Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, Philip Pemberton <ygroups@...> wrote:
>
> On 09/03/11 06:36, jmelson2 wrote:
> > I use Kodak digital sciences laser recording film, apparently made for laser recording of medical scans.  I don't know if regular B&W paper is red sensitive.
> 
> It's not -- but it is blue sensitive! Hence the use of a BluRay LD.
> 
I started building this thing in 1986, without any idea how to actually produce a beam that could be turned on and off.  In 1996 I got it running with a 670 nm laser diode, which solved a bunch of problems.

> How did you rig up the laser driver? High speed opamp to generate 
> constant current, then another one driving a transistor/resistor shunt 
> to short over the diode (and thus set the current through the diode)?
The litho imaging driver is dead simple, a CMOS output controls an NPN transistor, with a resistor around 82 Ohms for current limiting.
You change the resistor to change the current.  Pixels are now 5 us long, so extreme control of rise- and fall-time are not real critical.

I also have a grey-scale driver that has a 12-bit DAC that feeds an op-amp circuit.  It compares photodiode current to the intensity value and programs a pass transistor to get that photodiode current.
Pixels in that mode are 10 us long, seems to work pretty well.

Jon