Richard Clark wrote in
:

On Sun, 10 Jan 2010 14:57:43 -0600, Lostgallifreyan
wrote:

analog modulated laser diode driver

This is no slight accomplishment!

73's
Richard Clark, KB7QHC


Well, I have to be honest and say that I haven't truly finished it. It's
based on an LM317. I have this LM317 fetish based on admiration for the old
POW radio ideas, I really think it's cool that such a ubiquitous device can
be applied so perversely to such amazing unintended uses, and given its
chances of being found, designs based on it could be buuilt even if much of
civilisation were 'bombed back to the stone age'. On a more prosaic level,
it's dirt cheap, powerful, easy to build with, and contains useful features.

The driver worked in my initial test to 50 KHz, better than the 10K most
analog mod drivers seem to favour. I tested it with a square wave and the
edges were sharp with no overshoot if I didn't push it too hard, and did some
filtering with low ESR tantalums for supply decoupling. I later discovered
LTspice. I modelled it with that and quickly saw the same waveshapes my
100 MHz scope had shown me, which was a nice reality check that proved that
LTspice could likely similaute the design and let me improve it before
rebuilding. I devised a circuit that cannot be overdriven, attempts to do so
merely broaden the peak at max amplitude, it cannot overshoot, and will
modulate to 200 KHz. The proof is in the pudding but the truth is I never got
round to baking the pudding. Bought a slower version of the op-amp I needed
but got mightily disillusioned about the laser show thing and felt that I'd
either build it and gain nothing personally from it, or no-one would even
care. Given what I'd seen, I think this is genuinely true, few would. I
hadn't even decided if I was going for SMT or through hole, and it's a tough
call because available parts seem to force a hybrid... Like the pitch-to-MIDI
system I wanted to build as an adapted version of Doepfer's R2M control
system, it got put on the back burner. But I spent a couple of years pulling
together all the tooling and design files to do it, and when I find something
to motivate me, I intend to finish both these projects.

I think this is what's drawing me to SWL. The present stopped compelling me,
so in a way I'm drawing on something that endured from further past, as a way
to revive the spark that got me as far as I did get. I never found a way to
gauge what I do, because most times I find little to suggest that what I do
has anything that people can't find better elsewhere just by throwing money
at it, and as they usually have more than me I tend to think they're probably
right. But if I can find enough motivation just to do it even though it might
amount to nothing, I will do it because I don't know what else I'm good for.

Long spiel, but I think I needed to, as part of whatever I have to do to make
it happen. Like Mulder, I want to beleive, I guess..