My first PWM beam modulater/dimmer

[lasermaster1977]

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.

[masterpj]

how much control did this give you regarding dimming a beam? I assume it wasn't a very fast way of modulating a beam and came with a huge slew rate?

It's cool though!

[VDX]

... ich you'll move the rotating wheel or the beam, then you'll get modulation "PWM-ratios" from around 45% to 0%, looking at the grooves shapes ...

Viktor

[lasermaster1977]

how much control did this give you regarding dimming a beam? I assume it wasn't a very fast way of modulating a beam and came with a huge slew rate?

It's cool though!

I was trying to remember the slew rate of the motor travel. I could go from tiny dots of colored light to dimmed out when pushing the momentary push button in about two seconds. As best I recall, the slew rate of the notch in the blade transiting through the beam (from an initial 50% blanking duty cycle - on/off) until the beam was occluded was right around 10 seconds. So there was a decent amount of control over the blanking duty cycle and dimming, it just wasn't instant "in" instant "out". It was a good enough dimming effect to make things appear to fade off in the distance.

As my Lissajous PLL and free running sine, square, triangle patterns ranged from 45Hz to 450Hz, and AM modulation going 10x higher than that, the 36 toothed wheel provided blanking frequencies from 3-5 Hz to 2400 - 3600 Hz and my 18 toothed wheel yielded results of half of that.

The 1" 12volt DC can motors ran in the 4000-7200 rpm range unloaded. With the fly-wheel load effect of the 7" blanking wheel frequency stability was surprising good for a free running motor. After changing to a PWM motor driver is what really allowed the really slow and steady rotation rates (strobe effects) without sacrificing the higher rates.

I eventually made a 36 blade, 10" wheel out of aluminum planning to drive it with a large DC motor but by then I had Acousto-Optic modulators.

[lasermaster1977]

... ich you'll move the rotating wheel or the beam, then you'll get modulation "PWM-ratios" from around 45% to 0%, looking at the grooves shapes ...

Viktor

That's precisely how it was used, Viktor, and the very reason for the groove shape. The leading edge of each tooth was on radius to the circle.

[LaserCo]

Did you ever try adding a 2nd or 3rd wheel ? so the beam would pass thru both

[lasermaster1977]

Did you ever try adding a 2nd or 3rd wheel ? so the beam would pass thru both

Yes. I had two wheels with independent rotational rates that could be independently moved so the pre-scanner beam passed through one or the other or both. With both wheels in the beam and set to different harmonics of the scanned image, it resulted in some really dazzling effects.