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----
-title: "32-Channel LED tape driver"
-date: 2018-05-02T11:31:14+02:00
----
-
-Theoretical basics
-==================
-
-Together, a friend and I outfitted the small staircase at Berlin's Chaos Computer Club with nice, shiny RGB-WW LED tape
-for ambient lighting. This tape is like regular RGB tape but with an additional warm white channel, which makes for much
-more natural pastels and whites. There are several variants of RGBW tape. Cheap ones have separate RGB and white LEDs,
-which is fine for indirect lighting but does not work for direct lighting. Since we wanted to mount our tape in channels
-at the front of the steps, we had to use the slightly more expensive variant with integrated RGBW LEDs. These are LEDs
-in the 5050 (5.0mm by 5.0mm) form factor common with RGB LEDs that have a small section divided off for the white
-channel. The red, green and blue LED chips sit together in the larger section covered with clear epoxy and the white
-channel is made up from the usual blue LED inside a yellow phosphor in the smaller section.
-
-Since we wanted to light up all of 15 steps, and for greatest visual effect we would have liked to be able to control
-each step individually we had to find a way to control 60 channels of LED tape with a reasonable amount of hardware.
-
-LED tape has integrated series resistors and runs off a fixed 12V or 24V constant-voltage supply. This means you don't
-need a complex constant-current driver as you'd need with high-power LEDs. You can just hook up a section of LED tape
-to a beefy MOSFET to control it. Traditionally, you would do *Pulse Width Modulation* (PWM) on the MOSFET's input to
-control the LED tape's brightness.
-
-Pulse Width Modulation
-----------------------
-
-`Pulse Width Modulation`_ is a technique of controlling the brightness of a load such as an LED with a digital signal.
-The basic idea is that if you turn the LED on and off much too fast for anyone to notice, you can control its power by
-changing how long you turn it on versus how long you leave it off.
-
-PWM divides each second into a large number of periods. At the beginning of each period, you turn the LED on. After
-that, you wait a certain time until you turn it off. Then, you wait for the next period to begin. The periods are always
-the same length but you can set when you turn off the LED. If you turn it off right away, it's off almost all the time
-and it looks like it's off to your eye. If you turn it off right at the end, it's on almost all the time and it looks
-super bright to your eye. Now, if you turn it off halfway into the cycle, it's on half the time and it will look to your
-eye as half as bright as before. This means that you can control the LED's brightness with only a digital signal and
-good timing.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/pwm_schema.jpg" alt="A visualization of PWM at different duty cycles.">
-        <figcaption>Waveforms of two PWM cycles at different duty cycles.</figcaption>
-    </figure>
-
-PWM works great if you have a dedicated PWM output on your microcontroller. It's extremely simple in both hardware and
-software.  Unfortunately for us, controlling 32 channels with PWM is not that easy. Cheap microcontrollers only have `a
-handful of hardware PWM outputs`_, so we'd either have to do everything in software, bit-banging our LED modulation, or
-we'd have to use a dedicated chip.
-
-Doing PWM in software is both error-prone and slow.  Since the maximum dynamic range of a PWM signal is limited by the
-shortest duty cycle it can do, software PWM being slow means it has poor PWM resolution at maybe 8 bits at most.  Poor
-color resolution is not a problem if all you're doing is to fade around the `HSV rainbow`_, but for ambient lighting
-where you *really* want to control the brightness down to a faint shimmer you need all the color resolution you can get.
-
-If you rule out software PWM, what remains are dedicated `hardware PWM controllers`_. Most of these have either of three
-issues:
-
-* They're expensive
-* They don't have generous PWM resolution either (12 bits if you're lucky)
-* They're meant to drive small LEDs such as a 7-segment display directly and you can't just hook up a MOSFET to their
-  output
-
-This means we're stuck in a dilemma between two poor solutions if we'd want to do PWM. Luckily for us, PWM is not the
-only modulation in town.
-
-.. _`Pulse Width Modulation`: https://en.wikipedia.org/wiki/Pulse-width_modulation
-.. _`a handful of hardware PWM outputs`: https://www.nxp.com/parametricSearch#/&c=c731_c380_c173_c161_c163&page=1
-.. _`HSV rainbow`: https://en.wikipedia.org/wiki/HSL_and_HSV
-.. _`hardware PWM controllers`: http://www.ti.com/lit/ds/symlink/tlc5940.pdf
-
-Binary Code Modulation
-----------------------
-
-PWM is the bread-and-butter of the maker crowd. Everyone and their cat is doing it and it works really well most of the
-time. Unbeknownst to most of the maker crowd, there is however another popular modulation method that's mostly used in
-professional LED systems: Enter `*Binary Code Modulation* (BCM) <http://www.batsocks.co.uk/readme/art_bcm_1.htm>`_.
-
-BCM is to PWM sort of what barcodes are to handwriting. While PWM is easy to understand and simple to implement if all
-you have is a counter and an IO pin, BCM is more complicated. On the other hand, computers can do complicated and BCM
-really shines in multi-channel applications.
-
-Similar to PWM, BCM works by turning on and off the LED in short periods fast enough to make your eye perceive it as
-partially on all the time. In PWM the channel's brightness is linearly dependent on its duty cycle, i.e. the percentage
-it is turned on. In PWM the duty cycle D is the total period T divided by the on period T_on. The issue with doing PWM
-on many channels at once is that you have to turn off each channel at the exact time to match its duty cycle.
-Controlling many IO pins at once with precise timing is really hard to do in software.
-
-BCM avoids this by further dividing each period into smaller periods which we'll call *bit periods* and splitting each
-channel's duty cycle into chunks the size of these bit periods. The amazingly elegant thing in BCM now is that as you
-can guess from the name these bit periods are weighted in powers of two. Say the shortest bit period lasts 1
-microsecond. Then the second-shortest bit period is 2 microseconds and the third is 4, the fifth 8, the sixth 16 and so
-on.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/bcm_schema.jpg" alt="A visualization of BCM at different duty cycles.">
-        <figcaption>Waveforms of a single 4-bit BCM cycle at different duty cycles. This BCM can produce 16 different
-        levels.</figcaption>
-    </figure>
-
-Staggered like this, you turn on the LED for integer value of microseconds by turning it on in the bit periods
-corresponding to the binary bits of that value. If I want my LED to light for 19 microseconds every period, I turn it on
-in the 16 microsecond bit period, the 2 microsecond bit period and the 1 microsecond bit period and leave it off for the
-4 and 8 mircosecond bit periods.
-
-Now, how this is better instead of just more complicated than plain old PWM might not be clear yet. But consider this:
-Turning on and off a large number of channels, each at its own arbitrary time is hard because doing the timing in
-software is hard. We can't use hardware timers since we only have two or three of those, and we have 32 channels.
-However, we can use one hardware timer to trigger a really cheap external latch to turn on or off the 32 channels all at
-once. With this setup, we can only controll all channels at once, but we can do so with very precise timing.
-
-All we need to do is to set our timer to the durations of the BCM bit periods, and we can get the same result as we'd
-get with PWM with only one hardware timer and a bit of code that is not timing-critical anymore.
-
-Applications of Binary Code Modulation
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-BCM is a truly wondrous technique, and outside of hobbyist circles it is in fact very widely known. Though we're using
-it to control just 32 channels here, you can do much more channels without any problems. The most common application
-where BCM is invariably used is *any* kind of LED screen. Controlling the thousands and thousands of LEDs in an LED
-screen with PWM with a dedicated timer for each LED would not be feasible. With BCM, all you need to dedicate to a
-single LED is a flipflop (or part of one if you're multiplexing). In fact, there is a whole range of `ICs with no other
-purpose than to enable BCM on large LED matrices <http://www.vabolis.lt/stuff/MBI5026.pdf>`_. Basically, these are a
-high-speed shift register with latched outputs much like the venerable 74HC595_, only their outputs are constant-current
-sinks made so that you can directly connect an LED to them.
-
-.. _74HC595: http://www.ti.com/lit/ds/symlink/sn74hc595.pdf
-
-Running BCM on LED tape
-~~~~~~~~~~~~~~~~~~~~~~~
-
-In our case, we don't need any special driver chips to control our LED tape. We just connect the outputs of a 74HC595_
-shift register to one MOSFET_ each, and then we directly connect the LED tape to these MOSFETs. The MOSFETs allow us to
-drive a couple of amps into the LED tape from the weak outputs of the shift register.
-
-The BCM timing is done by hooking up two timer channels of our microcontroller to the shift registers *strobe* and
-*reset* inputs. We set the timer to PWM mode so we can generate pulses with precise timing. At the beginning of each
-bit period, a pulse will strobe the data for this bit period that we shifted in previously. At the end of the bit
-period, one pulse will reset the shift register and one will strobe the freshly-reset zeros into the outputs.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/olsndot_output_schematic.jpg" alt="From left to right, we see the STM32, one of the shift
-        registers, and the LEDs and MOSFETs. The LED tape is driven to ground by the MOSFETs, which are in turn directly
-        driven from the shift register outputs. The shift register is wired up to the STM32 with its clock and data
-        inputs on SCK and MOSI and its RESET and STROBE inputs on channel 2 and 3 of timer 1.">
-        <figcaption>
-            The schematic of a single output of this LED driver. Multiple shift register stages can be cascaded.
-        </figcaption>
-    </figure>
-
-
-Our implementation of this system runs on an STM32F030F4P6_, the smallest, cheapest ARM microcontroller you can get from
-ST. This microcontroller has only 16kB of flash and 1kB of RAM, but that's plenty for our use. We use its SPI controller
-to feed the modulation data to the shift registers really fast, and we use two timer channels to control the shift
-registers' reset and strobe.
-
-We can easily cascade shift registers without any ill side-effects, and even hundreds of channels should be no problem
-for this setup. The only reason we chose to stick to a 32-channel board is the mechanics of it. We thought it would be
-easier to have several small boards instead of having one huge board with loads of connectors and cables coming off it.
-
-The BOM cost per channel for our system is 3ct for a reasonable MOSFET, about 1ct for one eighth of a shift register
-plus less than a cent for one resistor between shift register and MOSFET. In the end, the connectors are more expensive
-than the driving circuitry.
-
-.. _MOSFET: https://en.wikipedia.org/wiki/MOSFET
-.. _STM32F030F4P6: http://www.st.com/resource/en/datasheet/stm32f030f4.pdf
-
-Hardware design
-===============
-
-From this starting point, we made a very prototype-y hardware design for a 32-channel 12V LED tape driver. The design is
-based on the STM32F030F4P6_ driving the shift registers as explained above. The system is controlled through an RS485_
-bus that is connected up to the microcontroller's UART using an MAX485_-compatible RS485 transceiver. The LED tape is
-connected using 9-pin SUB-D_ connectors since they are cheap and good enough for the small current of our short segments
-of LED tape. The MOSFETs we use are small SOT-23_ logic-level MOSFETs. In various prototypes we used both International
-Rectifier's IRLML6244_ as well as Alpha & Omega Semiconductor's AO3400_. Both are good up to about 30V/5A.  Since we're
-only driving about 2m of LED tape per channel we're not going above about 0.5A and the MOSFETs don't even get warm.
-
-.. _RS485: https://en.wikipedia.org/wiki/RS-485
-.. _MAX485: https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf
-.. _IRLML6244: https://www.infineon.com/dgdl/?fileId=5546d462533600a4015356686fed261f
-.. _AO3400: http://aosmd.com/pdfs/datasheet/AO3400.pdf
-.. _SUB-D: https://en.wikipedia.org/wiki/D-subminiature
-.. _SOT-23: http://www.nxp.com/documents/outline_drawing/SOT23.pdf
-
-Switching nonlinearities
-------------------------
-During testing of our initial prototype, we noticed that the brightness seemed to jump around when fading to very low
-values. It turned out that our extremely simple LED driving circuit consisting of only the shift register directly
-driving a MOSFET, which in turn directly drives the LED tape was maybe a little bit too simple. After some measurements
-it turned out that we were looking at about 6Vpp of ringing on the driver's output voltage. The picture below is the
-voltrage we saw on our oscilloscope on the LED tape.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about
-            100% overshoot during about 70% of the cycle time.">
-        <figcaption>Bad ringing on the LED output voltage caused by wiring inductance. Note that the effect on the
-        actual LED current is less bad than this looks since the LED's V/I curve is nonlinear.</figcaption>
-    </figure>
-
-
-Dynamic switching behavior: Cause and Effect
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-A bit of LTSpice_ action later we found that the inductance of the few metres of cable leading to the LED tape is the
-likely culprit. The figure below is the schematic used for the simulations.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/driver_output_ltspice_schematic.jpg" alt="The LTSpice schematic of one output of the driver,
-        taking into account the shift register's output ESR and the wiring ESL.">
-        <figcaption>The schematic of the simulation in LTSpice</figcaption>
-    </figure>
-
-As tested, the driver does not include any per-output smoothing so the ~.5A transient on each BCM cycle hits the cable
-in full.  Combined with the cable inductance, this works out to a considerable lag of the rising edge of the LED
-current, and bad ringing on its falling edge. Below is the voltage on the LED output from an LTSpice simulation of our
-driver.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/overshoot_sim_r0.svg" alt="The result of the LTSpice simulation of our driver output. The LED
-        current shows similar ringing to what we measured using the oscilloscope. Interestingly, the gate voltage shows
-        strong ringing, too.">
-        <figcaption>The result of our LTSpice simulation. This simulation assumes 1µH of wiring inductance and 50Ω of
-        output impedance on the part of the shift register. The ringing at the gate visible in the gate voltage graph is
-        due to feed-through of the ringing at the output through the MOSFET's parasitic Cgd.</figcaption>
-    </figure>
-
-We were able to reduce the rining and limit the effect somewhat by putting a 220Ω series resistor in between the shift
-register output and the MOSFET gate. This resistor forms an RC circuit with the MOSFET's nanofarad or two of gate
-capacitance. The result of this is that the LED current passing the wire's ESL rises slightly more slowly and thus the
-series inductance gets excited slightly less, and the overshoot decreases. Below is a picture of the waveform with the
-damping resistor in place and a picture of our measurement for comparison. The resistor values don't agree perfectly
-since the estimated ESL and stray capacitance of the wiring is probably way off.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30%
-        overshoot during about 20% of the cycle time.">
-        <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition damped the ringing somewhat,
-        but ultimately it cannot be eliminated entirely. Note how you can actually see the miller plateau on the
-        trailing edge of this signal.
-        </figcaption>
-    </figure>
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/overshoot_sim_r100.svg" alt="The result of the LTSpice simulation of our driver output with an
-        extra 100 Ohms between shift register output and MOSFET gate. Similar to the oscilloscope measurement the
-        ringing is much reduced in its amplitude.">
-        <figcaption>The LTSpice simulation result with the same parameters as above but with an extra 100Ω between the
-        shfit register's output and the MOSFET's gate.</figcaption>
-    </figure>
-
-A side effect of this fix is that now the effective on-time of the LED tape is much longer than the duty cycle at the
-shift register's output at very small duty cycles (1µs or less). This is caused by the MOSFET's `miller
-plateau`_. For illustration, below is a graph of both the excitation waveform (the boxy line) and the resulting LED
-current (the other ones) both without damping (top) and with 220Ω damping (bottom). As you can see the effective duty
-cycle of the LED current is not at all equal to the 50% duty cycle of the excitation square wave.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/asymmetric_iled.svg" alt="The result of an LTSpice simulation of the LED duty cycle without and
-        with damping. Dampening widens the LED current waveform from 50% duty cycle with sharp edges to about 80% duty
-        cycle with soft edges.">
-        <figcaption>Simulated LED duty cycle with and without damping. The damping resistance used in this simulation
-        was 220Ω.</figcaption>
-    </figure>
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/asymmetric_vgate.svg" alt="The gate voltages in the spice simulation above. The undamped
-        response shows sharp edges with the miller plateau being a barely noticeable step, but with strong ringing on
-        the trailing edge. The damped response shows RC-like slow-edges, but has wide miller plateaus on both edges
-        adding up to about 50% of the pulse width.">
-        <figcaption>The MOSFET gate voltage from the simulation in the figure above. You can clearly see how the miller
-        plateau (the horizontal part of the trace at about 1V) is getting much wider with added damped, and how the
-        resulting gate charge/discharge curve is not at all that of a capacitor anymore.</figcaption>
-    </figure>
-
-
-
-In conclusion, we have three major causes for our calculated LED brightness not matching reality:
-
-* Ringing of the equivalent series inductance of the wiring leading up to the LED tape
-* Miller plateau lag
-* The damping resistor and the MOSFET gate forming an RC filter that helps with wire ESL ringing but worsens the miller
-  plateau issue and deforms the LED current edges.
-
-Added up, these three effects yield a picture that agrees well with our simulations and measurements. The overall effect
-is neglegible at long period durations (>10µs), but gets really bad at short period durations (<1µs). The effect is
-non-linear, so correcting for it is not as simple as adding an offset.
-
-.. _LTSpice: http://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html
-.. _`miller plateau`: https://www.vishay.com/docs/68214/turnonprocess.pdf
-
-Measuring LED tape brightness
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-In order to correct for the nonlinearities mentioned above, we decided to implement a lookup table mapping BCM period to
-actual timer setting. That is, each row of the table contains the actual period length we need to set the
-microcontroller's timer to in order to get our intended brightness steps.
-
-To calibrate our driver, we needed a setup for reproducible measurement of the relative brightness of our LED tape at
-different settings. Absolute brightness is not of interest to us as the eye can't perceive it. To perform the
-calibration, the LED driver is set to enable each single BCM period in turn, i.e. brightness values 1, 2, 4, 8, 16 etc.
-
-The setup we used to measure the LED tape's brightness consists of a bunch of LED tape stuck into a tin can for
-shielding against both stray light and electromagnetic interference and a photodiode looking at the LED tape. We used
-the venerable BPW34_ photodiode in our setup as I had a bunch leftover from another project and because they are quite
-sensitive owing to their physically large die area.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/linearization_setup.jpg" alt="The led measurement setup consists of several PCBs and a
-        breadboard linked with a bunch of wires and a big tin can to shield the LEDs and the photodiode. A large sub-D
-        connector is put into the top of the tin can as a feed-through for the LED tape's control signals and the
-        photodiode signal. In the background the control laptop is visible.">
-        <figcaption>The LED brighness measurement setup. The big tin can contains a bunch of LED tape and the
-        photodiode. The breadboard on the right is used for the photodiode preamplifier and for jumpering around the LED
-        tape's channels. The red board next to it is the buspirate used as ADC. The board on the bottom left is a
-        TTL-to-RS485 converter and the board in the middle is the unit under test.</figcaption>
-    </figure>
-
-The photodiode's photocurrent is converted into a voltage using a very simple transimpedance amplifier based around a
-MCP6002_ opamp that was damped into oblivion with a couple nanofarads of capacitance in its feedback loop. The MCP6002_
-is a fine choice here since I had a bunch and because it is a CMOS opamp, meaning it has low bias current that would
-mess up our measurements. For many applications, opamp bias current is not a big issue but when using the opamp to
-directly measure very small currents at its input it quickly swamps out the signal for most BJT-input types.
-
-The transimpedance amplifier's output is read from the computer using the ADC input of a buspirate USB thinggamajob. In
-general I would not recommend the buspirate as a tool for this job since it's ADC is not particularly good and it's
-programming interface is positively atrocious, but it was what I had and it beat first wiring up one of the dedicated
-ADC chips I had in my parts bin.
-
-The computer runs a small python script cycling the LED tape through all its BCM period settings and taking a brightness
-measurement at each step. Later on, these measurements can be plotted to visualize the resulting slope's linearity, and
-we can even do a simulation of the resulting brightness for all possible control values by just adding the measured
-photocurrents for a certain BCM setpoint just as our retinas would do.
-
-.. raw:: html
-    
-    <figure>
-        <img src="images/driver_linearity_raw.svg" alt="">
-        <figcaption>
-            A plot of the measured brightness of our LED tape for each BCM period. The brightness values are normalized
-            to the value measured at the LSB setpoint (brightness=1/65535). Ideally, this plot would show a straight
-            line with slope 1. Obviously, it doesn't. The bend in the curve is caused by the above-mentioned duty cycle
-            offset adding an offset to all brightness values. Shown is both the raw data (light), which has essentially zero
-            measurement error and a linear fit (dark).
-
-            The plot is in log-log to approximate how the human eye would perceive brightness, i.e. highly sensitive at
-            low values but not very sensitive at all at large values.
-        </figcaption>
-    </figure>
-
-While it would be possible to fully automate the optimization of BCM driver lookup tables, we needed only one and in the
-end I just sat down and manually tweaked the ideal values we initially calculated until I liked the result. You can see
-the resulting brightness curve below.
-
-.. raw:: html
-    
-    <figure>
-        <figure class="side-by-side">
-            <img src="images/uncorrected_brightness_sim.svg" alt="">
-            <figcaption>
-                Calculated brightness curve for the uncorrected BCM setup. As you can see, at low setpoints the result
-                is about as smooth as sandpaper, which is well in line with our observations. At high setpoints the
-                offset gets swamped out and the nonlinearity in the low bits is not visible anymore.
-            </figcaption>
-        </figure><figure class="side-by-side">
-            <img src="images/corrected_brightness_sim.svg" alt="">
-            <figcaption>
-                Brightness curve for the corrected BCM setup extrapolated using actual measurements. Looks as buttery
-                smooth in real life as it does in this plot.
-            </figcaption>
-            </figcaption>
-        </figure>
-    </figure>
-
-.. _BPW34: http://www.vishay.com/docs/81521/bpw34.pdf
-.. _MCP6002: http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf
-
-Controlling the driver
-----------------------
-
-Now that our driver was behaving linear enough that you couldn't see it actually wasn't we needed a nice way to control
-it from a computer of our choice. In the ultimate application (our staircase) we'll use a raspberry pi for this. Since
-we already settled on an RS485_ bus for its robustness and simplicity, we had to device a protocol to control the driver
-over this bus. Here, we settled on a simple, COBS_-based protocol for the reasons I wrote about in `How to talk to your
-microcontroller over serial <serial-protocols>`_.
-
-To address our driver nodes, we modified the Makefile to build a random 32-bit MAC into each firmware image. The
-protocol has only five message types:
-
-1. A 0-byte *ping* packet, to which each node would reply with its own address in the
-   first 100ms after boot. This can be used to initially discover the addresses of all nodes connected to the bus. You'd
-   spam the bus with *ping* packets, and then hit reset on each node in turn. The control computer would then receive
-   each device's MAC address as you hit reset.
-2. A 4-byte *address* packet that says which device that the following packet is for. This way of us using the packet
-   length instead of a packet type field is not particularly elegant, but our system is simple enough and it was easy to
-   implement.
-3. A 64-byte *frame buffer* packet that contains 16 bits of left-aligned brightness data for every channel
-4. A one-byte *get status* packet that tells the device to respond with...
-5. ...a 27-byte status packet containing a brief description of the firmware (version number, channel count, bit depth
-   etc.) as well as the device's current life stats (VCC, temperature, uptime, UART frame errors etc.).
-
-Wrapped up in a nice python interface we can now easily enumerate any drivers we connect to a bus, query their status
-and control their outputs.
-
-.. _COBS: https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing
-
-Conclusion
-----------
-
-.. raw:: html
-
-    <figure>
-        <figure class="side-by-side">
-            <a href="images/olsndot_schematic.png">
-                <img src="images/olsndot_schematic.png" alt="A picture of the LED driver schematic">
-            </a>
-            <figcaption>The LED driver <a href="images/olsndot_schematic.png">schematic</a></figcaption>
-        </figure><figure class="side-by-side">
-            <a href="images/olsndot_pcb.png">
-                <img src="images/olsndot_pcb.png" alt="A picture of the LED driver PCB layout">
-            </a>
-            <figcaption>The LED driver <a href="images/olsndot_pcb.png">PCB layout</a></figcaption>
-        </figure>
-    </figure>
-
-Putting some thought into the control circuitry and software, you can easily control large numbers of channels of LEDs
-using extremely inexpensive driving hardware without any compromises on dynamic range. The design we settled on can
-drive 32 channels of LED tape with a dynamic range of 14bit at a BOM cost of below 10€. All it really takes is a couple
-of shift registers and a mildly bored STM32 microcontroller.
-
-Get a PDF file of the schematic and PCB layout `here <olsndot_v02_schematics_and_pcb.pdf>`__ or download the CAD files
-and the firmware sources `from github <https://github.com/jaseg/led_drv>`_. You can view the Jupyter notebook used to
-analyze the brightness measurement data `here <http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Run_analysis.ipynb>`__.
-