The RTI Build Experience

This is the second post in a series, written by  Kevin Falcetano.  See the first post for an introduction.

Though time-intensive, the RTI build was relatively simple. It was made so simple because the hard stuff; i.e. circuit design, dome construction, control box construction, and programming, were already done by Leszek Pawlowicz. This made the project more like a LEGO set than a massive undertaking.

I began with the control circuitry. The basic idea behind it is that it was necessary to control each LED individually, but it had to be a type of LED that was bright enough and had a quality output of consistent white light. This, unfortunately, meant that individually addressable LEDs (that may be controlled with a single data cable and an Arduino library for the control protocol) just would not do. So this is where Leszek’s build comes in. It calls for an eight by eight matrix of LED’s to be controlled by a special series of circuits connected to an Arduino Mega.

This allows for a total of 64 LEDs to be driven off of 16 digital pins, where the columns are the positive lead and the rows are the negative lead. The arduino’s power supply cannot drive this on its own, and that is why the circuitry is much more complicated. What is required is essentially the use of transistor-like gates on the positive and negative ends of the matrix so the current can be directed through one specified LED at a time.

One such set of gates is the highside MOSFET circuit, built on an Arduino Mega Shield:

An Arduino shield is a piece of hardware meant to be mounted on top of an Arduino microcontroller unit to interface with it. In this case, this was a blank board used to connect eight highside MOSFET circuits to the digital pins that would open the “gates” on the MOSFETs on the positive end to the desired column.

These components all had to be hand soldered to the shield board, but after quite a long period of time, everything was in its place without much of a hitch. The positive end of each MOSFET (the source) goes in parallel to the positive end of the power supply. Each gate (actually called the gate) gets connected to a corresponding arduino digital pin to be switched open as necessary, and finally, the negative ends of each MOSFET (the drain) get connected to corresponding pins of the positive ethernet cable that leads to the LEDs in the dome. The resistors and transistors shown on the board exist to convert the MOSFET for highside control. The other side of the shields connects the other eight Arduino digital pins to the row “gates” on the negative end, to make a complete circuit that controls each LED.

Those negative gates, controlled by CAT4101 LED drivers, were soldered onto a different board along with a beeper and USB control pins:

In this case, there is a normal resistor for baseline current limitation, which acts as a bottleneck for the maximum current through the LEDs. Alongside each resistor is a variable resistor to further reduce the current and vary/tune the intensity of each LED row. These CAT4s allow the negative side of the LED matrix to selectively connect to ground by way of the ground ethernet cable. Once all was soldered, it was a matter of mating this board, the Arduino Mega, and the MOSFET control shield.

It also helped to have a separate power distribution board, with positive and negative rails for the 5V Arduino power supply, and positive and negative rails for the 9V power supply that drives the LEDs.  After connecting the power leads up and stripping and adding pins to eight LEDs and two ethernet cables, it was time for the test: The moment of truth for my soldering and circuit building skills.

Aside from one cold solder joint that I had to fix, everything worked.

As you can probably see from the top left corner of the above image, there is a large pile of LEDs that all have wires soldered on, pins crimped, and heat shrink tubing applied to  them. This was a very fun intermediate step that requires no further explanation but will now be mildly complained about. That level of repetition is not actually fun, but very necessary.

Speaking of, the next thing on the list was painting the LEDs black. Since the goal of good RTI is to eliminate as many environmental variables as possible, the inside of the LED dome must be as non-reflective as possible, so as to eliminate internal reflections that could accidentally cause multiple other, albeit dimmer, light sources within. This means even the area around the actual LED chips had to be painted black, with the only other exception being a small portion of the positive pad so as not to mix up the polarity when it came time to wire everything together.

Now that this bit was out of the way, it was dome prep time. The domes we got were clear, which helped with marking the outside for LED positions and then transferring the marks to the inside. I did this using some clever geometry math mostly outlined by Leszek in his documents. I, however, had the idea to melt the marks into the inside of the dome using the soldering iron, which caused a rather unpleasant smell to fill up the room that may or may not have been an indicator of toxic fumes. This step was necessary so that the marks showed up even with the paint over them. Next was just that: painting both the small and big dome using exactly one entire can of matte black spray paint.

Above is an image from the spray booth inside Regis Center for Art, where I applied multiple coats of the paint in a highly impatient fashion. Everything turned out fine, since the drips in the small dome would be inconsequential to the build.

After everything was dry, it came time to mount the LEDs inside the big dome, since it was the first one we decided to have done to make sure everything was working as intended. Each LED was hot glued in place with the positive ends of all LEDs pointing in the counterclockwise direction.

The above photo shows the placement of the LEDs as well as part of the following step: the stripping and crimping of many chains of wires that connect up the LED matrix. Each LED has an attached male dupont pin head, whereas each chain of connectors has properly spaced female dupont pin heads. Every concentric circle of eight LEDs represents a negative row, and every vertical set of eight LEDs (offset to help with lighting coverage) represents a positive column.

With all of the rows and columns wired up, the open ends of each string of wires were to connect with the male dupont pins I crimped onto the ends of both the positive and negative ethernet cables’ individual wires. Every part of the control circuitry was made to correspond with matching pins 1 through 8 on the ethernet cables, such that it becomes incredibly easy to know which Arduino pin corresponds with which row and column number and which LED. The next step was then to clean up the wiring and turn the dome into a more self-contained piece.

Behind this piece of special matte tape (3M Chalkboard Tape) that has about as much stick as a thirty-year-old Post-It, are the connections to the ethernet cables pushed down and hot glued in place. With the finishing touches in place on the dome, a successful test was run and it was time to contain the control circuitry in a cozy project box.

The Arduino, MOSFET shield, CAT4101 board, and power board were all hot glued in place after drilling the requisite holes for the bells and whistles. The two potentiometers were wired in and bolted on to control the LED on time and delay, as this can vary between different shooting scenarios. A reset button, preview button, and action button are all shown above, and used to operate the reset, single light preview, and main action activation functions in the Arduino’s programming. This image was taken before I added the main 2.5mm jack used to snap the camera shutter and the switches that select the operation modes. At the right side of the box are the ethernet jacks for power to the dome (top is positive and bottom is negative), and the USB connection for cameras without a way of connecting to the 2.5mm shutter switch. Last to mention is the OLED screen that displays relevant information that helps with eliminating mistakes and debugging when repairs and/or changes must be made.

On the subject of the control box, I personally made changes to the construction in a few ways:

• I accidentally busted a switch because the soldering iron was too hot, so I removed the sound on/off function which ended up being fine since I think I messed up the wiring of the beeper anyway.
• I completely removed the USB shutter, servo shutter, and IR/Bluetooth shutter options because we only really need a 2.5mm auto shutter and a manual mode.
• I added a small transistor circuit to short the 2.5mm jack to ground using an Arduino digital output in order to actually make the 2.5mm auto shutter function work.
• I added a three way switch to reduce the number of switches, but increase the functions possible.

This is the front of the final form of the box:

After the box was wiring up the small dome. The steps were the same as the big one but just scaled down, so I’ll spare you the boring details other than the fact that this one only contains 48 LEDs, with eight columns and six rows.

With everything complete, I also had to change some of the Arduino code, which was provided by Leszek with his hackaday project. The changes to the code are outlined below:

• Removed sound function and USB and IR/Bluetooth shutter functions
• Added dome mode switch, so either dome can be used without reprogramming the control box
• Added a new white balance preview function for calibrating the camera because the old one was tedious
• Added new display functions to show new modes above
• Added a new function to snap 2.5mm shutter
• Added a function to skip intro screen because of impatience and changed the intro screen to display relevant version info of the new software build.

Oh, and one last minute addition before this goes up: I added a remote preview button so that the exposure settings and focus can be adjusted from the computer without getting up to turn the preview function on and off, making the setup process a bit less tedious. This feature is just a wired button that plugs into the control box at the front and is completely optional since I kept the original button in place.
The final post is a conclusion and thoughts on the lessons learned from this build.