My first PWM beam modulater/dimmer
Can't believe I still had this. (I can't wait to see if it still works. I got this new BGA-based driver to try on it.)
This baby is made of pure shirt cardboard about a 1/16th of an inch thick and a full 7" in diameter, spray painted with Krylon Flat Black. There is no imitation shirt cardboard on this beauty, let me tell you.
The notched modulator wheel was hot glued onto a 5/16" O.D.aluminum collar with an 1/8" I.D. that was secured by hex set-screw to the end of a 12 volt, 1" diameter DC motor shaft. The speed of the wheel motor was controlled by a uA791 1-amp linear power op-amp, and later by a 555 timer configured as a PWM speed controller. I actually had two modulator wheels with different numbers of teeth that I could move into the beam independently.
The "tear" at the center of the wheel is from when it was eventually separated at the hot glue joint from the motor shaft collar.
The modulator wheel motor was mounted by rubber-band on a tapped and threaded Teflon block that rode back and forth on a 1/4" - 32 tpi, horizontal lead screw. The lead screw was mounted in brass bushing bearings at the ends and was driven via gear reduction by another 1" 12V DC motor. This allowed the block and wheel motor to ride back and forth on the horizontal lead screw by varying amounts, into and out of the 1 Watt mixed gas laser beam. The axis of the wheel motor was at the same height as the laser beam. As the notches of the wheel were driven further into the beam the blanking duty cycle changed from 50:50 to 1:40 or so, and slowly became more "Off" than "On". So there was a variety of beam segmentation lengths possible, until the effect was "dots of light", or fade-to-black.
If the wheel ever stopped when "in the beam" (which joyfully never happened during a planetarium show, but did in rehearsals only a few times), smoke was released from the wheel.
It was the poorest man's beam modulator, the mother of this invention arose out of necessity, but it worked and worked for many years in the planetarium, eventually replaced by AO modulators on each scanned beam.
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Everything depends on everything else