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From: miso on 5 Dec 2008 00:38
On Dec 4, 5:50 am, Dungeon Dave <howl...(a)the.moon> wrote:
> > You already have the 12V supply, so I would keep it as part of the
> > project.
>
> okay. It's one less headache, I guess.
>
> > For one thing, this prevents you from having to deal with the
> > mains. There are probably several LEDs in series, plus a resistor. I
> > don't think they would use 12V per LED since the drop across the
> > resistor would be high.
>
> Nor me, but the transformer claims it's chucking out that and these are
> somewhat superbright LEDs. I'll get a multitester on the DB and measure
> the voltage at the parallel end, see if it really is that much.
>
>
>
> > What you want to do is pulse width modulate the power to the LED. That
> > is, let duty cycle control the brightness.
>
> > Most cheesy PWM circuits consist of a sawtooth generator which you can
> > make out of a 555, and a comparator.
>
> okay.. I recall some diagram that had a strobe-like effect to simulate
> dimness, although it looked more like a square wave (I could probably pop
> some caps across the output points to smooth out the wave).
>
> Ta for the help!
>
>

No, a rectangular pulse is what you want. Your eye will average the
intensity. The 200Hz suggested frequency is plenty fast.
From: Preacher Kane on 7 Dec 2008 08:58
>
> You already have the 12V supply, so I would keep it as part of the
> project. For one thing, this prevents you from having to deal with the
> mains. There are probably several LEDs in series, plus a resistor. I
> don't think they would use 12V per LED since the drop across the
> resistor would be high.
>

Nope - they're paralleled up.. and just checked the voltage. 13.55V at the
supply end!
From: Dungeon Dave on 7 Dec 2008 08:53
From: Rich Grise <rich(a)example.net>

> This is my recommendation, but I wouldn't use a 2N3053 and 2N2955 -
> I'd use something more like a PN2222 and a TIP32, because they're much
> more recent designs, with lots of beta and stuff.

okay.. so just replace 3053->2222 and 2955->TIP32?

Roger that.

> If you want to
> switch the low side, lose the 3053 and just drive a TIP31. They're
> less than a buck at digi-key.

I don't know why, but I'd prefer to have the switch at the high side -
something about having floating grounds worry me.
>
> And yes, 200 Hz is high enough to make the flicker go away. :-)
>
And yes, my idiotic eyesight never actually READ the site carefully and
noticed that! Major DOH! Excuse my noobness/impatience; it's strangely
nice to be on the other side of the clueCurtain once in a while!

> Have Fun!
> Rich

Ta!
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