, PLA feed issues. Nothing there, so I took a closer look at the piece. I noticed an interesting pattern:

Notice how the problem started at a certain layer – that is not really interesting; notice instead how the layers above the break point are basically intact, just shifted over to the left (along the X axis). A couple of possibilities:

  1. A problem with the SD card or the X3G file on the SD.
  2. Corrupted firmware (on how it controls the X axis)
  3. Problems on the X axis travel (bad lubrication, loose drive belt)

First I took a look at 1 and 2 – the software problems. I looked at those first because those are the simplest to fix. I reformatted the SD card, re-built the X3G file and reset the printer to factory defaults. Then I kicked off the same print job and noticed the problem in action (thankfully it was very easy to reproduce). Here is a video of the issue:

Hear the THUNK THUNK? That is the head either stopping without deceleration, or step skipping. This is what happens on the print:

The squiggly line is where the steps are skipped. This means that you loose alignment between layers. You can see it more clearly in this larger photo:

So this makes the printer basically useless. Bummer. I started digging around and found a great YouTube video by forbinone that describes the same problem and gives a very simple fix. Basically, the problem is that the X-axis servo cable, from being too tightly crimped in the factory, and the thousands of forwards-backwards cycles, causes some of the cores inside the cable to break. That means skipped steps. The solution of course is to replace the cable.

Once you remove the right-side MDF from the Replicator 2, you can see the problem in the X-axis stepper cable (light grey ribbon cable) – it is crimped far too tightly.

Mark the point where the kink in the cable forms (in the diagram above, the problem space is at the blue mark on the cable near the top-right of the picture), and then remove the little bar holding the cable in place to free the grey cable. Ensure the black ribbon remains in place (it is for the homing switch and does not suffer this problem), and replace the little bar.

Now you just trim away the problem section of cable, and splice in some new ribbon cable. To ensure the splicing did not lead to any shorts across the four wires, I decided to use some 0.1″ headers.

Then add in the new section. Be sure to add enough that there is no tension in the wire – there should be enough play to allow the X-axis gantry it to move smoothly forwards and backwards without getting tangled anywhere. It is critical to ensure you keep the polarity correct – that is, don’t accidentally flip the splice so that the leftmost cable on one end of the original ribbon is now connected to the rightmost cable of the ribbon. That could badly damage the printer.

To ensure the new longer section did not get tangled into the rest of the machine, I cable tied it to the front-right vertical beam.

To finish up, I added some insulation to the joints, and bolted the MDF back to the machine.

It definitely looks a little war weary, but after the operation, it runs perfectly. I did the same 4 hour part again, and it came out without any issues:

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This story begins with a simple email about rotary encoders, and ends with me shipping my first paid commission.

It’s a big personal milestone, but really not as a big a deal as you might think in terms of just getting it done. The device I designed and delivered was the Rabbit Engineering Model I1, an interface module that lets flight sim cockpit builders (like my customer), home arcade cabinet builders, and anyone who wants to build a custom controller to connect a variety of buttons, switches, potentiometers, and rotary encoders to their computer by USB. You can get an overview on the product page, and order one for yourself.

By the way, if you get the chance to make a project for someone who has a real problem that you can solve for them, I strongly recommend you do it – this has easily been the most satisfying spare-time project I have worked on.

In early August, I got an email from Mr. G (he has a real name, but let’s keep it anonymous) asking about the rotary switches I used in my custom SF2 controller. He asked simply if I knew of a place that sold them cheaply. After  a couple of mails, I suggested he use rotary encoders together with a microcontroller like the TeensyUSB. Alas, Mr. G did not have the skills to pull that off. So I thought, hey, why not help a fellow flight simmer out and build it for him? I offered to do it for the cost of parts plus a small markup. He accepted. We had a commissioned job!

I had already had experience on similar projects and experiments such as the FSX MCP panel, the iPac distribution box, the SF2 controller, the Arduino Leonardo keyboard experiments, and the Multijoy_retro; so I was confident I could pull it off in a month or so.

When doing any kind of engineering project it’s important to set a clear spec (specification) by working with the customer – so we had a long exchange about exactly what the machine would do, I offered some options (with the relative costs), and Mr G. settled on what solved his problem and fit his budget. A simple agreement email locked us both in (I would deliver by end of August, and he would pay the agreed amount), and the three week clock began to tick down.


Essentially, it would be an interface device that would have as many as 42 buttons, switches, rotary encoders or potentiometers connected and converted to key presses, joystick buttons and/or joystick axis events. It would connect to the host by USB without drivers, and the configuration would be programmable by a config file on an SD card. Here is the final spec we agreed upon.

  • 42 configurable inputs, all can be used as binary IO, each using one of two modes:
    Push-Hold – the event occurs as long as the button is held down (for push buttons).
    Push-Nohold – the event occurs only once, regardless of push duration (for single events coming out of toggle switches)
  • All can be used to sense 21 rotary encoders (2 inputs needed per rotary). Clockwise rotation sends one event, Counterclockwise rotation sends a different event
  • 6 can be configured as analog inputs, sensing a 10K potentiometer (linear or logarithmic).
  • Inputs can generate any of the following events:
    Key press/release – single key, or with one modifier key (shift, ctrl, alt)
    Joystick button press (one of 32 buttons)
    Analog inputs can be mapped to one of 6 joystick axes
  • Configuration of the inputs is done by modifying a text file in an SD card which is read when the device is plugged into the USB port.

I really tried to convince him on an option to have expansion cards that would add more inputs using shift registers, because it was a feature I wanted to work on; but it broke his budget. In the end, you build for the customer and not yourself, so his decision stood.

Now the design phase – I had already played around with rotary encoders and the TeensyUSB, but only using interrupts. To satisfy the spec I would need more than the handful of interrupts that the Teensy provides, so I began to experiment with polling modes. This was kind of a risky/dumb move on my part, because if I failed to figure it out I would have had to break the spec/cancel the project/upset the customer, but believe me, that fact was very motivating. After a couple of evenings slaving over a hot C++ compiler, I got that working; turn clockwise and you get one input set, turn counterclockwise and get another, with minimal side effects (turn the rotary too fast and you miss steps, which is a bug the Saitek Pro Flight Multi Panel also has!). The other interesting feature, PushNoHold (holding down a button triggers only one input event) was easily done using the standard Arduino Bounce library.


The next “interesting” part in terms of code was SD card reading, which really should not have been hard, but gave quite a few hassles. Ultimately it was all related to the type of SD card – I needed to use an SDHC type rather than the SD types I was assuming I could use. Remember kids, try different disk formats if you’re having disk reading issues.

The big technology choice for this project was the microprocessor to use. I am a big fan of the TeensyUSB line, and I already have experience with it, so I knew it would be the one. I chose the Teensy 2++ in the end because of its large number of input pins and RAM; To satisfy the configurability requirement in the spec, I would need storage per input and 2KB would not be enough.


One painful (hardware) bug I hit was around pin 6 on the Teensy 2++. This is connected to the LED, and so behaves differently from the other pins. In order to turn this into a normal input, I had to add a pull-down resistor, and have the input  connect to 5+ rather than to ground like the other pins. Rather than try to reconfigure a big part of the project, I decided to document the strange behavior in the manual (a trick I learned from Clive Sinclair and his scientific calculator).

Writing the code was fairly straightforward, but my days as a C# programmer have left me soft and flabby in terms of pointer handling, so I got slapped around a bit by C++. One fine evening, at about 5am, I finally got all the code running and tested. This project actually has the most code of any I have done (mostly around the config file parsing).

Time to move onto the physical design. There were two aspects to this: Provide an interface to the input pins, ground plane, and +5 supply (needed for potentiometers) in a way that would make for simple integration of the Model I1 into the electronics of the customer’s project; and then a protective shell and means to physically attach it into the customer’s project (by gluing, bolting, or whatever).

The interface was the most important part – I decided to simplify things by having three clusters of connectors: The input pins (two long headers), a ground plane (a pair of headers with way too many pins), and a power rail with +5 and ground right next to each other. This makes adding input devices simple – just add some male 0.1″ headers and away you go.


And here it is in the shell. The tolerances were tighter than in previous projects, because I wanted to be as compact as possible (the bigger this device, the bigger the customer’s enclosure would have to be). Why yes, that is our old friend hot glue there.


The manual (more on that below) explains how to wire and connect each type of input device (potentiometer, button, etc). I used color coding on the headers to ensure that connecting was as error-free as possible.

Here it is painted (the white numbers are not on the device – this is actually a screen grab from the manual):

Then came the testing. Testing is not my favorite part of any build, but on this project which would be going out into the wide world and would be hard to fix once deployed, I took some extra time and built a little test rig with the various types of input devices attached. Well, “test rig” is a kind term, it’s a chunk of MDF with some holes. Still, it did the job OK.

Then I tested every type of input on every pin. Good times. Grudgingly, I admit it was worth it because I found two software bugs and one short circuit.

The plastic shell was designed in OpenSCAD and 3D printed. As usual, it began with a sketch (which I made in Las Vegas while there for Black Hat 2014/Defcon 22 – always carry a sketch book with you!).

I wanted to give it a designed look, but at the same time this would live inside a beige box one day, so spending too much time was not necessary. It is essentially a flat box with mounting holes, but I used hex bolts instead of screws to give it a little interest.

Then the printing (I think total print time was around three hours):

And presto! We have a shell:


I embossed the Rabbit logo on the back, as there was a huge expanse of plastic.

I also printed a matching shell for the SD card disk drive.

For physically mounting the Model I1 to a project, there are two options – you can bolt it in using the bolt holes in the shell, or you can screw in the provided “gluing feet” – you then glue the feet to your project. The idea is that you don’t want to glue the device itself to your project, because if you need to move/remove, the force could damage it. So instead you can just unscrew the device from its feet, and then grunt and pull at the feet all you need without risking the delicate electronics (a trick I developed for the Multijoy_retro).

Then came the writing – this beast would need a detailed manual, because I had to document not only how to connect input devices, but also the file format for the configuration file (which is fairly complex due to all the options you can have). If you thought testing was bad, then writing is the worst, but a well documented product really makes a huge difference – not only does it give an impression of quality and thoroughness, but it saves you a lot of time and energy from all the customer questions/emails you have avoided.

Once all that was done, it was time for the finishing touch – update the Rabbit Engineering site with the product information, and then build a nice packaging experience. When you are building for a paying customer, their experience of their product starts from the very moment they open the box, so that deserves some design and attention also. Sadly, there is no cardboard 3D printer, so a lot of Xacto and sticky tape action followed.

The manual goes on top. This is essentially the first contact the customer has with the device they just bought, so you want to create a “Woah, cool!” reaction.

Then , when they remove the manual, they see all the parts neatly laid out in the box.

Pack all that into into a shipping box, and off it goes to its new home in the wilds of North Carolina. Apparently it’s wild there, I’ve never been, so I will take their word for it.

In the end I was able to get the project done just three days short of the promised date. From conception to shipping was just over 20 days – this was possible mostly because I had already solved all of the underlying problems before I began. If you need a Model I1 for your own project, you can order one up from our online store.

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A larger project I want to complete as a RabbitEngineering.com milestone is a standalone computer, in 1980s style – complete with external storage, controller and screen. Top priority for this project is innovative and appealing design. I decided to begin with the controller (mouse replacement).

I decided to not use a mouse or trackball – those are not innovative enough; instead, I decided to model my controller on a well-established interface that is not used in computing, but popular in some areas of aviation – the HOTAS throttle. This is an example, from the McDonnell Douglas F-15 fighter:

The throttle is used to control the thrust of the engines by moving it forwards and backwards, but in the 1960s cockpit designers, realizing that a pilot’s hand needed to rest on the throttle for most of a flight, began to add controls for common functions – RADAR settings, the air brake, radio selectors, etc. This was done by adding first buttons to the throttle, and later 4 and even 5 way digital joysticks, in places where the fingers rested naturally.

I decided to use the throttle as the basis of my design. I wanted to keep the physical design minimalist and geometric, so I first settled on a simple cylinder with a flattened base. I 3D printed out test form in white PLA, and marked with sharpies where my fingers rested (this is a good technique for noting required adjustments to 3D printed parts).

Immediately a problem became apparent – the top part, where the fingers curl around the form, was too wide, leading the hand to be overextended and uncomfortable. Back to OpenSCAD. The tube was at least not wasted – I now use it as a pencil holder on my workbench.

For the next form, I narrowed the top, but kept the base wide for stability – in essence, the new form is a triangular prism with spheres for vertices. Again, I printed in white PLA and marked where the fingers naturally lay using sharpies.

Front view:

Left view (where the thumb rests, and the control for the mouse cursor will be):

Right view:

Once that was done, it was time to measure the locations of all the buttons and transfer them into OpenSCAD to begin modelling the complete shape. The final controller would have the following electronic components:

  • A two-axis analog joystick to control the cursor (for compactness, I used the part used in the Nintendo PSP, which is available at Adafruit). Controlled by the thumb.
  • A 5-way digital joystick for left click (depress the joystick), mouse wheel up and down, and scroll page left and right (also from Adafruit). Controlled by the index finger.
  • 6mm tact switch for middle click. Controlled by the middle finger (from Adafruit)
  • 6mm tact switch for right click. Controller by the ring finger (from Adafruit)
  • A TeensyUSB 2.0 programmed as a USB keyboard and mouse as the HID interface. The teensy can be programmed with Arduino libraries, but is much more compact and has better USB device emulation (you can also get these from Adafruit).

A new technique I wanted to try on this project was to have plywood inlays in plastic parts. On the Nespoise I had mixed black PLA and plywood to a very nice effect, but I wanted to try and hide the edge of the plywood this time. Because OpenSCAD lets me export to STL which is consumed by both Makerware (for printing PLA) and CamBam (for CnC cutting), I thought it would be fairly straightforward to do. Here is the shell from the outside (minus buttons):

The shell consists of an upper and a lower half, joined by two #6 imperial screws. The top half is the complex part, with the buttons/joystick openings, screw posts, and hinges:

Internally it’s more compact that any other project I’ve worked on – to ensure all the parts deconflict correctly, I rendered a number of strange cross sections (OpenSCAD makes this very easy). Here are some of the more interesting ones:

I had never worked with hinges and only had limited experience with moving parts like d-pads, so I decided to make test prints of just these areas. Turned out this was a good idea, as the tight tolerances I originally put in caused annoying sticking of the parts.




 Once all the bugs in the top half were resolved, it was time to print the complete shell. This required the use of supports which I don’t often use, and I ended up with some untidy/incomplete areas in the print (for example, one screw post only printed halfway before breaking down). Given it was a 5 hour print, I decided I could live with the bugs, especially given they were all inside the body (the external surface was smooth). I then glued/screwed the switches and joysticks in, and got ready to wire them in place.

I then spent some time programming the firmware for the Teensy. It’s better to do the bulk of this before final assembly, because you might discover short circuits, bad switches, or other hardware related bugs during programming, and you want to avoid having to disassemble the project to resolve those.

Here are all the plastic parts (with most of the electronics) ready to test.

I then cut the plywood inlays and they snapped nicely into place, and I added a little superglue to ensure they stayed in place. Time to add the remaining electronics before further testing. Here are some views of the top half ready to close – it is a tight fit.

Here are all the parts labelled. For the power light, I used the “natural PLA lens with logo bezel” trick I invented for the NESPo and used in the NESPoise, this time with the Rabbit Engineering logo.

On the bottom half I added two 1 Oz fishing weights, to make the controller feel more solid on the table. I wanted to avoid users from mistakenly moving it around as they would a mouse. The extra weight plus rubber non-slip feet makes it adhere to the table surface nicely.

Time for final assembly – here it is next to the second form:

Now to test it for usability. I found the cursor speed needed some tweaking, so I made some changes to the firmware. Teensy makes this a simple and painless process.

Here are some views of the completed controller, showing the power light illuminated:

Here it is in use, to give an idea of how the hand fits:

You can grab all the files to make your own at Thingiverse.




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When the Xbox team launched the Xbox One, they all got a special edition of the Xbox One which was white and stated “I MADE THIS”. Here’s your chance to say you also made an Xbox One! And you won’t have to fork out $3000 on EBay either.

Download the files to print your own from Thingiverse.


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I recently picked up an Atari 1200XL from a guy at work, and got into the mood to return to new mini machines. Instead of the 1200XL (which was a bit of a bomb), I decided to do the far more popular 800XL. And of course you need some media, so I did the Atari 1050 disk drive also.

As usual, you can pick up the files to print your own at Thingiverse.


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