>> Are you using a PID servo loop to run your motor at fixed speed?
No, there is a classic phase locked loop using the 4044 phase detector, but the "oscillator"
is a DC motor turning an encoder. So, the 4044 feeds an active filter and then a
boosted op-amp driving Darlington transistors to run the motor. The motor is a
Yaskawa motor made for moving the print head back and forth on a Qume
daisy wheel printer from ages ago. A nice, smooth motor optimized for low
speed operation. I had to adapt a much better encoder on the back of it,
though. So, the encoder produces 1024 pulses/rev (I only one one of the
quadrature channels), and then multiply this with a 74LS297 digital PLL
by 20 to generate 20480 pulses/rev. The encoder index pulse is used to
gate the stream of pixel clock pulses that clock the bits into the laser.
The motor/drum spins at 600 RPM.
>> I
am thinking of using a Blue Ray UV laser diode instead of the red
laser to allow use of blue sensitive film and a red room safety light.
Though I have not looked at the availability of sheet film these days.
I finally used up my stock of Agfa red laser film and went to Kodak PRD
film. I think this is used in the typesetting industry. The old Agfa film
was 7 mil, this stuff is 3 mil, so I had to change the shim on my drum
to bring the diameter of the emulsion out a little more to get it at that
20.480" circumference. Just luckily, I had the right thickness mylar
sheet left over from the bad old days with the crepe tape on mylar
sheets! These phototypesetter films are made for red lasers and
HeNe lasers. I buy mine on eBay, there are a bunch of guys
selling unopened film that is nearing the expiration date. if you keep the
stuff in good conditions, you can use it DECADES past that date.
>> What sort of lens did you end up using and did you do any spot rounding?
It is a 13 mm multi-element lens, probably quite similar to the objective
lens in a microfiche reader. But, the optical system is more complicated.
Right in front of the laser, I have a 3 mm sphere lens glues into a
disc with a hole in it. Right at the front of that, I have a piece of
aluminum foil with a pinhole in it, to block off the light coming through
the edge of the sphere. This assembly is at one end of about a 4" tube
from the objective lens. About in the middle of the tube is a double-concave
lens. This makes the spot size smaller. Well, the spot size is so small
that this lens may not be necessary. I have to actually defocus the
thing slightly so that I don't leave gaps between the raster lines.
>> What
did you use to drive the pixel video to the laser diode? I was just
thinking of shunting out the lasers current source supply with a FET or
transistor across across the laser diode.
The pixel clock is 5 us (200 KHz) so it is not critical stuff at all!
I use a 2N2222 and a series resistor, and a little trimpot to set the
current. Not a closed-loop system. (I do have a closed-loop
analog driver that can be used with gray-scale film to record
images, it uses the sense diode in the laser package.)
Jon