CASA CTL-BM 270mW RGV Laser Show Teardown and Review

Inside the CTL-BM

The CTL-BM is constructed in a reasonably well-crafted anodized aluminum housing.  Although the screws will not tolerate many insertion/removal cycles as they are only lightly gripping the sides of the housing, it would be easy to repair a stripped screw by using a slightly larger size self-tapping screw.  The faceplate (the one on the front, with the laser aperture) is freed by removing the six machine screws holding it on, and the top of the device (with the handle) is freed by removing one screw in the center of the rear of the device.  At this point, the lid slides forward out of it’s channel in the sidewalls to reveal the guts of the device.  Keep in mind that without the faceplate, the sides of the device are a little wobbly and prone to bending out.  Try not to pick it up by the sides if the face and top are removed.

Click the pic for a full-size version.

Annotated Internals of the CTL-BM

The device is fairly modular, which is nice to see for the homebrewer, as it suggests that cobbling together a bunch of our own modules might actually provide similar performance.

Bottom of the picture (front of the device) is the optics stage, with the three lasers and the XY stepper.  Top left we see a big chunky 7V transformer used to power the blue PCB containing the laser drivers/TTL modulators, which resides in the middle top.  Top right brings us to the green 5V power supply board (mounted upside down above the control board), and the control board containing the Winbond 8051 clone and the Toshiba ULN2803 darlington array used to drive the steppers.   Most of the control board is obscured by the green power supply board, but it really only contains the 8051, 2803 stepper drivers, and an RS485 level shifter chip to do the DMX level translation from the 3-pin XLR port on the back (marked DMX) to the 8051′s serial port.  For a little primer on DMX to serial conversion including a homebrew USB-DMX interface to run this beast, check out our article on A Homebrew DMX Interface.  Keep in mind that ANY low-voltage serial interface could be used for a DMX connection, whether it’s a micro or a hacked up USB-RS232 Converter.

Let’s continue with the teardown, shall we?

CTL-BM Optics I – Laser Mixing

We’ll start by describing the optics stage.  The optics are constructed of three discrete laser diodes mounted and hand-aligned onto a quarter-inch piece of aluminum for rigidity.  They are coupled into one beam using two dichroic mirrors/filters, which are really just little squares of plate glass with a dichroic coating on them.  These “dichros” are also hand aligned, and secured in place with a screw (from bottom) and glue to fix the final alignment in place.  The first dichro is aGM (Green/Magenta), and the second is a YB (Yellow/Blue), and by transmitting or reflecting various wavelengths, the three beams are combined into one.  This is a classic laser-color-mixing stage and should be familiar to those “skilled in the art”.

Let’s take a moment to talk about the dichroic filters.  They are called dichroic because they appear as two different colors depending on whether you are looking at them through transmitted or reflected light.

Di-chroic = two-colors.

We referred to them as the GM and YB dichro’s, which denotes their transmitted/reflected colors, respectively.  So the GM filter will look green if you hold it up and look through it, and will look magenta if you set it down on a dark surface and look at the reflected light.  The YB will look yellow when held up and will reflect blue.  You can buy these from an overpriced optics dealer or through ebay, but we are presently researching what can be done using dichroic art glass, which is much much cheaper.  More details on this in the future.

The green beam shoots directly from the far left straight through both dichros to the XY steppers, and both the GM and YB dichro’s pass the 532nm beam with no reflection.  In theory – you’ll see later that there’s a bit of parasitic reflection that gets hidden within the housing.  The RED beam is mixed in first by bouncing it off a GM dichro. The green passes through and the red is reflected at a 45 degree angle to combine with the green, creating a “yellow” beam that continues on to the YB dichro.  At this point, both red and green will pass right through the YB dichro, and blue(violet) will be reflected so we mix in a blue beam by bouncing it in off the YB dichro.  If all three beams were on, then we would have a “white” beam continuing from the YB dichro over to the steppers.  It is in this manner that the various colors of the laser can be generated, and the ORDER DOES MATTER.  If you look at a transmittance/reflectance plot of these types of dichros, you will see that the GM dichro has some reflectivity at the 400-ish nm region.  So you don’t want the blue passing through the GM or you will lose beam from parasitic reflection.  The YB, however, happily transmits anything above yellow ( Green:532, Red:650) so it’s natural to mix in RED first, BLUE last.

Looking back to figure 1, we can see now how to generate the “7-colors” available to this laser projector.  The 8051 enables and disables each of the laser diodes, and by the combination of each of the individual beams, we can create the following colors.

• Red – Red only
• Green – Green only
• Blue – Blue only
• Yellow – Red and Green
• Magenta – Red and Blue
• Aqua – Blue and Green
• White – All three beams

Not bad, and it looks cool.  They’ve achieved the desirable “white “beam, so at the $200 price point it’s a nice entry-level laser projector with enough capability for the wow factor the kiddiez (liek us) are looking for.

As a side note, much higher color resolution should be achievable if the TTL modulation rate were high enough to pulse each color for a short time while the XY steppers were in a fixed position.  This might be too much data for the little 8051 to churn out, but we can imagine that if each “tick” of the XY stepper positioning were divided into just TWO timeslices, we could shoot a beam that was yellow for half of the time and red for the other half.  The color change would be way faster than our eye could resolve and the resulting beam would appear to be orange.  With more timeslices and more combinations, the output spectrum of even a simple device like this one could be huge.  Who knows why they didn’t do it – not enough 8051 cycles, TTL modulator too slow, or XY stage too jumpy?  Any of those could prevent higher-order modulation, but it’s an interesting thing to think about for our own developments.

Next, what it looks like during operation – beam pix!  Beware, the next two pages have many large pics, so the load times will be abit longer..