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From: Dungeon Dave on 3 Dec 2008 13:52
Hi, all. Relative newb here - relative in the sense that I used to dabble
in some electronics as a youngster and did some electronics theory in A-
level physics but that don't make me an expert as such.

Okay, so onto my problem. I bought a set of these the other day:

http://www.wickes.co.uk/Kitchen-Lighting/Accent-Ten-LED-Indoor-Light-
Kit/invt/190803

(actually, that doesn't look like the right package, but it'll do for the
purpose of this query)

- they're a string of white "nail" LEDs that are intended to mount in
thin plinths, powered by a plug-in transformer that delivers 12V to a
matchbox-sized distribution point. Since the DB just consists of 13
powered sockets in parallel connection, I'm guessing each LED has a
resistor in series to provide the correct voltage across the LED itself
(they may be bright, but I ain't heard of any LED that can take the ommpf
that 12V will pack and survive for too long!)

During the day, these lamps are bright enough, but I want to subdue them
for evening and night usage (I'm planning on leaving them on as
nightlights on the stairs, etc), so thought of just using a 12V dimmer of
some kind. Bill Bowden's site came up in a few google searches, and was
thinking of employing something like:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer.gif

(the site directed me to this forum)

However, it had me thinking: firstly, I have a transformer to drop
240AC->12DC, then another circuit on to lower the 12V... would I benefit
from combining the two into one? Build something that delivers a
rectified 2-12V output (looks like 5W max, BTW)? If so, are there
diagrams/kits (Vellmen, RS Electronics etc) readily available?

Secondly, I'd like to combine this with a timer circuit in some way, so
that it'd fire up at 100% brightness at one set time, dim to 20% (or
whatever preset I use) at a second time, then turn off completely at a
third time. I was toying with the idea of linking in a serial feed from a
USB controller then firing control information from my server (Linux,
running 24x7) so I could easily schedule something that determines what
brightness at what times of day I require, letting that ramp up/down the
lighting as I see fit. I've seen various projects online that cover this
info, but before dipping my toe in there, are there any more readily-
available circuits that can do this kind of timing-based control? Could I
hack away at a central heating controller? I'd prefer to have the self-
contained wall-mounted jobby if possible.

Okay... I know expecting someone to design something that exactly fits my
needs is asking a lot, given I've just strolled in here. However, I'm
guessing that if I'm going ot be reinventing the wheel through my
ignorance, I'm hoping someone can at least put me on the right road.

Thanks in advance, people!
From: miso on 3 Dec 2008 15:43
On Dec 3, 10:52 am, Dungeon Dave <howl...(a)the.moon> wrote:
> Hi, all. Relative newb here - relative in the sense that I used to dabble
> in some electronics as a youngster and did some electronics theory in A-
> level physics but that don't make me an expert as such.
>
> Okay, so onto my problem. I bought a set of these the other day:
>
> http://www.wickes.co.uk/Kitchen-Lighting/Accent-Ten-LED-Indoor-Light-
> Kit/invt/190803
>
> (actually, that doesn't look like the right package, but it'll do for the
> purpose of this query)
>
> - they're a string of white "nail" LEDs that are intended to mount in
> thin plinths, powered by a plug-in transformer that delivers 12V to a
> matchbox-sized distribution point. Since the DB just consists of 13
> powered sockets in parallel connection, I'm guessing each LED has a
> resistor in series to provide the correct voltage across the LED itself
> (they may be bright, but I ain't heard of any LED that can take the ommpf
> that 12V will pack and survive for too long!)
>
> During the day, these lamps are bright enough, but I want to subdue them
> for evening and night usage (I'm planning on leaving them on as
> nightlights on the stairs, etc), so thought of just using a 12V dimmer of
> some kind. Bill Bowden's site came up in a few google searches, and was
> thinking of employing something like:
>
> http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer...
>
> (the site directed me to this forum)
>
> However, it had me thinking: firstly, I have a transformer to drop
> 240AC->12DC, then another circuit on to lower the 12V... would I benefit
> from combining the two into one? Build something that delivers a
> rectified 2-12V output (looks like 5W max, BTW)? If so, are there
> diagrams/kits (Vellmen, RS Electronics etc) readily available?
>
> Secondly, I'd like to combine this with a timer circuit in some way, so
> that it'd fire up at 100% brightness at one set time, dim to 20% (or
> whatever preset I use) at a second time, then turn off completely at a
> third time. I was toying with the idea of linking in a serial feed from a
> USB controller then firing control information from my server (Linux,
> running 24x7) so I could easily schedule something that determines what
> brightness at what times of day I require, letting that ramp up/down the
> lighting as I see fit. I've seen various projects online that cover this
> info, but before dipping my toe in there, are there any more readily-
> available circuits that can do this kind of timing-based control? Could I
> hack away at a central heating controller? I'd prefer to have the self-
> contained wall-mounted jobby if possible.
>
> Okay... I know expecting someone to design something that exactly fits my
> needs is asking a lot, given I've just strolled in here. However, I'm
> guessing that if I'm going ot be reinventing the wheel through my
> ignorance, I'm hoping someone can at least put me on the right road.
>
> Thanks in advance, people!

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.

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.
From: Jasen Betts on 4 Dec 2008 00:05
On 2008-12-03, Dungeon Dave <howling(a)the.moon> wrote:
> Hi, all. Relative newb here - relative in the sense that I used to dabble
> in some electronics as a youngster and did some electronics theory in A-
> level physics but that don't make me an expert as such.

> During the day, these lamps are bright enough, but I want to subdue them
> for evening and night usage (I'm planning on leaving them on as
> nightlights on the stairs, etc), so thought of just using a 12V dimmer of
> some kind. Bill Bowden's site came up in a few google searches, and was
> thinking of employing something like:
>
> http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer.gif

555-based PWM lamp dimmer, a good choice.

> (the site directed me to this forum)

> However, it had me thinking: firstly, I have a transformer to drop
> 240AC->12DC, then another circuit on to lower the 12V... would I benefit
> from combining the two into one? Build something that delivers a
> rectified 2-12V output (looks like 5W max, BTW)? If so, are there
> diagrams/kits (Vellmen, RS Electronics etc) readily available?

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.

> Secondly, I'd like to combine this with a timer circuit in some way, so
> that it'd fire up at 100% brightness at one set time, dim to 20% (or
> whatever preset I use) at a second time, then turn off completely at a
> third time.

> I've seen various projects online that cover this
> info, but before dipping my toe in there, are there any more readily-
> available circuits that can do this kind of timing-based control?

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.

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.


From: panteltje on 4 Dec 2008 08:15
On Dec 3, 7:52 pm, Dungeon Dave <howl...(a)the.moon> wrote:

> However, it had me thinking: firstly, I have a transformer to drop
> 240AC->12DC, then another circuit on to lower the 12V... would I benefit
> from combining the two into one? Build something that delivers a
> rectified 2-12V output (looks like 5W max, BTW)? If so, are there
> diagrams/kits (Vellmen, RS Electronics etc) readily available?

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.

> Secondly, I'd like to combine this with a timer circuit in some way, so
> that it'd fire up at 100% brightness at one set time, dim to 20% (or
> whatever preset I use) at a second time, then turn off completely at a
> third time. I was toying with the idea of linking in a serial feed from a
> USB 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

From: Dungeon Dave on 4 Dec 2008 08:50

>
> 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!
>

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