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From: Dungeon Dave on 4 Dec 2008 09:01
> One thing to consider when chosing between a switched (pulse width
> controlled) dimmer, and some linear system, is that in pulse width
> modulated
> LED supplies you get a sort of stroboscopic effect when your eye scans
> accross
> the LEDs (you see more then one, some distance apart).
> If you do not like this, then use DC.

Given they're not going to be near anything else strobing (overhead
lights @50Hz, monitors/TVs etc) I wasn't too bothered about the strobing
effect, but I know what you mean. Would upping the PWC frequency to
something much higher (maybe in 200Hz range) disguise the strobe effect?

[OS-based controller]
>
> As you are not very experienced, maybe it would be simpler to use the
> serial port.
> For example to write to an antique UART, and use the 8 data lines to
> for
> example drive a resistor R2R network to make an analog voltage to
> drive the brightness,
> for example drive a 2N3055 emittor follower.
>
> That will save you from programming a PIC.
> Here is a simple serial communications program for Linux:
> http://panteltje.com/panteltje/pic/io_pic/ptlrc-0.3.tgz
>
snagged, untarred and read the README... mmm.. nice! Thanks for that, I
may toy with a few proof-of-concept experiments and that util.
From: Dungeon Dave on 4 Dec 2008 09:11
[Bill's diagram]
>
> 555-based PWM lamp dimmer, a good choice.

Can't go wrong with a 555... where would we be?

(don't say "taking a pair of scissors to a 556"...)
>

>
> Bill's circuit doesn't reduce the voltage (much) what it mainly does
> is reduce the on-time of the LEDs (by blinking them at a rate that's
> too fast to see) this is a very power-efficient way of dimming
> lights.

I read that reducing the power output (strobing) is the more efficient
method than splitting it and dissapating the unwanted power elsewhere,
but as we're talking a total of 1W for my lights I didn't know if it was
worth it. I'll have to do some costings.. perhaps I should have done that
first.

(/me ponders a blog...)
>
[timer]
>
> an irrigation controller perhaps ? this is way more complicated than
> Bill's PWM dimmer. it may be easier to hack a few electronic
> timer-plugs and use their timers to control on-off and brightness.

mm... never thought of that. Will investigate, ta.
>
> OTOH it would be easy to wire a light-sensor a variation of bill's
> dimmer circuit so that when it sees "dark" it dims the lights and just
> use the timer plug to turn them off when you want off.

Essentially I wanted to have the lights at 100% during evening/dusk so
some LDR-based circuit could trigger the "on" cycle, but then wanted them
to dim to "comfort light" after a pre-determined point (20% from midnight
onwards?), so what was an illuminating effect doesn't become too dazzling
to sleepyheads. Bit of a strange requirement, I know.

okay, that's given me some pause for thought - ta for that info!
>
>

From: PeterD on 4 Dec 2008 09:17
On Thu, 04 Dec 2008 13:50:10 GMT, Dungeon Dave <howling(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

I can just see one preparing a ham sandwich with a strobe light. The
blood from the cut-off fingers would be something to see. <bg>

>, 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!
>>
From: ehsjr on 4 Dec 2008 12:51
Dungeon Dave wrote:
>>One thing to consider when chosing between a switched (pulse width
>>controlled) dimmer, and some linear system, is that in pulse width
>>modulated
>>LED supplies you get a sort of stroboscopic effect when your eye scans
>>accross
>>the LEDs (you see more then one, some distance apart).
>>If you do not like this, then use DC.
>
>
> Given they're not going to be near anything else strobing (overhead
> lights @50Hz, monitors/TVs etc) I wasn't too bothered about the strobing
> effect, but I know what you mean. Would upping the PWC frequency to
> something much higher (maybe in 200Hz range) disguise the strobe effect?

Yes. Build the Bowden circuit - it runs at 200 Hz so there
is no need to up the frequency. You were also concerned about
the square wave output - that is not a problem, at all. Get
the dimming portion of what you want working first, sort out
the timing part later.

Be sure to check that the voltage coming from the supply
included with the lights is 12 volts DC.

Ed

<snip>
From: Rich Grise on 4 Dec 2008 13:56
On Thu, 04 Dec 2008 17:51:16 +0000, ehsjr wrote:
> Dungeon Dave wrote:
>>>One thing to consider when chosing between a switched (pulse width
>>>controlled) dimmer, and some linear system, is that in pulse width
>>>modulated
>>>LED supplies you get a sort of stroboscopic effect when your eye scans
>>>accross
>>>the LEDs (you see more then one, some distance apart). If you do not
>>>like this, then use DC.
>>
>> Given they're not going to be near anything else strobing (overhead
>> lights @50Hz, monitors/TVs etc) I wasn't too bothered about the strobing
>> effect, but I know what you mean. Would upping the PWC frequency to
>> something much higher (maybe in 200Hz range) disguise the strobe effect?
>
> Yes. Build the Bowden circuit - it runs at 200 Hz so there is no need to
> up the frequency. You were also concerned about the square wave output -
> that is not a problem, at all. Get the dimming portion of what you want
> working first, sort out the timing part later.
>
> Be sure to check that the voltage coming from the supply included with the
> lights is 12 volts DC.

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. 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.

And yes, 200 Hz is high enough to make the flicker go away. :-)

Have Fun!
Rich

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