Cyclops is a high-power LED driver that enables precise control of light power for optogenetic stimulation. The circuit was developed by Jon Newman while in Steve Potter's lab at Georgia Tech in order to complete his thesis work, which required the delivery of ultra-precise, continuously time-varying light waveforms for optogenetic stimulation [1,2]. This was, and still is, not possible with commercial hardware for optogenetic stimulation. Since its first use, the circuit has been improved in terms of speed, precision, programmability, and ease of use. This document provides construction, usage, and performance documentation for the Cyclops LED driver. This document evolves with the repository. To view old revisions, checkout tags or old commits using their SHA.
Note Github does not render alt text specified in Markdown figures as captions. Therefore, if you are viewing this document on Github, you will need to hover over figures to see their captions.
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Contributors
Table of Contents
- Attribution
- Features
- Performance Specifications
- Usage
- Construction
- LED
- Quality Control Procedure
- License
- References
It has been a long road to design and test the Cyclops to the point where it is in active use in many neuroscience labs around the world. This process has been a lot of work but also a very rewarding learning experience. I am very happy that this device may enable your scientific endeavours and I hope it will eventually be one small module in a of growing set of high-quality, open-source, and afforable tools that facilitate your research and enable an open, community-oriented approach to neuroscience.
I receive no monetary compensation from the sale of these devices. It would mean a great deal to me if you would consider referencing the following paper (for which the Cyclops was developed) in published work that makes use of the Cyclops.
J.P. Newman, M.-f. Fong, D.C. Millard, C.J. Whitmire, G.B. Stanley, S.M. Potter. S.M. Potter. Optogenetic feedback control of neural activity. eLife (4:e07192) 2015. doi: 10.7554/eLife.07192 [link]
For instance, in your methods section:
Optical stimuli were delivered using the Cyclops LED driver (Newman et al., 2015; www.github.com/jonnew/Cyclops).
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- Ultra-precise
- High power
- Up to 1.5A per LED
- Wide bandwidth
- ~2.5 MHz -3 dB bandwidth
- Maximum 200 ns 1.0A rise-time
- Current and optical feedback modes
- Built-in waveform generation
- Over-current protection
- Modular
- Arduino compatible
- 4 synchronizable optical channels
- Accepts external analog, gate, or trigger inputs
- External stimulus sequencer
- External digital trigger
- TTL logic level
- External analog waveform generator
- 0-5V analog signals
- Internal 12-bit DAC
- Synchronized across up to 4 drivers
- Powerful Arduino library
- Programmable triggering logic
- Respond to USB input
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The following oscilloscope traces give indicates of the circuit's precision and speed. Note that time series traces are not averaged - these traces display per-pulse temporal characteristics. Optical characteristics and optical feedback signal for the Cyclops driver were provided by a Thorlabs PDA36 amplified photodiode set to 0 dB of transimpedance gain. Measurements were performed a single Osram golden dragon LED.
The following traces are the same as the previous ones except that the amplified photodiode was used to provide optical feedback. The slowdown compared to current feedback is due to a speed of the photodiode. A faster amplified photodiode would provide crisper rise and fall times
The current-feedback mode -3dB bandwidth was determined by applying a
flat noise signal over 50 MHz with mean = 1.0V and Vpp = 500 mV into the
EXT port with maximal current gain. It occurs at around 2.5 MHz.
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The cyclops is a device that is capable of transforming voltage signals (e.g. sine waves, square pulses, etc.) into optical signals from high-power LEDs. Voltage signals to drive the device can be generated internally using an on-board digital to analog converter or can be delivered from an external source, such as a function generator or stimulus sequencer. The cyclops provides numerous measurements of circuit operation that can be recorded during an experiment such as LED current and stimulus reference voltages. The device can be controlled over a USB interface using its Arduino library. The device also can be configured to drive commercially available LED modules from Thorlabs and Doric.
Below we provide an explanation of the operational modes of the device and the different ways it can be used to generate optical stimuli. Refer to the above diagram to locate the physical switches, dials, and connectors corresponding to verbal or iconic descriptions device settings.
To use current feedback mode, push the F.B. MODE slide switch to the
CURR position (
). Using the circuit in current
feedback mode ensures that the forward current across the LED is
precisely regulated according the voltage at the VREF pin. This
configuration is a standard method for driving LEDs because the
relationship between current and LED irradiance is smooth and monotonic.
This means that more current across the LED will generate more light
power (while staying within the LED's maximum ratings, of course).
However, the relationship between current and irradiance is not linear.
For most LEDs, it looks like a logarithmic function. Additionally, the
efficiency of the LED is inversely related to its temperature. So, as
the LED operates and heats up, the amount of light it produces drops
even when the current is held constant. The severity of an LED's
temperature dependence and current/irradiance nonlinearity depend on the
type of LED (roughly, the color and who made it). These properties
should be clearly documented in the LED's data sheet. With a quality LED
and proper thermal management, the effects of temperature and static
current/irradiance nonlinearity are fairly minimal and can be ignored in
most situations.
To use auxiliary feedback mode, push the F.B. MODE slide switch to the
AUX position (
). When extremely stable, linear
control of light power is required, the auxiliary feedback input can be
used to used to compensate for the temperature dependence and static
nonlinearity of the current/irradiance relationship of the LED. For
example, when the auxiliary voltage is supplied by an amplified
photodiode that is somewhere indecent to radiation from the LED, or is
sampled from the fiber transporting LED light, the gate voltage is
adjusted such that the measured light power matches a DAC-supplied
reference voltage. This is the case in the circuit diagram. This
configuration is referred to as optical feedback mode. The
PDA36A
adjustable amplified photodiode from Thorlabs is a good option for
supplying optical feedback. However, you can make your own amplified
photodiode for a fraction of the price, and a design is included within
the cyclops repository. Optical feedback completely linearizes the
relationship between a supplied reference voltage and the light power
produced by the LED by compensating for the current/irradiance
nonlinearities and temperature dependence.
There are three ways to generate light signals using the driver. The behavior of each of these options is dependent on the feedback mode being used. The behavior of each input option is described in relation to the feedback mode of the driver.
The test button is
always available and will override all other input modes. Using the
TESTbutton the behavior of the circuit is:
Source
the current specified by the MAX CURR.dial.
Generate
the optical power specified by the h * mW level that is specified
by the MAX POWERdial. The intensity of the LED will be dependent on the auxiliary feedback signal used which defines the 'h' parameter.
External input mode is
engaged when the SOURCEswitch is moved to theEXTposition and user supplied voltage waveforms are present at theEXTBNC input. If the user attempts to supply more than 5V to theEXTinput, the circuit will clamp the input signal to 5V. UsingEXTmode, the behavior of the circuit is:
- Source
the current specified by (EXT Voltage / 5V) * MAX CURR.
- Generate
the optical power specified by (EXT Voltage/5V) * h * mW. The
intensity of the LED will be dependent on the auxiliary feedback
signal used which defines the 'h' parameter.
The internal digital
to analog converter (DAC) is engaged when the SOURCEswitch is moved to theDACposition and can be used to generate pre-programmed waveforms and waveform sequences triggered by a digital pulse to theTRIGinput. This feature relies on optional Arduino installation and programming the device using its API. Using theDACmode, the behavior of the circuit is:
- Source
the current specified by (DAC Voltage / 5V) * MAX CURR.
- Generate
the optical power specified by (DAC Voltage/5V) * h * mW. The
intensity of the LED will be dependent on the auxiliary feedback
signal used which defines the 'h' parameter.
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TODO
If you have questions during device assembly, please direct them to the open-ephys forum so that others may benefit. Pull requests improving this documentation are welcome.
-
This google sheet contains a reasonably up-to-date parts list. However, it is manually managed and therefore prone to errors. It is recommended that method 2 be used instead
-
Fully assembled cyclops PCBs can be purchased from Circuit Hub
This also includes an up-to-date parts list for each PCB with optimized prices. Note that these parts are for a single PCB without the enclosure, power supply, etc. Those parts can be found on the "Circuit Hub Kit" tab of the Google spreadsheet above.
Most of the parts can be purchased from Digikey, but there are a few components that need to be bought from other sources such as Newark, Adafruit, and Samtec. All vendor, part number, and quantity information is listed on the BOM. If you are having trouble getting a part, check the Google Sheet as there are alternative suppliers listed for some parts.
The cyclops PCB can be constructed by purchasing from one of the pre-uploaded options:
- OSH Park - made in America, excellent quality. Minimum of 3 boards per order
- Seeed Studio - made in China, very good quality. Minimum of 5 boards per order.
Alternatively, the gerber files can be uploaded to the PCB fabrication service of your choice. The layer of each gerber file is identified by its file extension:
*.GKO = board outline
*.GTS = top solder mask
*.GBS = bottom solder mask
*.GTO = top silk screen
*.GBO = bottom silk screen
*.GTL = top copper
*.G2L = inner layer 2 copper
*.G3L = inner layer 3 copper
*.GBL = bottom copper
*.XLN = drill hits and sizes
PCB stencils, which are useful for applying solder paste to the boards, can be purchased from a service like OSH stencils using the gerber files located in ./cyclops/stencil/. If you plan to hand solder the board, or don't mind dispensing solder paste yourself, then you do not need to purchase these stencils.
The BOM includes several optional components, which are not in the pre-populated Digikey cart. These include:
- An extruded aluminum enclosure, which houses the completed board. The enclosure is recommended because the large voltages and current transients used to drive high power LEDs can cause capacitive and inductive interference with nearby recording equipment. Acrylic front and rear panels can be purchased from Ponoko using the links supplied in the BOM. The instructions below show how these plastic pieces are modified to provide proper electrical shielding.
- An M8-4 connector. This is a rather expensive connector that allows cyclops to drive Thorlabs LED modules or Doric LED modules.
To assemble a Cyclops board, you will need the following materials
-
A soldering iron and, if possible, a hot-air reflow device.
- At minimum, a soldering iron regulated to ~370 deg. c) will do the job.
- In addition to the iron, a hot-air rework tool or reflow oven are recommended and the assembly instructions below assume you are using one of these two options. A low cost, high-quality hot-air rework station can be purchased from SparkFun here.
-
Copper braid ('solder wick') for solder removal (e.g this)
-
Liquid flux (
no-cleanvariants are easiest since they don't have to be thoroughly removed after use) -
Solder paste (e.g. this)
-
Stereoscope or loupe (optional but nice for tracking down shorts.)
-
Isopropyl alcohol for cleaning flux off the board (e.g. this; optional)
-
An anti-static mat (e.g. this; optional but recommended to protect your work...)
PCB component population and soldering is fairly straightforward and requires standard surface mount construction techniques.
- A tutorial on hot-air soldering can be found here.
- A great tutorial filled with general tips and tricks for surface mount soldering can be found here.
The following steps provide a visual guide to construct your own board. The goal is to create a fully populated PCB like this one:
Following board construction, you should run through the electrical tests outlined in the next section before applying power.
-
Place the bare PCB on a flat surface, preferably one that is static dissipative or anti-static. Alternatively, the board can be mounted in a PCB vice.
-
The silkscreen layer on the PCB (white text) has almost all the information you will need to properly populated the PCB. However, its a good ideal to to open the cyclops design in EAGLE. This will allow you to get detailed information on components before placing them on the board.
You can then the
informationtool to get detailed information on each component, e.g. to ensure you are placing the correct value resistor or capacitor.
-
After cleaning the surface of the board with isopropyl acholhol or similar, apply solder paste to each of the pads. For an excellent series of tips on effective methods for dispensing solder paste, see Mike's video on the subject. Do not apply solder paste to through-holes or the pads shown outlined in red in the following image. These will be hand soldered later in the assembly process.
The correct amount of solder paste to apply is `enough'. Each component contact should sit in a small amount of paste, but blobs of paste that envelop the component pad or pin may later result in a short. The following images show examples of good and bad solder placement.
If you need to pause at any point, you should store place the PCB in the fridge to prevent the flux in the solder paste from breaking down.
-
Populate all top-side surface mount components on the board. There is a single surface mount switch on the back of the board that will be hand soldered later. Additionally, all through hole components (e.g. power jack, BNC connectors, etc) will be populated later. Start by placing the integrated circuits (ICs). Use the stereoscope or loupe to ensure that pads are making contact with the pins of the placed components. Precise component alignment is not necessary. Components will self-align during the reflow process.
-
After placing the ICs, place the passive components (resistors, capacitors, inductors, diodes, and ferrite chips).
-
Next, reflow solder the board. We use a homemade reflow oven constructed from a toaster oven, Arduino board, reflow oven control shield, and mains relay. You can make a similar one, use a commercial reflow oven, or use the hot air station. Reflow the solder paste on the board using your oven or hot air gun as described in the links above.
-
After the solder has cooled, examine solder pads using the stereoscope of loupe for solder bridges between pins, solder that has not melted, or pads lacking a decent solder joint. Fix any issues using a standard soldering iron If there are solder bridges present, get rid of them using some solder wick before moving on. Solder through-hole components in place using a standard soldering iron. A low cost reflow oven can be made form a toaster oven as shown here. This link also contains useful information on the basics of the reflow soldering process,
-
Each board has an address (0 through 3) that is defined by two solder jumpers and the location of a ferrite chip. This allows cyclops boards to be stacked to share a power supply while being driven by a common microcontroller. For each board that will share a microcontroller, a unique address must be specified and the solder jumpers and ferrite chip must be soldered in appropriate positions to reflect this address. See the picture below to better understand this addressing scheme.
TODO: Pictures of circled 0 ohm jumpers
-
Flip the board over and install the final surface mount component, the
AUX<>CURRslide switch, by hand soldering. -
Next, populate all electromechanical components. This can be soldered in place with a standard soldering iron and a large chisel tip.
Note: The the barrel power jack (name:
POWER, value: PJ-063BH on the schematic) should be mounted on the bottom of the board. It fits on both the top and the bottom, and will properly supply the board with power if mounted on the top. However, if the barrel jack is mounted on the top side of the board, it will not fit inside the enclosure. -
If you are not planning on putting the PCB in an enclosure, jumper the solder points for the power switch together using AWG 20 (~1.8 mm diameter) braided copper wire or thicker.
TODO: Picture of jumpered switch solder points
-
Install the heatsink on the PCB
Flip the PCB so that the bottom is exposed. Locate the large white square surrounding the exposed copper pad indicating the heatsink mounting location. Remove the paper backing from the heatsink to expose the adhesive surface Press the heatsink into place on the PCB.
-
[Optional] Install the Arduino
TODO: Image of desoldering and mounting
Remove the power jack from the Arduino using a hot air gun or desoldering tool. Press the Arduino the dedicated headers.
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To construct the enclosure, you will use the following materials
Parts
- Assembled Cyclops PCB (1x)
- Extruded aluminum enclosure (1x)
- Laser-cut enclosure panels (1x front, 1x rear)
- Rocker switch (1x)
- Hookup wire (2x, 15 cm lengths, 20 Ga or thicker)
- Shrink tube (2x)
- Gain knob (1x)
- Toothed washer (1x)
- Jam nut (1x)
- Light pipes (3x)
- Button cover (1x)
- Screws to install panels (8x)
- [optional] Conductive coating for EMI suppression (e.g. this).
Tools
- Soldering iron
- Hot air gun or lighter
- Phillips head screwdriver
-
Assemble the power switch
- Strip about 1 cm of insulation from each end of the hookup wires
- Thread the stripped portion of the wire halfway through each of the switch's solder terminals. Fold the wire back, so that the stripped part is touching both sides of the terminal.
- Solder the hookup wire to the terminals. Make sure the solder flows between the wires' copper braid and the switch terminals.
- Slide the heat shrink from the back of each wire, over the solder joints. Hit then with a hot air gun or pass a lighter underneath them to shrink them into place over the solder joints.
-
Prepare the panels
- Pull the paper backing off the panels.
- Optional: Spray the inside of each panel with conductive EMI suppression. Additionally, you may want to file the inner edge of the enclosure to reveal the bare alumnimum to ensure good contact between the coating the rest of the enclosure. Allow the coating to dry before proceeding.
- Press the power switch into position on the back panel. Orient the switch so that the 'on' symbol (-) is toward the middle of the panel and the 'off' symbol (o) is toward the outside. This will make the panel easier to mount on the enclosure. Hold the panel close to hole you are pushing the switch into as the panel material is pretty thin and could break if it is bent too much.
- The switch will snap into place.
-
Install the heatsink on the PCB if you have not already done so
- Flip the PCB so that the bottom is exposed.
- Locate the large white square surrounding the exposed copper pad indicating the heatsink mounting location
- Remove the paper backing from the heatsink to expose the adhesive surface
- Press the heatsink into place on the PCB
-
Solder the power switch to the PCB.
- Solder the power switch to the two indicated terminals on the PCB.
- Make sure you do this after you have installed the switch in the rear panel.
- The wires can be soldered to either solder point, orientation does not matter since this is a SPST switch.
- Optional: If you are using an EMI suppression coating, you should solder a third, fine gauge hookup wire to one of the GND test points on the board that you will eventually use to make electrical contact with the enclosure.
-
Install the light pipes on the PCB.
- Insert the light pipes in the mounting positions in the front of the PCB.
- Squeeze them into place firmly using some needle nose pliers or sturdy forceps.
-
Install the front panel on the enclosure
- Using 4 of the 8 mount screws that came in with the enclosure, install the front panel on the enclosure.
- Optional: If you are using an EMI suppression coating, strip the end of the fine wire that was soldered to GND and insert it into one of the screw holes before inserting the screws. When the screws are turned in, this will provide electrical contact with the enclosure.
- Note: The holes should not have to be tapped. The screws should just kind of jam themselves into the holes. Godspeed.
- Note: The screws that come with the enclosure are what I would call "cheese grade". It is possible to break these screws so be cautious when turning them in. If you think you will break the screws, then you might have to drill out the holes with twist drill to loosen them a bit. I have not had to resort to this yet.
-
Install the PCB in the enclosure.
- Slide the PCB into the box using the middle mounting rail.
- Note: When the BNC connectors come through the front panel, you will need to push them down a bit, slightly flexing the PCB to get the light pipes through the front panel. This is required to hold the light pipes in place.
- Note: If you have installed an Arduino on your board, make sure you have removed the barrel jack power connector on the arduino first. If you leave it on the Arduino, it will press into the floor of the enclosure and possibly cause a short.
-
Install the rear panel.
- Loop the two hook up wires and push them into the enclosure on top of the PCB.
- Use the panel the push the remaining wire into the enclosure.
- Use the remaining 4 screws to install the rear panel.
-
Install gain knob and button cover.
- Slip the toothed washer over the gain dial.
- Tighten the jam nut on the gain dial's threads until just past finger tight using a pair of needle nose pliers.
- Note: Do not over-tighten using a combination spanner or you will strip the threads.
- Turn the dial on the front panel fully counter-clockwise.
- Slip the gain knob over the dial with the tick mark pointed slightly below the 0 position.
- Tighten the set screw on the side of the knob to lock it into place using an Allen key.
- When you turn the knob fully clockwise, the tick mark should be pointing close to the 1.5A position.
- Press the red cover over the
TESTswitch until it snaps into place.
Congratulations, you are the proud owner of a high-precision, high-power, high-speed LED driver that will make commercial drives feel a bit ridiculous for costing so much and very self-conscious about their performance characteristics.
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There are several things to consider when determining the type of LED you wish to drive with the Cyclops and the configuration of the LED.
- Will optical stimulation be used in-vivo or through a microscope?
- Will it be performed on freely behaving animals?
- Do you need to perform bilateral stimulation?
- Will you need to incorporate an amplified photodiode into your stimulator to measure optical power or use use optical feedback to produce ultra precise light waveforms?
The answers to these questions will determine the type of LED you use, how it is coupled to the preparation (e.g. collimated for the back aperture of your microscope or, fiber coupled for in-vivo stimulation), and weather or not it needs to be commutated in some way. The following provide a few simple options for LED configurations, but there are many more to consider for your experiments.
Regardless of which you choose, the following is always true: keep the cabling to the LED as short as possible and 'fat' enough to handle high currents (AWG 18 or thicker). The currents and voltage used to drive high power LEDs are many orders of magnitude (like 6 or more...) greater than those recorded during electrophysiology experiments. Also, the Cyclops is a fast circuit. Fast circuits hate long cables because they introduce appreciable delays and parasitics that can adversely affect operating characteristics. Very long cables will introduce ringing into light waveforms with fast edges! Ideally, the LED should be right next to the device. I typically mount my fiber coupled LEDs directly into the banana sockets on the back of the device using copper-clad printed circuit board so that my 'cables' are about 2 cm in length.
Anders Asp has contributed the following PDF document containing detailed instructions for fabricating a bilateral, commutated fiber-coupled LED for use in freely moving animals that works with the Cyclops driver:
The cyclops can be used to Drive Thorlabs fiber-coupled LED modules. You will need to install the M8 4-position connector in expansion port B to drive these LEDs.
TODO: Pictures/instructions for M8 installation process in Thorlabs configuration
The cyclops can be used to Drive Doric fiber-coupled LED modules. You will need to install the M8 4-position connector in expansion port B to drive these LEDs.
TODO: Pictures/instructions for M8 installation process in Doric configuration
The cyclops can be used to Drive Thorlabs collimated LEDs for microscope-based stimulation. You will need to install the M8 4-position connector in expansion port B to drive these LEDs. See Thorlab fiber-coupled LED instructions for instructions.
\FloatBarrier \newpage
The following procedure can be performed on assembled boards to ensure functionality.
- Insert alligator clip across power switch solder points
- Insert device into PCB clamp
- Power from 15V, 1.5A capable bench-top power supply.
- Power indicator LED turns on.
- Using a multimeter, probe the 12V, 2.5V, -5V, and -1.25V test points
- 12V good
- -5V good
- -1.25V good
- While probing the 2.5V test point, use a ESD-safe screwdriver on the trimpot to get exactly 2.5V.
- Seal the pot with a dab of hot-glue.
- Set MDO3000's AFG to produce 1-5V, 100 Hz, 10% duty cycle square wave.
- Insert LED/amplified photodiode test fixture into banana sockets, IDC connector, and AUX BNC port.
- Insert AFG output of MDO3000 output into VCTL BNC port of device
- Bring CURR output of device to Ch1 of MDO3000
- Bring VREF output to MD3000
- Triggering on VREF Channel set scope to measure rise and fall times
- Bring front panel potentiometer to 50% position.
- Input switch to EXT source
- Examine wave shape and rise/fall times in current FB mode. Rise/fall times < 300 ns. No ringing on waveform.
- Examine wave shape and rise/fall times in optical FB mode. Rise/fall times < 300 ns. No ringing on waveform.
- Return FB switch to curr position
- Return input switch to OFF (middle) position
- Bring gain potentiometer to full on position
- Briefly tap on the TEST button.
- Ensure that the >1A indicator LED turned on during pulse.
- Return gain potentiometer to zero position
- Remove all power connectors.
- Remove alligator clip.
- Initial and serial number the board using sharpie on the large power trace on the right side of the board.
- Enter board serial number into the spreadsheet.
Cyclops
LED Driver by Jonathan P. Newman is licensed under a
Creative
Commons Attribution-NonCommercial-ShareAlike 4.0 International
License.
Based on a work at https://github.com/jonnew/cyclops.
Copyright (c) Jonathan P. Newman All right reserved.
The code associated with the Cyclops project is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
The code associated with the Cyclops project is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this code. If not, see http://www.gnu.org/licenses/.
\FloatBarrier \newpage
[1] J.P. Newman, M.-f. Fong, D.C. Millard, C.J. Whitmire, G.B. Stanley, S.M. Potter. S.M. Potter. Optogenetic feedback control of neural activity. eLife (4:e07192) 2015. doi: 10.7554/eLife.07192
[2] T. Tchumatchenko*, J.P. Newman*, M.-f. Fong, S.M. Potter. Delivery of time-varying stimuli using ChR2. (* - equal contributions, co-first authors) Front. Neural Circuits (7:184) 2013. doi: 10.3389/fncir.2013.00184