Tuesday, January 24, 2017

Harmochord restoration with a 3D printer (mostly Dad's work!)

In the summer of 2016, we stumbled across an interesting find at a pawn shop in Verdun:

It's basically an accordion with an electric pump instead of the manual bellows, designed as a table-top instrument! Single bank of reeds for the keyboard, and a relatively small array of bass notes compared to the full size accordions

After trying it out in the store and verifying that at least every key makes a sound, we decided it was worth the $20 and the effort to lug it back home on the bus (it probably weighs around $15kg or so...)

The things that were apparent immediately:

- It was "Made in West Germany", which makes it probably the most interesting instrument I have owned thus far...
- pump appears to work fine
- 3 black keys are missing
- a few of the bass notes would stick when pressed

Things we found out afterwards:

- The "briefcase" actually was supposed to contain 3 legs, which allows the instrument to be self standing. Those were missing as well. (oh well...)
- slight leakage in some of the bass notes, causing either weaker sounds or other tones when some notes were pressed

As a keyboard player, the first thing I wanted to do was to get replacements for the black keys. Never got around to it with busy school and family schedule, but when my dad stayed with us for a bit, he took on the project.

This is mostly a documentation of the result of his efforts!

Here's a close-up of the missing keys:

A 3D printed model was made in SketchUp:

It was a basic 2D profile that was pulled upwards, and then a small rectangular extrusion was added (top left of picture).

Here's the key being printed and the finished product:

Here are keys in their final location. Note that the key bed (not 100% sure if that's the right term here) isn't in the best of shape, so things aren't super aligned and some of the existing keys also aren't completely straight either.

Finally, a short demo of how it sounds:

There are a few more things that need to be fixed. My dad managed to get rid of the sticky behaviour of the bass notes, but the leakiness of some of the notes are still there. Here are some shots of the mechanism when it was open, for curiosity's sake:

So at some point, I might tackle that... (but there's also a good chance it won't happen until the next time Dad visits! ;)

Tuesday, October 25, 2016

Raspberry Pi Robot with PiShield, Part 3: Fast video streaming

I fiddled for a good part of the day trying to find a low-latency local network video streaming solution to implement a "FPV"-like control for the robot using a RPi camera module. It turns out a bunch of existing solutions (like VLC streaming etc) have really high latencies. This is OK for things like a security camera, but not so much for realtime control.

Finally, this rather hacky solution using netcat appeared to work best:

1. On the client (viewer/controller, running on a Mac) side, run:

 nc -l 5000 | mplayer -fps 30 -cache 1024 - -x 640 -y 360

 2. On the RPi side, run:

 raspivid -n -t 999999 -fps 12 -rot 180 -o - | nc IP_OF_VIEWER 5000

 netcat is used to listen and send; raspivid grabs the video and pipes it to netcat, and on the other side netcat pipes the received data and spits it into mplayer.

 This was the only way I could get less than 1s of latency on the video feed, although it still wasn't great. Having a good USB wifi dongle also helped a bit. Finally, changing the dimensions of the video using raspivid actually made it much slower, probably due to software resizing of each frame on the fly compared to just sending out the raw feed from the camera.

 Next steps: get some more interesting sensor info from the PiShield, and get a separate power source since there seem to be random system freezes when the motor and Pi are both running off the same power bank.

UPDATE Oct 26:

with the advice of a very helpful redditor who suggested gstreamer, I gave it a shot this evening.

On Raspbian, gstreamer1.0 is currently already in the official repos, so no need to add custom sources as these earlier instructions.

For OSX, I just downloaded and installed the latest compiled version from the official source. It also handily tells you that by default the path for the commands are at


From here, we can either have a tcpserver on the Pi that a client can connect to, in which case:


raspivid -t 0 -h 720 -w 1080 -fps 25 -hf -b 2000000 -o - | gst-launch-1.0 -v fdsrc ! h264parse !  rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=minibian.local port=5000

Mac (note explict path to gst-launch-1.0 command, since I haven't added it to path):

/Library/Frameworks/GStreamer.framework/Commands/gst-launch-1.0 -v tcpclientsrc host=IP_OF_RPi port=5000  ! gdpdepay !  rtph264depay ! avdec_h264 ! videoconvert ! osxvideosink sync=false

Now UDP would be faster, and in this case it would be similar to the netcat example where the RPi would define the UDP "sink" using the IP address and PORT of the viewer (on the Mac), which would yield the following:

RPi (note destination should be explicitly defined by the source this time):

raspivid -t 0 -h 720 -w 1080 -fps 25 -hf -b 2000000 -o - | gst-launch-1.0 -v fdsrc ! h264parse !  rtph264pay config-interval=1 pt=96 ! gdppay ! udpsink host=IP_OF_VIEWER port=5000


/Library/Frameworks/GStreamer.framework/Commands/gst-launch-1.0 -v udpsrc port=5000  ! gdpdepay !  rtph264depay ! avdec_h264 ! videoconvert ! osxvideosink sync=false

Eye-balling the two versions, I feel like the UDP version is slightly snappier, but considering that my network is not very congested, I have a feeling the difference wouldn't be as much as if I had a lot of traffic running. Regardless, this is a HUGE improvement over the existing solution! Should try to do some timing tests to see if I could get a better measure of the actual latency - perhaps using a photo of a stopwatch like this guy did (which was one of many sources I originally consulted for the netcat solution above as well)!

finally, here's a test on the actual video latency of the setup above: looks like its somewhere between 150 and 200 ms.


Yet another option, is to use RPiCamWebInterface. In the end I didn't get around to do the latency measurement, but it feels *almost* as fast as the previous solution. The other bonus is the client user interface is far nicer!

Here's a sample video showing the stream in action:

Tuesday, October 18, 2016

Raspberry Pi Robot with PiShield, Part 2

The previous post showed the basic functionality of the Raspberry Pi Robot. The Arduino-based motor driver takes in commands to drive the robot in different directions, can be controlled remotely in a serial terminal via SSH using the screen application.

This is what the system looks like:

The Raspberry Pi talks to the Arduino via USB serial. The movement commands are sent from the Pi to the Arduino, currently, 'w', 's', 'a', 'd' to toggle movement, and ' ' (space) to stop. there are also ASCII numbers 1-6 that correspond to PWM values to control the speed of the motors. While it would be possible to control the motor driver with the GPIO pins of the RPi directly, I had that other circuit all hooked up from a previous project, and all it required was a single USB cable to the Pi, so I went with that instead of rewiring everything.

This time, we want to start doing something a bit more fun. Here's a preview of the end result:

The premise of the example is quite simple: get too close to the robot, and it runs away from you!

We use Python this time to implement the sensing behaviour. The source code that implements the above is available here. I call this one the PyPiBot as it runs on Python... ;)

Basically, what it does is the following:


1. Open SPI device for getting sensor data from the PiShield 
2. Open serial port (for sending commands to the motor driver)

Main loop:

1. Get data from sensor port
2. If sensor value is greater than a certain threshold trigger a move forward routine:
   - send a 'w' to the robot, which puts it in forward mode
   - wait a second (sleep)
   - send a ' ' to make it stop
3. Check for ctrl+c and quit the program

What you notice about this particular script is that it is blocking, which means that while the robot is in the move forward mode and the script sleeps, you can't respond to anything else. We'll tackle that in another example in the future!

Friday, October 14, 2016

Raspberry Pi Robot with PiShield, Part 1

Using the base from Tippy, I've started building a Raspberry Pi powered, sensor enabled robot. Currently, you can SSH into it and control it via the serial port. The original Arduino-based motor driver from Tippy is still there, but in theory we could just send out PWM and control signals from the Pi directly.

Main components:

- Raspberry Pi Zero with inline 2-port USB adapter:
- - Port 1: wifi dongle
- - Port 2: Arduino USB serial
- PiShield Sensor Interface (current Kickstart project here)
- 5000 mAh OCHO mobile power bank (1A and 2.1A outputs).
- Arduino and Sparkfun motor driver. Its been a few years since I ordered that part. This one has two channels with PWM input for each channel for speed control. There are now ones that fit directly on the Raspberry Pi as well which could be more convenient to use...
- Tamiya Track and wheel set. Really handy little platform for making a basic tracked robot. One of the cheapest options out there too.

My idea is to add sensors to the PiShield, and then have it react to local environment conditions in addition to receiving wireless commands. The software I'm running on the Pi is the latest minibian, which usually boots up and gets an IP from my router in about 10~15 seconds.

Could this have been done on an Arduino or even an ESP8266 based board? Sure - but its kinda neat to be able to run linux on-board in a relatively compact package. Stay tuned for more updates!

p.s. apologies to /r/cablemanagement. This part is clearly not my strong suit... ;)

Wednesday, October 05, 2016

5V power supply shootout

As I've got quite a few Raspberry Pi's in employed in various settings, one constant issue is finding the right power source for them. The Pi3, for example, is supposed to require up to 2.5A under heavy use (I presume a lot of wifi activity + fully loaded CPU/GPU, which is probably not likely to happen all the time). However, it's always good to know where the limits lie. I've been running a Model B on a wifi dongle + USB webcam as a streaming home surveillance system for a while now, on a "measly" 1A adapter without any issue (its a Nexus 4 charger, results below explain the quotes around the word "measly"...), which is probably living a bit dangerously perhaps... Anyway, here's some tests I did recently, along with lots of pictures to show for it!

First up, the test equipment. Here I'm testing a Nexus charger (1A), Samsung Tablet charger (2A), a 2.2A supply from Element14, and a cheap "2A" supply from a dealer on AliExpress. Near the top left are the key components for the test, which will be displayed in a larger image following this one:

Here are the interesting pieces of test equipment. On the left is a "USB charge doctor". They're available for a couple bucks on AliExpress. (e.g. link here.) There are a number of models out there at different prices, but their main feature is to be able to sit between a female USB outlet and a device connected by a male cable, and measures the voltage and current. Another cool feature of this particular one is it also has a resettable energy counter. This basically sums the amount of energy that has gone through, and is good at things like measuring how much capacity is left in your battery, or what the effective output capacity of a mobile power bank is. For example, on our aging Nexus 4, if I deplete the battery completely and charge it (system turned off), it measures about 1600mAh, which is quite a bit below its factory spec of 2100mAh. Our Nexus 5, much used but in newer condition with fewer cycles, reads about 2000mAh (out of 2300 new).

Plugged into the USB power meter, is a funny device with two large green resistors on it. This is a switchable dummy load that can be set to draw either 1A or 2A from the USB port it is connected to. Using basic Ohms law we can see that the resistors are most likely 5Ohms each, and the 1A mode just makes use of a single resistor (drawing 5V/5Ohms = 1A) while switching to the 2A mode, puts two in parallel (5V/2.5Ohms = 2A). There's also a handy two colour LED that lights up either green in 1A mode or red in 2A mode, assuming the power source doesn't crap out completely (see later tests).

The next funny device is a hacked microUSB female to female USB adapter. This is required in order to use the power meter with the chargers that have microUSB ports hardwired to them. Here are the chargers: Note that two of them are hardwired. I use a 1m Aukey microUSB cable so that they can be used with the same funky adapter (explained later). Aukey, along with UGreen, Anker, and a few other "respected" brands, provide quite high quality products from the AliExpress marketplace at a price that is a compromise between so-cheap-you're-surprised-they-even-work (and most often they don't) and arbitrarily-expensive-north-american-department-store levels.

Below shows the setup for the cellphone chargers. Since 

I could simply plug the USB power meter and dummy load directly into the charger (and is in fact what I'd normally do). However, I deliberately introduced the extra USB cable so that I could use my home-made adapter in between. This is because when I was making it, I realized that the wires of the USB extension cable I sacrificed, as well as the traces on the female microUSB adapter board was a bit on the thin side. If the resistance was significant, it may cause a noticeable voltage drop when using this adapter, and would have a negative impact on the observed performance of the charger. This way, any potential degradation would be consistent throughout all our tests. (The flip side of course means that I wouldn't be able to make a very convincing critique of the absolute performance, only how they measure up against each other....)

 So then, with a wired charger, this is what the setup looks like:

After that wall of text and pictures, lets finally get down to some results!!

Nexus 4 charger. 1A setting. 4.62V, 0.82A. Note the green light.

Nexus 4 charger, 2A setting. 3.94V, 1.37A. The power meter will probably crap out at some voltage threshold, but here we see its still kicking away, drawing way beyond its rated capacity...

Next we have the Samsung 2A charger. Here we see it at 4.6V, 0.83A for the 1A setting, and 4.53V, 1.65A for the 2A setting.

Onto the 2.2A rated charger from Element14: at 1A, it measures 4.83V at 0.9A

And at 2A, 4.5V and 1.65A

Finally, we have the cheapo 2A charger: even at 1A, the voltage drops down to 3.97V, and outputs a measly 0.7A

 And when trying to put a 2.5 Ohm load to draw 2Amps: .... uh oh.

Well, at least the red light gives off a feeble glow, showing that its at least trying. The voltage has dropped so low that the power meter no longer turns on. At some point I could easily verify this threshold on a variable power supply at some point...

Finally, just for fun, I got a 5000mAh Mediasonic Ocho power bank. It has a 1A and 2.1A port.

On the 1A port, I get 4.56V at 0.84A

Trying to draw 2 amps on the 1A port, we get a dropout. However, what you may barely make out in the picture below, is that the voltage is just on the threshold for the power meter to almost turn on. This suggests that its doing better than the cheap wallwart above!

On the 2A port, we have no trouble at least keeping things going. 4.27V @ 1.56A. Considering that these guys are supposed to at least register a "charging" status on power hungry tablets, its not surprising that it at least manages to chug along, even if the voltage dips quite a bit... for about $10cad from NCIX when on sale, these guys are an amazing deal.

So, there you have it! Here's all the info consolidated in one place:

Charger      Rated   1A             2A
Nexus 4      1A      4.62V 0.82A    3.94V 1.37A
Samsung      2A      4.60V 0.83A    4.53V 1.65A
Element14    2.2A    4.83V 0.90A    4.50V 1.65A
Cheap0       2A      3.97V 0.70A    N/A
Mediasonic   1A      4.56V 0.84A    N/A
Mediasonic   2.1A    4.64V 0.8A     4.27V 1.56A

Comments and final thoughts

The voltages across the board seems a bit on the low side. This is definitely related to the adapter I built. Due to the thin wiring of the cable and small traces, the adapter's resistance will create a.) a voltage drop and b.) reduce the amount of current (Ohms law here we go again...). As mentioned earlier, I wanted to get a sense of the relative performance of these chargers with respective to each other, and if I removed the adapter for the chargers that don't need it, it would skew the results quite a bit against the wired chargers.

I was especially interested in getting some sort of measurement of these wired chargers that I was especially interested in, as prior to the making of this adapter, I had no way at all of testing their performance in any way (other than to see if a Pi manages to boot and/or display the "low voltage rainbow square").  At the same time, I could unquestionably say that the cheap adapter is totally off in terms of rated performance, not even coming close to performing at its advertised values.

For a more definitive view on the absolute performance of these guys, I should probably look into building a microUSB adapter with beefier wiring to reduce the amount of losses through this part. It might also be good to actually compare the performance of the cheap USB power meter with a decent bench meter to see if there are any discrepancies there as well. 

As one last point - the cheap USB dummy load gets pretty HOT. I would not recommend it for any prolonged tests, and perhaps invest in a headsinked and actively cooled version at some point.

Tuesday, October 04, 2016

Kickstarter is Live!!!

I have a habit of back filling this blog, typically in a sneaky way such that when you visit, it looks like the content has been gradually updated over the years. However, currently due to PhD stuff, family/baby stuff, and various other things, I haven't managed to backfill adequately to get rid of the obvious gaps, so this post will look somewhat unexpected... but here goes: after starting this project about two years ago with Infusion Systems, the PiShield is finally live on Kickstarter!!

Wednesday, July 06, 2016

Jeux d’orgues on iPad with Yamaha CP33

Jeux d'orgues is a neat application that contains a number of French organ samples (link here). There's also an iOS version and Android version (called Opus 1). Using a class compliant USB-MIDI adapter or alternatively directly connecting to the USB-MIDI port, it is possible to play the instrument using external controllers.

Here we see it in action with my Yamaha CP-33. The neat thing is the app supports MIDI CC/PC mappings to trigger different stops, which means I can change them using the built in patch keys of the keyboard!