Replacing the middle button on the Logitech M570 Wireless Trackball (or, yes, I really do love this trackball that much)

I've been using a wireless trackball for a while now, and it's turned out to be one of the best purchases I've ever made.  It's especially excellent for working on the train, since i don't like the trackpad and there isn't room for a normal mouse.

The wireless trackball I'm using is the Logitech M570.  It has a combination scroll wheel/middle button; however, over time this button began to get flaky.  There would be weeks where the button wouldn't respond at all, or would only respond to exceptional force. Looking online, this seems to be a common problem; the device is somewhat cheaply made, and the middle button is not the usual high-performance switch, but some lower-quality part.

I set out to determine whether the button could be replaced; I was successful, and my procedure for replacing it is outlined below.  The new button is slightly stiffer than the old, but it works consistently.

Button Replacement Procedure

Disassembly

First, pop the blue trackball out.  Then, there are five screws holding the shell together; remove them.  Note that one of these screws is beneath the sticker in the battery compartment (shown below).

With the shell off, you'll need to remove the circuit boards.  First, detach the ribbon cable leading to the trackball reader (the gold bit in the middle of the image below); to do this, you'll have to pull up the plastic locking connector.  Then, remove the four screws which keep the circuit boards on the lower shell, and pick up the circuit board assembly.  Take care not to bend the battery wires as you remove them from the lower shell.  The middle/scroll wheel button we're going to replace is just in front of the ribbon cable.

Old Button Removal

In removing the old button, we need to be careful not to damage the rest of the circuitry.  Note that our task is made easier by the fact that we don't care what state the old button ends up in.  The method I used to avoid damage to the surrounding circuit is shown below; first, I used a diagonal cutter to cut the two forward leads on the button.  Once this was done, I gently rocked the button up and down to weaken and then break the remaining two leads of the button and remove it (I did not cut these leads, as there were difficult to access without risking damage to the rest of the circuit).

Once the button is mostly removed, I used copper solder wick to clean up the four holes that the leads of the button formerly inhabited.

New Button modification and installation

If you were paying close attention during the old button removal, you'll notice that the leads were pretty much vertical.  The buttons I had on hand were very similar in size and function (normally open, leads paired length-wise); however, their leads were gull-wing SMD style (MOUSER link).  Before I can install it, I need to manipulate the leads into a more vertical configuration, detailed in pictures below.

First, push the leads down.

Then, use pliers to straighten the leads downward.

Then, gently insert the button into the cleared holes and solder it into place; be sure that it is flush with the surface of the circuit board.

Re-Assembly

Just reverse the steps in Disassembly.  It is important to make sure that the little plastic power button in the lower shell is lined up with the switch on the circuit board before screwing everything back together (the switch is the silver-and-black affair on the lower right of the picture above, just in front of where the battery clips attach to the circuit board).

It is interesting to note that Logitech uses Nordic Semiconductor radios for their wireless links (at least for the new-ish unified receiver).

My initial investigation of the MARLOK key (or, how many times do I have to mess up using a pipelined ADC before I'll learn my lesson permanently?)

As I mentioned in the previous post, my university uses a somewhat rare access control technology called 'MARLOK': each user is issued a key, whose identity is encoded in a series of holes in the shank of the key (kind of like an old punch-card).  The key is inserted into a reader next to a door, and if the user has privileges to the door, it is momentarily unlocked.

I was interested in reading the key (and possibly eventually building something to emulate the key, in a reader), so I had a couple of little boards made to mount infrared emitters and detectors at the appropriate distances to read the three tracks of the key.

Initial Research

Before doing all this, I tried to look up the MARLOK system on the internet.  Details were sparse; the only information of any substance is here, where someone reports that each key encodes 24 bits of ID.  Otherwise, there's very little info out there; nothing on the structure of the encoding, how the clock is embedded/recovered, etc.

The Current Test

The sensor (with key inserted) is shown below.  The emitters are Kignbright APT2012F3C; they are wired in parallel with a 460Ohm current limiting resistor, leading to a total of 5mA passing through them collectively.  The detectors are Everlight PT19-21C/L41/TR8 phototransistors; they are connected to the positive rail, and then through 2.2kOhm resistors to ground.  The voltage is sampled at the top of the resistors by a microcontroller, then sent out at 100Hz through the serial port.

The key has three tracks; the black plastic is infrared-transparent.  The metal of the key is punched with square holes to allow for IR light to travel through the key.

The ends of two of the tracks were totally accidentally exposed, to show the structure of the holes (shown below).  As you can see, the holes are square.  From the data I show later, the middle track is a clock signal; this makes sense, as the middle hole is offset from the exposed side track by a half-hole-width, so that transition on the clock track signals when to sample the data tracks.

Below is a plot of the three track traces; the left is when the sensor was seated at the base of the key, and the right occurred as I slid the sensor off the key.  Green is the middle track, red and blue are the side tracks.  Note that it took a little messing around to get this data; originally, the LED current was three times what it was for these trials, and it saturated the clock channel (since, being in the middle, it got illumination from all three LEDs).

This next is the same information, but at the end of the sensor removal, as the sensor comes of the end of the key.

From the data, I saw that the middle track was the only one that was always on; from this (and the fact that its holes were offset from the other tracks' holes), I assume it is the clock signal.

Looking at the traces over a full sensor-removal, I saw the following patterns of holes:

01111110011101
11111111111111 END OF KEY>>
00001001110111
As i said above, the middle track was the only one that was always 'on'.  Additionally, since the first clock has both 'on', and the last has both 'off', and the internet says these keys encode 24 bits, and there are 14 total clock pulses, I assume that the end-of-key data track bits are always 'on', and the base-of-key data track bits are always 'off', which likely helps in level determination and data recovery.

Moving Forward

As evidenced in the data traces, I'm having difficulty keeping the key straight in the sensor; I'll need to do something to mechanically narrow the passage.  Once this is done, it'll be possible to recover sampling times from the clock transitions.  It might also be possible to improve the signal level by masking the LEDs so that they don't bleed over into the other channels as much.

Data Recording Software

To get this data, I created a microcontroller firmware that samples the three channels, then packs each 12bit sample into a message packet (which includes a start sentinel nibble for frame alignment) and sends it over the serial port.  I then created a general-purpose serial port data-extractor and -viewer for MATLAB, which is available here.

Final bugs and V2 (or, yes, I actually labeled the USB pins in reverse order. Stop laughing)

I've finally figured out what was wrong with the sleep mask charger and USB circuits: the pins on the connector were reversed.  So, hilariously, every time I plugged it in, i was connecting V+ and GND backwards, leading to some considerable heating in the charger IC.  Remarkably, the whole thing worked flawlessly once I flipped the connector.

Since I worked that out, I've designed version 2 of the sleep mask and helmet boards.  Additionally, I designed boards for a few other projects, including an instrumented glove, and MARLOK key reader and a pulse oximeter.  I've begun to populate the boards, but I'm out of some things, so I'm waiting for another Mouser order.  Unfortunately, the component names ended up on the silkscreen, making it thoroughly messy.

Sleep Mask V2

After figuring out what I'd done wrong with the USB connector (as well as playing around with the REM detector), I moved forward with redesigning the sleep mask controller board.  Most importantly, I significantly shrunk the board (the old version was very... clunky).  I used smaller versions of the microcontroller, FET, headphone jack and opamp.  Additionally, I added the option for a lowpass between the microcontroller DACs and the headphone amplifier, in the hopes that I can reduce power consumption.

I also wired up a new mask (the old one was... clunky.  Also gaudy... very gaudy).

Helmet V2

The new version of the helmet implemented the analog-power-off changes, in addition to adding a charger circuit.  However, it ends up being about the same size of the old board, owing to the use of smaller FET, opamp and microcontroller.

Instrumented Glove

I've long been interested in the idea of a joint-angle-sensing glove for computer interface.  Additionally, I overheard someone around the lab implying that they would like to take such joint data over a whole day of normal hand-use.  So, I thought that I could use some of the spare board-space on a circuit which would read at least 22 channels of joint angle data, and record them to an SD card.  The flex sensors will be home-made, and will change their resistance with flexion (like in this article).

I decided to go for a controlled-current topology for resistor-sensing (rather than a divider) to get a linear read on the resistance.  The resistor sensors are connected to the header at the top of the board; they connect to V+ on one side, and to the constant-current sink on the other side, through a matrix of FET switches; 5 banks of 5 sensors each give me 25 potential sensor channels for a mere 10 digital outputs.  This circuit also includes a battery charger.

MARLOK key reader

I've long hated the MARLOK key; it seems like a perfect storm of high-cost and low-convenience and I don't understand why you would use it instead of an iButton system or RFID.  In any event, I have a MARLOK key and I was curious about how the information (and clock) were encoded in it.  There isn't much information online about the format; all I was able to find was that the information is encoded in the sequence of holes drilled in the shaft of the key.

I designed a pair of board upon which I could mount IR emitters and IR phototransistors to read all three tracks of the key; I added a few spare holes to weld the two sides together using wire scraps.

Pulse Oximeter Sensor

Finally, I wanted to try out reading pulse (and possibly blood oxygenation) non-invasively.  I created a sensor board based on "A wireless reflectance pulse oximeter with digital baseline control for unfiltered photoplethysmograms" by Kejia Li and Steve Warren.  The sensor is reflectance-mode, which means that you only need access to the surface of the body; in opposition to the transmission mode, where the emitter is opposite a protruding bit of anatomy (like a finger) from the detector.  The emitters and detectors are in the upcoming order, so it's just the board and cable for now.

REM detector + display updates (or, scrolling displays are almost as cool as fezzes)

As I remarked in a recent post, I've nailed down the cause of my pesky REM detector noise issue.  Additionally, I started sprucing up the display subsystem, using a DMA channel to significantly reduce the processor load needed.  I've taken another couple of steps forward, in that I've implemented a fix to the REM noise issue, and I've refined the display subsystem so that it can easily produce text and primitive graphics; these refinements are shown immediately below.

The bottom 3/4 of the display is the REM differential signal level, scrolling from left to right.  The numbers on top are the 32-bit integer holding the device's seconds counter (left) and the IR illuimnator level (right).

Not very interested waveform-wise, but a clearer picture.

The video shows the scrolling display in action; since the sensor is pointed at the couch arm, there's not much going on, except when I rotate it.

Implementation details, vaguely

On the display side, this is really only a slight complication of what I outlined previously; there's a local buffer that I draw on, and periodically I set the DMA channel to transmit the contents of that buffer to the display via the serial transmitter peripheral.  There's a little bit of trickery translating the signed ADC inputs into the bytes that get painted into the display buffer, but it's overall quite straightforward.  A zipped archive of the project files is here.

On the REM detector side, I'd determined that the reason I was getting 'bias noise' every four samples was that the PWM switching transients were making it into the ADC signal, and because the PWM and REM counters were not integer multiples of each other, they would only 'line up' every four samples.  To rectify this scenario, I modified the code to make sure that the sample timer is always set up to be an integer multiple of the PWM timer period.  I also set up the clocking of the timers to go through the XMEGA's event system, so that I could keep the timers in sync.  A plot of the signal below shows no abnormal patterns in the noise, so I think this has taken care of the problem.

Moving Forward again

So, my next step has to be nailing down the REM detector.  Specifically, I'm going to slightly modify the hardware so that I can quarter the PWM duty cycle.  This way, I can maximize the settling time between the PWM switching time and the ADC sample time.  After this is done, I'm going to assess whether this has had the effect of decreasing the noise; either way, I'm going to collect some real-life sleep data, and get cracking on whether I can assemble a filter/classifier that can detect REM.

Of course, this completely avoids the issue of the busted charger circuit; unfortunately, I still haven't secured access to a hot-air rework station...

Using DMA to automatically transfer display data (or, it's RTFM, not 'Haphazardly Cast Your Eyes Left to Right Over The Manual While Thinking of Something Else')

As an aside from the process of perfecting the REM detector, I decided to start to clean up the display-painting parts of the firmware.

It takes half a kilobyte of data to completely fill the display (128-by-32 off-or-on pixels) and the data is sent through one of the chip's serial peripherals.  Since there is so much data to transmit per display update, it takes a LOT of processor cycles to do so; it takes even more cycles to do so by waiting and polling the serial peripheral instead of setting up an interrupt (as I did to debug the display).

Since the process of getting that data out to the display could be fairly straightforward, it seemed like the perfect excuse to take advantage of the AVR XMEGA microcontroller's onboard DMA feature.  And since I made a couple of errors along the way (and couldn't find a straightforward explanation of my problem online), it seemed appropriate to describe the situation here.

What is DMA?

Briefly, DMA (direct memory access) is a way of taking the load off of the processor by 'automating' data transfers.  Specifically, the DMA controller transfers a block of data from one location in memory to another while the processor executes other tasks.  Without a DMA controller, any large data transfer would take up processor time as the processor accessed and copied each byte of data individually.

There are many situations where you would want to transfer a lot of data from one location to another, or a small amount of data from (or to) one location a great many times.  In my scenario, I have a buffer in local memory that represents what the display will look like; the buffer is local so that manipulating it (compositing the image, blanking regions, adding text) is easier.  However, that buffer data needs to be periodically sent (serially) to the display hardware.  Ideally, the DMA will automate the process of taking each byte of the buffer in turn and sending it to the serial peripheral when it's ready to accept the bytes.

Implementation on the XMEGA

The XMEGA A microcontroller that I'm using has four independent DMA channels.  Each of them has a lot of configuration, including the ability to specify the source and destination data addresses and the ability to set what triggers the data transfers.

My problem was that I didn't read the manual closely enough; I thought that setting the channel trigger to the serial peripheral meant that every time the send register was empty (the trigger source), that a single byte would be sent.  In the default mode, however, the trigger causes the DMA to transfer an entire block of data as quickly as possible; since the serial peripheral sends out bytes a LOT slower than the rate that the DMA controller pushes bytes, this meant that each transfer only resulted in a few randomly-selected bytes of the buffer actually being sent to the display.  This caused me some consternation until I re-read the manual and realized my error; this fast operation is useful when copying data into SRAM or other fast destinations, but completely inappropriate for slow, single-byte destinations like the USART peripheral.

For slower destinations which will need to signal the transmission of each byte (or each burst of 2, 4, or 8 bytes) one at a time, the 'Single-Shot Data transfer' mode is used.  This mode completes a single burst, instead of a whole block, with each DMA channel trigger activation.

Since I want to transfer the complete contents of the display buffer to a single address on the serial peripheral, I need the destination address to be fixed and the source address to increment during the transmission, and reset at the end for the next update.

The actual C code I used to initialize the DMA controller and channel on the ATXMEGA128A4U is shown below; USARTC1 is the serial peripheral I'm using as my transmitter (it's been set up in master SPI mode for my display module, and then used to initialize the display module) and debugBuffer is the 512-byte-long stretch of internal memory that I'm using as my display buffer.

//set up a DMA channel

//enable the DMA controller

DMA_CTRL = DMA_ENABLE_bm;

//set the burst length to 1 byte

DMA_CH0_CTRLA = ( DMA_CH_SINGLE_bm | DMA_CH_BURSTLEN_1BYTE_gc );

//set the following: source address incremented, reload after each block; destination address fixed (reload after each block)

DMA_CH0_ADDRCTRL = ( DMA_CH_SRCRELOAD_TRANSACTION_gc | DMA_CH_SRCDIR_INC_gc | DMA_CH_DESTRELOAD_TRANSACTION_gc | DMA_CH_DESTDIR_FIXED_gc );

//now set the DMA trigger source to the USART data register being empty

DMA_CH0_TRIGSRC = DMA_CH_TRIGSRC_USARTC1_DRE_gc;

//load the block transfer count register with the number of bytes in our blocks (that is, 128*4 = 512)

DMA_CH0_TRFCNT = 512;

//now put in the initial source address; should be the memory address of the first byte of the display buffer

DMA_CH0_SRCADDR0 = ( (uint16_t) debugBuffer >> 0 ) & 0xFF;

DMA_CH0_SRCADDR1 = ( (uint16_t) debugBuffer >> 8 ) & 0xFF;

DMA_CH0_SRCADDR2 = 0x00;

//now specify the destination address; the transmit register of the USARTC1

DMA_CH0_DESTADDR0 = (( (uint16_t) &USARTC1_DATA ) >> 0) & 0xFF;

DMA_CH0_DESTADDR1 = (( (uint16_t) &USARTC1_DATA ) >> 8) & 0xFF;

DMA_CH0_DESTADDR2 = 0x00;

Once the DMA channel is set up (and assuming the USART and display have been initialized), I can update the display with the current contents of the buffer by simply enabling the DMA channel:

DMA_CH0_CTRLA |= 0b10000000;

Project Updates (or, I am probably not dead)

So, it recently came to my attention that I haven't posted in several months.  This is due, in large part, to my actually getting work done on my PhD.  This is also due to the fact that the next steps are relatively... un-glamorous and un-postable.  Specifically, I have a small issue with the REM detector, and I need to figure out what's wrong with the battery charger circuit.  On the up side, I was able to significantly reduce the standby power consumption of my helmet flasher so... little victories.

Charger Debugging

As it stands, the Lithium-Polymer battery charging circuit does not work.  This is ironic, as it was the only sub-circuit that I did not mock up and test before ordering the circuit boards (I even mocked up the 3.3V buck converter, which only had three components).  I did this because, well... I was just using the reference implementation.  I've gone over the design (and the physical artifact's correspondence to it) with a fine-toothed comb; at this point, I'd like to swap the chip to see if that's the issue.  Unfortunately, I do not own a hot-air rework station, so that is easier said than done.  Additionally, the chip gets REALLY hot and sources a bunch of current when you plug it in, which complicates debugging, as I can only leave it plugged in for short periods.  Further, even if I could simply replace the chip, I'd be leery of doing it, as the new chip could easily fry as well.  This is triply annoying, as I would like to add the charger to my helmet and run signal glove in their next iteration, but can't do that until I know I've got the circuit right.

REM Detector Hiccups

The specific problem with the detector is that, for certain levels of illumination, the the detector output follows a 'negative bias' for every fourth sample; this is illustrated below.  Looking at the 'noise' and mean of the signal after separating it into four down-sampled signals (that is, the first sub-signal is every four samples of the original, starting with the first sample of the whole record; the second is every four starting with the second, etc.), it appears that this really is just a constant 'bias' term added only to every fourth sample.

This is an example of the detector output against my hand; green is the differential signal, dark blue is the single-ended signal, and light blue/cyan is the illuminator amplitude.  As you can see, the signal appears to get much more 'noisy' as the illumination level increases, until it abruptly decreases at a certain level.

Zoomed in view of the 'noisy' segment from above; we see that the extra 'noise' is due to every fourth sample being much lower than the other three.  This pattern is also borne out in the single-ended signal.

My current hypothesis is that this is due to my use of low-passed PWM outputs to give myself some extra low-bandwidth analog outputs to control the REM detector illumination level and the differential signal bias level.  Specifically, since the sample-specific noise only occurs at certain levels of illumination, and then suddenly stops once the level raises above that level, I am lead to believe that it's something to do with the PWM switching time lining up with the ADC sampling time.

If that's the case, I'll have to decrease the passing of switching transients by reducing the corner frequency of the low-pass and/or increasing the carrier frequency of the PWM.  Of course, increasing the carrier frequency will reduce the resolution of the LP-PWM channels; however, since I don't use the full resolution anyway, it wouldn't be much of a sacrifice.

Helmet flash controller standby power reduction

Going back to the helmet flasher project, I noticed that the AA batteries were getting drained in a manner that was less-than-consistent with usage.  I recalled that I had been less than diligent in regards to standby power usage, so I figured I could shave off a few milliamps by taking a closer look.

There were two main methods I figured I could use to reduce standby power use: put the controller into a deeper sleep state while in standby, and modify the hardware so that I could remove power from the op-amps used int he constant-current LED drivers.

The first approach, going into a deeper sleep state, was the first I implemented since it didn't involve modifying the hardware.  Before making my changes, I inserted a 1-Ohm 1% precision resistor to measure the standby current draw.  I set the device to go into "power down" when going into the standby state, with wake-up accomplished by state change on any of the buttons.  Additionally, I set the device to go into "standby" while the timers count and wait to move on to the next flasher state.  The source is available here, in case it is useful to someone wanting to see how a non-human primate would implement the preceding.

After implementing the sleep state changes, the standby power consumption went from 2.1mA to 1.7mA.  Good, but we're not finished.  Since the on-state power consumption is in the range of 100-200mA, it was impossible to detect if my changes had an effect on on-state power consumption.

The next step was to switch off the power to the linear constant-current LED driver block.  The op-amps used in that block draw current even when the LEDs are off, so it made sense to try to switch the power off when the device was in standby.  Since the analog block was currently fed directly from the power rail, it was necessary to cut that source first.

Since the op-amps only draw a few milliamps (far less than the 30mA the controller pins are rated for), it made sense to feed them from one of the controllers output pins; this way, I didn't need to add any additional FETs.  After doing this, and making the appropriate changes in the firmware, the standby power consumption fell from 1.7mA to less than 0.1mA.

Overall, these changes reduced standby power consumption from 2.1 to 0.1mA; a more than 95% reduction.  This makes me much more comfortable with leaving the batteries in when I'm not using it.

Next Steps

So, the most immediate next steps involve fixing the REM detector problem outlined above and making the charger circuit work.  After that, though, there are a couple of immediate next steps:

Finally finalize the sleep mask hardware

The current version of the sleep mask hardware is... clunky.  Pointy.  Eye-pokey, even.  Since it was a first prototype, I didn't put all the effort in the world into optimizing the layout, or the parts (I just used the components I had on hand, instead of sourcing the absolute smallest I could find).  Once I'm confident that the hardware is up to snuff, I can source the absolute smallest components (especially the controller and FETs/op-amps and the passives) and re-design the board.  To do that, though, I need to rectify the problems listed above (REM four-sample noise, charger broken).  Additionally, I'll need to take a few nights' worth of data and specify a classifier/filter that can detect 'REM' to my satisfaction; if the hardware needs revision to get to that point, I'd rather do so before the next hardware revision.

On a somewhat unrelated point, I'm going to hack up the headphone amplifier hardware to see if I can't make it more efficient.  As it stands, the white noise component consumes the lion's share of the power for the device.  Looking at the output of the headphone amp, the 'square edges' of the DAC output are preserved to the output; this might be causing greater power consumption than necessary.  If I introduce a high-pass before the headphone amp and shave those square edges off, I might be able to significantly reduce my power consumption.

Update the helmet flasher and glove turn signal hardware; make a decision about the helmet

As I've said above, I'd like to make the turn signal glove and helmet flasher run on rechargeable lithium-polymer batteries.  To do that, I need to make sure that the charger chip and circuit work as advertised.

Additionally, I need to make some decisions about the helmet flasher.  As it stands, it steps the battery voltage up to 16V to drive the LEDs in series.  This was done partially since I originally had intended to add some EL wire to the helmet; EL wire requires ~150V AC, and my intent was to switch current through some step-up transformers.  The transformers could be smaller/have a lower turns ratio if my switched DC voltage was larger.

This step-up is expensive (3$ for the controller alone, to say nothing of the related caps and inductor).  However, the series wiring of the LEDs allows the brightness/current of the LEDs to be more consistent.  Maintaining the step-up would also mean that I wouldn't have to re-wire my existing helmet.

Design the sleep mask PCB version 2

As I said above, once the sleep mask hardware is finalized, I'll source smaller components, and then redesign to the circuit board.  I figure it will be installed above the nose in the mask, with leads going down to the REM detector and red LEDs.  I'll also incorporate power control of the analog block, as I did for the helmet above, to reduce standby power consumption.

Order new boards + components; assemble + test

Just what it says: pick the new smaller components, order them and the boards, then build everything.

Software development of the sleep mask

At this point, the hardware for the sleep mask should be more-or-less finalized.  All that will be left is putting together the firmware.

I haven't given a lot of thought to the design of the interface or the overall design of the firmware.  However, I have given some thought to potential features I can try out, including:

• REM-relative wake-up alarm: only wakes you up if you are at the tail end of an REM cycle (or you've reached some no-later-then-this time).  I haven't checked the science behind this, but I've heard that you wake up more refreshed if you wake up at the end of a cycle, rather than during the deep sleep in the middle.
• Sleep induction using entrainment: again, I haven't looked into how rigorous the science is behind this is, but some advocate the use of binaural beats or isochronous pulses to induce lower-frequency EEG states, assisting the user away from consciousness.
• External cues for lucid dreaming induction: the original purpose.
• Slowly increasing LED illumination to ease wake-up: just what I said, improve wake-up by gradually increasing illumination within the mask along with the natural dawn.
• REM logging: save the timestamps of REM periods, allow them to be transferred over USB.  Share on facebook?
• USB bootloader: I know that Atmel provides one, I just need to see where it's hosted and play with it.

Bicycle helmet complete (or, the afore-promised inconsequential blinking-light projects)

I have finally completed the bicycle helmet flasher project (that I added as an afterthought to the REM sleep mask PCB).  The design and fabrication of the controller module was outlined in a previous post; here I show the results of finally installing the lights into my existing helmet.


Additionally, the source has been slightly modified (available here); the device now has three buttons, one each for brightness of the forward and rear lights and the third setting the mode (off, both flashing, rear flashing with forward steady on).


The outside of the helmet is shown below; first from ahead, then from behind.  The LEDs are mounted within the hollows of the helmet, using folded-over staples as an initial anchor, then doused with two-part epoxy.





The helmet in action is shown below:

The helmet is shown from the inside below; the LEDs are wired in series.  The leads of the LEDs go through tough plastic strips which are anchored to the helmet with bent-up office staples; everything is further stabilized with a healthy helping of two-part epoxy.

The interface consists of three buttons adhered to the side of the helmet (visible in the video above).  Everything on the helmet should be water-proof (though the controller module and battery pack are not currently water-proofed, as they are very much prototypes).

Moving forward, I would want to optimize/shrink the controller module, and incorporate a lithium-polymer battery pack with USB charging; hopefully, this would allow the whole battery/controller module to fit within the hollows of the helmet (rather than having a tethered AA battery pack hanging off of the back, as it is currently)

Sleep mask prototype assembly and initial testing (or, now that the hardware's built, it's only 90% of the project left to do)

The next step in the development of the Sleep Mask was fabricating the 'final' prototype, and doing some initial testing to make certain that there were no shorts.

The front and back of the assembled board (with battery and display attached) are shown below.  The smallest surface-mount parts (including the charger and headphone amplifier ICs) were placed on syringe-applied solder paste and 'baked' into place using a cheap electric skillet; the remainder of the parts were applied manually, using a conventional iron (the reason for this roundabout assembly method is detailed in a previous post; long story short, I bought the wrong paste).

Being extra-cautious, I checked each of the solder joints on the tight-pitch ICs before adding the rest of the components.  Being extra-paranoid, I introduced cuts into the power traces, so that I could monitor current usage as I re-connected different subsystems (this is evident in the backside image).

Front of assembled board; display not yet mechanically fixed

Back of assembled board; display not fixed; connector to mask lights/sensor at bottom

With everything reconnected and no programming loaded into the controller, the device drew 4.6mA; with phones plugged in (and, again, no programming and thus no signal being output), the device drew 44.5mA.  Even with the tiny battery I have connected now (450mAh), this is low enough to allow for a full night's use on a charge; of course, this doesn't take into account the power used by the IR REM sensor illuminator, the display or the extra power which may be expended to generate actual sounds with the headphones.  However, power use appears to be dominated by the audio system, so I am not too concerned right now.

I connected the assembled board to the mask (shown twice below); additionally, the display is scotch-taped to the board to secure it mechanically (but reversibly so).

The only things left for the hardware are to fix the board and battery to the mask and to install the red LEDs in the eyecups (these will be used to flash at the user during alarm conditions).  I'll also need to install a header to allow for repeated programming.  All that's left for the software... is everything.

Or course, this is the roughest sort of prototype, meant to prove the concept and develop the basic REM detection algorithms and the framework of the eventual overall program architecture.  In addition to about a million changes to the overall mechanics of the mask (formed neoprene base? injection-molded face for the buttons/display), the main board itself would undergo a lot of beneficial changes, mostly to decrease its size.  As I've commented before, the components I've used are ones I have a stock of locally; as such, they are rated for far more current/voltage/power/dissipation than needed for the current application.  Additionally, the controller is the easy-to-hand-solder TQFP, rather than the absolute smallest package available.  As a result, I suspect that a future version of the device, with all the same functionality but reduced part size, could be as small as one quarter of the area of this version.

Project updates (or, trading a few milligrams of epidermis for a few milligrams of reflowed solder)

Due to travel (and actual, legitimate research), I've not been able to progress on these projects much in the last few weeks.  Additionally, getting the boards from Seeed took a little while (though it was worth it, 10 boards for 15$ is nothing to sneeze at; they're shown immediately below).

Today, I got back into things by trying out a little hot skillet reflow.  Going off of resources at SparkFun and this instructable, it seemed the cheapest method available to me.  To apply the paste, I didn't have the time, money or patience to do solder paste stencilling (shown in the previous links); so, I applied the paste manually, as at this site.  Unfortunately, I didn't realize that the paste formulations are different between stencil and syringe application; I loaded some stencil paste from Sparkfun (here) into a syringe and it was very tough to get it to come out.

One thing to be aware of with the stencil-type solder paste: it behaves a lot more like wet sand than any sort of easily-coaxed gel.  Syringe-type paste might behave a little better/differently.

In any event, I was able to reflow the majority of the components on the (hastily thrown together) helmet flasher board, shown below (apologies for the poor picture quality).  I have seen heating-element control boards for toaster ovens and skillets to get the perfect heat profile; in my case, cranking the thing up to max temp and waiting for the solder to turn shiny sufficed.  Note the blue wire fix; I forgot to connect the enable line for the step-up to a free pin on the controller.



Debugging revealed only two small errors in the reflow; two of the pins on the stepup controller were bridged (easily separated) and one of the resistors in the current controller didn't reflow (also easily rectified).  The step-up produces 'high voltage' (16.5V), the pots have all been manually set (one to set the high voltage level, the other two to set the maximum constant-current levels) and the controller talks to my programmer.

The next steps for this quick project are A) create a simple program for this thing, and B) assemble the in-helmet parts of the project (lights, switches and 2xAA battery pack installed, wiring routed).  There's also the more pie-in-the-sky goal of implementing the EL drivers (but I haven't quite sourced the transformers yet; not enough of my CFL bulbs have gone out yet).

Of course, just because the project has barely started doesn't mean I'm not already thinking about version 2; specifically, I'd want to implement the following changes:
A: source smaller components, with specs sized more appropriately for this project.
B: add a LiPolymer battery and charger circuit to allow the controller module to be more monolithic and allow it to be charged over micro USB.
C: figure out a better connector solution between the helmet and the controller; the 0.1" headers I'm using were chosen for inventory convenience.

Implementation of a supervised discretization algorithm (or, I'm almost certain that I had a reason for this when I started)

Oftentimes, you have a set of data and want to use it to make a prediction about the future (e.g., predicting tomorrow's weather, based on today's weather) or about an unobserved system (e.g., predicting the presence or absence of metal ores underground, based on the local magnetic field).

In order to make that prediction, we need to set up some sort of mathematical framework describing the possible relationships between the data and the prediction.  If we have first-principles knowledge of the system under question, we can make a model of the system and use the available data to set any free parameters the model has.  However, we often have no idea about the system between the data and the prediction.  In these cases, we need to propose a sufficiently complicated and unbiased model so that, after setting the model's parameters according to the observed prediction/data relationships, the model accurately reflects the unknown system between the data and the variables to be predicted.

There is an enormous literature describing many different structures for generic predictors.  However, many of these rely on the input data being discretely-valued; that is, instead of taking on any value in a continuous range (like a distance, 12m, 5.43m, 1m, 0.333205m), they can only take on a discrete number of values (like 'number of fingers' is an integer between 0 and 12, inclusive).  In order to leverage these discrete-input predictors, it is necessary to 'discretize' continuous inputs; that is, to assign ranges of values to a single class (e.g., temperatures 0 to 10 degrees are now '0', 10 to 25 are now '1', 25 to 40 are now '2', etc.).

There is a smaller literature describing and analyzing methods for chosing how to perform this discretization.  In the future, I might put together a post reviewing these methods (at least, from a layman's perspective, as I am not a member of the machine learning research community).  Here, I will share my implementation, in MATLAB, of the method created/communicated by Marc Boulle in 2006; this was the method I found the most compelling after performing a review of the discretization literature.

The implementation

The algorithm is described (and derived, and analyzed, and experimentally compared with other methods) in "Boulle, Marc (2006). MODL: A Bayes optimal discretization method for continuous attributes. Machine Learning, 65:131-165."  The algorithm consists of a criterion which allows one to compare different potential partitionings of the data and a method for attempting to find the best partitioning, based on this criterion.

My implementation uses two 'linked lists' (in quotation marks, because they are implemented as the MATLAB generic array data type, instead of a distinct linked-list data type).  The first list contains the data on the partition intervals in the data; this information includes the total number of instances of the sample data in the interval, the number of instances of the sample data from each output class in the data, and the identity of the first an last member of the data set in each interval.  The second list points to adjacent pairs of the intervals in the first list, and is sorted according to how much the criterion would be improved by the merger of the pointed-to intervals.  The algorithm proceeds by merging the 'best' pair of adjacent intervals, then updating the lists to reflect that merger.  The algorithm is called 'greedy', as it always chooses the most immediately obvious 'best' merger, even though that may not lead to a universally optimum solution.

My implementation of this algorithm (including some support functions) is included in this archive.

A quick test

Here's some example data I generated, along with the optimal discretization returned by the algorithm (as implemented in MATLAB).  The 1000 sample data points were drawn from equal-variance gaussian distributions whose means were different and determined by their output class value.  The 1000 sample data points are plotted below; the y-axis is an individual data point's continuous value, and the point's color corresponds to its output class value.  The horizontal lines show the edges of the  intervals determined by the algorithm; as you can see, the classes (colors) are well-segregated by the edges.  Intuition suggests that there would be four edges (separating the five output classes); in the case below, there is an additional edge separating the contentious transition region between the dark blue and cyan classes.

Project updates (or, why is it that the least interesting parts of a project make up most of the effort?)

In the last few weeks (since I tested out the OLED display), I've been getting all of the last little details in place to move forward with the sleep mask project.  Specifically, since the circuits have been finalized, I have been laying out the printed circuit board and making certain that I have all of the necessary components on hand (and putting together an order for those I do not).

The cheapest service I could find is Seeed Studio's Fusion PCB service.  For a mere 10$ you get 10 5x5cm boards; an extra 15$ gets you 5x10cm.

After completing the board layout (shown below), I found that it was more than 5x5cm.  It is larger than I had hoped, but still reasonable relative to the size of the sleep mask.  A large part of its... largeness... is due to the fact that I was designing based on the parts I already had in my inventory.  Those parts, in turn, were chosen to be usable across many projects; as a result, they are usually rated for much higher voltages, currents, and dissipated energies than are strictly necessary for this project.  This is okay for a prototype, but any future hardware revisions will involve specifying more appropriately-sized resistors and capacitors.

Since I was going to have to pay for an extra 5cm of board, I decided to make the best of it and add a circuit for a project I've had on the back burner for a while.  Specifically, I want to build some flashing lights into my bicycle helmet for safety; some of the lights will be ordinary LEDs, but eventually I want to build some EL wire into the helmet to give a real Vegas feel.  To do this, I need 'high voltage' (about 20V) to step up to 120V using transformers.  While I am still collecting the transformers (I take them out of burned-out CFL lightbulbs, as transformers or even bare magnetics of appropriate size have proven difficult to source), I am going to move forward with getting this board, including the 20V step-up and LED constant-current drivers, layed out and ordered.  It is also shown in the image below.

The REM sleep mask board is on the left; the microcontroller is in the middle, with the micro USB connector above, battery charger above right, buck converter right, REM detector bottom left, headphone amplifier left and OLED display top left.  The helmet flasher/boost board is on the right; boost top left, EL wire switches bottom left, microcontroller bottom right and LED drivers top right.

SPI OLED A-OK (or, I would like to apologize to my readers for the preceding title)

I've reached another milestone in the development of my sleep mask: the OLED module works.

This section of the project was relatively straightforward: basically nothing more complicated than establishing a serial connection to the device and starting it up appropriately.

The Hardware

This is identical to my original design, which was, in turn, copied from the module datasheet.  This is the same hardware as is sold by Adafruit Industries; I acquired mine from another source, without the breadboard-friendly PCB attached.  It has an on-board capacitor charge pump to provide the high-voltage (~7.5V) necessary to drive the OLED pixels.  The serial interface is identical to the Serial Peripheral Interface (SPI) with a Command/Data select line and a Reset line in addition to the usual Chip Select line.

The Software

The stripped-down testing firmware I used is posted here.  It's not much to look at; it just starts up the SPI on-chip peripheral and sends the necessary command bytes (while manipulating the control lines appropriately) to start up and activate the display module.  It then starts sending out data bytes to change what is shown on the display.  The commands sent are outlined in the module controller's data sheet (the Solomon Systech SSD1306).

The controller continuously updates the pixels in the display by reading from an internal display memory.  When data is written to the device, it is used to update the contents of this display memory.

The folks at Adafruit have also implemented some software to drive this display module.  My software is largely the same, with one noticeable difference.  The controller contains twice as much display memory as needed for the module (to make it capable of driving larger displays); when writing to this memory over the serial link, the memory is all written over in turn before restarting at the beginning.  The Adafruit software just writes zeros onto that second half of the driver memory; however, there is a command which allows you to set the limits of the memory to be written.  By setting this command to only write over the usable half of the memory, my software doesn't need to write the entire memory every time, only the half that is actually visible.  In this way, I don't need to devote as much of my computational resources to updating the display.

The Goods

The test hardware setup is shown below.  As usual, I used my oh-so-refined soldering and fabrication skills to gain access to the tiny pads on the end of the display's ribbon connector.  The connections are relatively simple; a few decoupling caps between power pins to ground, some pass-throughs for the serial link, and the two capacitors for the charge pump.

To prove that I actually got this to work, here is a short video of the device (and ATXMEGA controller) as power is applied; first the display is told to turn all the pixels on, then it displays from the display memory (which is, initially, full of noise; this is in contrast with the data sheet, which states that the RAM should be blanked after a reset cycle).  Then, the controller starts sending alternating frames of display data.

The firmware source containing the specification of the alphabet/symbols is here.

REM detection hardware, firmware and software tests (or, I honestly had some doubts that this would work so well)

On the REM-detecting, white-noise-generating, potato-julienne-ing project front, I have finalized the hardware (and toyed with the firmware) for the REM detector subsystem.

To recap this project: I am developing a sleep mask which will be able to detect the REM (rapid eye movement) phase of sleep, and wake the user up at the 'optimal' point in their sleep cycle.  Additionally, it will be able to record the timing and duration of REM sleep phases (potentially useful for improving sleep) and it will be able to generate white, pink or red noise through speakers at the ears to improve sleep.

I mocked up the hardware and some firmware which sends acquired REM detector samples over a serial channel; I also had to put together some software to acquire, interpret and plot this serial information.  The results of this effort have allowed me to finalize the hardware design for the REM detector subsystem.  As it stands, all I have left to finalize of the hardware is the display and the USB interface hardware; once these two things are nailed down, I can design and order the boards and fabricate the hardware, moving to the firmware-only phase of the project.

Finalized hardware

The hardware has been modified from the schematics I presented originally.  These changes are due primarily to two factors: the microcontroller has an internal gain (negating the need for a second external gain stage) and the desire to used a switched-emitter topology (that is, the illumination of the eye for REM detection will only be on for a small percent of the time to save power).

As seen in the schematic below (the left op-amp), the current output from between the phototransistors on the mask is fed into a transimpedance amplifier whose output is fed directly into an ADC pin on the microcontroller.  This single-ended signal can eventually be used to set the emitter amplitude.  This signal is also fed into the positive side of a differential ADC (with gain); the negative side is fed from a lowpassed PWM output.  This negative input can be used to bias the differential ADC channel to maximize dynamic range; the lowpass converts the oscillating PWM signal into a DC signal whose level is the rail voltage times the Duty Cycle of the PWM waveform.  The transimpedance amplifier feedback resistor is set to 40kOhm to maximize signal amplitude while preventing saturation under normal (and even some abnormal) usage conditions.

The driver circuitry has been significantly improved in this schematic.  Specifically, an op-amp is used to 'linearize' the current control.  Above, a sense resistor is used in negative feedback to set the current through the infrared emitter; the set point is determined by a lowpassed PWM fed through a voltage divider.  Without the 'linearization' of the sense resistor and negative feedback, the highly nonlinear nature of the emitter's current/voltage characteristic meant that only a few of the possible PWM output levels were 'useful' (that is, corresponding to levels of current we would want to drive our emitter with).

Additionally, the op-amp makes driving the emitter simpler; its high input impedance simplifies the design process for the lowpass-PWM easier.  Additionally, it makes it simple to add an enhancement N-FET to allow for switching the emitter on and off (by tying the control input of the op-amp to ground).

Firmware/Software for debugging

To debug the REM detector hardware, I needed to implement a firmware to sample the REM detector input channel and transmit that data to my laptop.  I also needed to create software on my computer to acquire and plot the transmitted serial data.  The firmware and software are contained in this zip archive.

The big questions I needed to answer were: what do I need to do to keep the differential channel biased appropriately, and how long does the emitter need to be on to ensure that the phototransistor signal is stable before sampling.

The firmware samples the single-ended and differential ADC channels.  It then updates the PWM bias setting on the negative input of the differential channel to keep the differential signal centered.  It then formats a couple of serial bytes according to the ADC data and sends them to the computer.

To plot this serial stream, I made a function in MATLAB that opens the serial port and continuously samples the incoming bytes, breaking up the stream into sequences, translating them into floating-point numbers and displaying them on the screen, as seen below.  The blue trace is the single-ended ADC channel, the green trace is the differential channel, and the red is a moving-average of the green.

I waggled my eyes at the beginning and middle of the plotted waveform; you can see that the signal is well-modulated by eye-waggling, which is necessary for detection of REM.

I used a USB oscilloscope (the Hantek DSO-2090, highly recommended) to check out the settling time for the phototransistor signal in response to switching the emitter.  At the end of the day, I established that a switched emitter could be timed to allow for detection of REM using the specified circuit, so I have finalized that circuit and am moving on to validating the USB and display subsystems.

Plotting bars, whiskers and bridges in MATLAB (or, too much time spent getting the spacing just right)

I needed to plot the mean and variance of some data, and indicate pairs of data points which were significantly different.  I searched and searched, but there was no easy plot function in my environment of choice (MATLAB), so I made one myself.

The look of the plot is shown below; basically, you've got clusters of mean+variance data (or whatever you want to represent with a bar and whisker).  Within each cluster, there are pairwise relationships you want to indicate.  Relationships can be further specified by a variable number of marks above the bridges.

The function is here.  It takes three arguments: the first two are (number of clusters)-by-(number of bars per cluster), and represent (respectively) the height of the bars and the length of the whiskers.  The third argument is (number of bars per cluster)-by-(number of bars per cluster)-by-(number of clusters) and indicates whether a bridge should be drawn between a pair of bars; a nonzero value indicates that a bar should be drawn, and integers greater than one indicate that extra indication (in the form of circular marks) should be made above the bridges.

It's not the prettiest, but it works and it got the job done well enough for me.  Besides, any fine-tuning is going to be done in your vector editor of choice, so all that's important is getting the actors arranged on the stage.

Reverse engineering a motion-capture file format (or, the answer to my prayers... a week ago)

So, in a surprising turn of events, I am posting about something that I actually did for my research.  Part of the work I do involves motion capture; I use cameras and strobes and markers affixed to bony landmarks on the rat hindlimb to record the motion of the limb in space during behavior.  One of the motion capture files that I recorded was corrupted with noise, and could not be un-corrupted using the programs and tools from the system manufacturer.  Being the clever and industrious fellow that I am (read: I didn't want to do the analysis that I actually had scheduled), I spent a day to completely reverse-engineer the motion capture data file format and use that knowledge to create a program which completely removed the corruption from the file in question and allowed normal data analysis to occur.

The Problem

As I said above, part of the analysis I am doing for my research involves recording the kinematics of hindlimb locomotion.  The system that the lab purchased to get this data is passive and camera-based; that is, bits of shiny stuff (markers) are affixed to the subject and illuminated, and the grayscale images of the shiny stuff are used to infer the location of bony landmarks in space over time.

Since the shiny stuff is so shiny, it's usually easy to set a threshold on the grayscale images to get rid of non-marker sources in the image.  However, sometimes there is something else similarly shiny in the image; in those cases you 'mask' the offending pixels (always setting them to zero).  Of course, that also means that, if a legitimate marker moves into the masked area, it will not be detected.

My situation was that I had masked the offending reflections in the image... but then had to shift the treadmill around a bit.  As a result, some large reflections were present in the data.  They were such that the post processing (converting the grayscale images into labeled 3-D trajectories) just wasn't working; bits of whiteness from the reflections were being erroneously labeled.

Unfortunately, the system we use does not have a native facility for re-masking data after it's been recorded. So, I needed to roll my own.  To do this, I needed to understand the native data file format.

DISCLAIMER: The system we use is the Vicon Nexus.  This is NOT RECOMMENDED by them.  DO NOT USE THIS INFORMATION TO DO ANYTHING.  In fact, stop reading now.  I make no guarantees as to the usability or safety of the software provided here.

The Solution

I opened my my trusty hex editor (HxD by Maël Hörz) and took a look at a couple of the raw data files.  Long story short, they all shared almost identical initial segments (the first 770 bytes, specifically) which I assume are header information and contain ASCII sub-strings with the camera type and specs in plain text.  There were also two 4-byte-long sections of this header which described A: the number of images frames in the file and B: the offset (number of bytes from the beginning of the file) at which the 'index' began.  This header was followed by the second section (the largest by far) which contained the grayscale image and blob center data.  The final section was the 'index', which contained a series of 12-byte-long records describing the frame numbers (first four bytes) and the offset at which each frame began in the file (last eight bytes).

Sections of the data segment were arranged hierarchically; each object on a given level started with two bytes of 'start sentinel', four bytes describing the length of the object, and four bytes giving some other important number (e.g., camera number, number of blobs in a frame, number of grayscale scan lines in a blob).  The top level for each frame was, well, the frame; that is, all of the data taken during one sample.  Below the frame level was the camera subframe level; each of those contained the data for the given frame from one of the cameras.

A bit of indirection below the camera subframe, and we come to the meat of the file: the grayscale image data.  Each subframe specifies how many bytes long it is, and how many 'blobs' it contains.  Blobs are just contiguous sections of non-zero in the grayscale image.  Each blob then specifies how many bytes it contains, and how many horizontal scan lines of grayscale data it is made up of.  These lines then specify the X- and Y-coordinates of their left-most pixel, how many pixels long they are, and then proceed to actually post the grayscale data.  Using the file read and write commands makes traversing this hierarchy simpler, because the file pointer helps to keep track of where you are.

I keep things vague, because I don't want to ruin the fun for anyone else, and because I am a coward.

The Goods

Using my detailed notes on the structure of the data file and its many headers and start sentinel codes, I implemented several useful functions to make it possible to quickly and painlessly re-mask my data.  These functions are included in this .zip archive; the files are MATLAB m-files and use mostly generic, easily-ported syntax.  One of the functions uses the MATLAB sparse matrix data type; I have kept the non-sparse version of the code in the comments, so porting should be straightforward.

I reiterate from above: DO NOT USE THIS UNLESS YOU KNOW WHAT YOU ARE DOING.  This is in no way endorsed by Vicon.  Always make a backup.  Et cetera.  Contact me if you have concerns and absolutely need to re-mask some bad Vicon data.  I post this only in the spirit of giving, in the hope that someone in the future, in the same spot that I was in a week ago, will be helped by my efforts.

The functions are as follows:

• extractFrameIndices: This function extracts the offsets for all of the frames in the record.
• makeSparseFrameRaster: This function extracts a specified frame from the record, for viewing.
• remaskInPlace: This function dances through the record, zeroing out all of the pixels in the record the user desires
• testFunc: An example masking function that I used to test this out (also, coincidentally, exactly the masking function that I needed applied to my data)

The re-masking function is designed to be as mutable as possible; the form of the masking can be as complicated as the user desires, since a handle (MATLAB version of pointer) to the masking function is passed to remaskInPlace, rather than parameters defining a restricted domain of masks.  The masking function receives as inputs from the calling function the X- and Y-location of the pixels in question as well as the index of the current frame, allowing the masks to be functions of time as well as space.  Additionally, user-specified parameters can be passed transparently through remaskInPlace to the masking function.