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L.E.D.`s - series or parallel?

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Superuser
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Re: L.E.D.`s - series or parallel?

@Baldrick1  No problem, I use the same excuse normally😀

TBH I when I saw your figures , I had to use the calculator to make sure I wasnt getting it wrong

@shutter another useful link https://www.allaboutcircuits.com/tools/led-resistor-calculator/ it also explains polarity

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Community Veteran
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Re: L.E.D.`s - series or parallel?

@Baldrick1  Thanks for the "new improved table" !... I get confused when I am trying to read those colour bands too ! ...it must be an "age problem"... at least we have that as an excuse.

Thanks again...

much appreciated

 

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Re: L.E.D.`s - series or parallel?

@MisterW   thanks for the formula to work out how long the battery pack will last !.

this is getting to be really educational....  !   all I wanted to do was backlight a sheet of paper ! ..

Will post some pics of the finished item when I have done it. !

Thanks for your help and input, too..

 

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Seasoned Pro
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Re: L.E.D.`s - series or parallel?

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Hero
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Re: L.E.D.`s - series or parallel?

@shutter 

In the distant past I was working as an electronics development engineer. These were the days when if you wanted a powersupply you sat down, designed it then bread boarded a prototype. Then the colours came naturally to me. If I needed say a 2k7 resistor I would go staight  to the rack for Red/violet/red without thinking. If alternatively you asked me what colour number 8 was I would have to think about it.

This was simply me trying to do mental arrithmetic as I was too lazy to set up a simple spreadsheet. The mental agility obviously isn't what it used to be!

Sorry for the confusion.

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Re: L.E.D.`s - series or parallel?

@Mook   Thanks... but I do have an old one of these... ( about 40years old, in fact ! )

COLOUR CODE WHEEL.png

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Re: L.E.D.`s - series or parallel?

Hi Guys !..

Update on progress.... I made up a "Light Bar".. with 4 leds and worked out the resistance required via the linky Baldrick1 posted... it was 10 ohms... just happened to have 20 4.7 ohms resistors kicking around, so did 4 sets of 2 x4.7 in series... and appended them to the positives of the led`s... bunged on the 4.5v batterypack.. and all was bright!..

then I started to fit the lightbar in to the box...but could not get enough light in the right places under the "screen"... so I am going to have a rethink, about how many led`s I need... possibly another 4 in a row opposite side ...so that is the mk 2 version for sometime in the next week or so.. !

 

 

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Superuser
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Re: L.E.D.`s - series or parallel?

and worked out the resistance required via the linky Baldrick1 posted... it was 10 ohms.

@shutter  not sure where you get 10 ohms from, with a single led and a 4.5v supply it should be 47 ohms!

10 ohms is going to give about 90mA which will brick the led fairly quickly I would imagine!

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Re: L.E.D.`s - series or parallel?

Hmm  !   dunno !  .. can`t seem to "reproduce" the result... even by manipulating the voltage.. the nearest I get now is 25 ohms...  using 4.0v as the power, as I measured it from the 3AA battery power pack.

All were very bright, probably looked as bright as if just connected direct to the power pack ( its got a 9v type plug on connector, so easy to "short" the led across the terminals.

Anyway.. I shall do the calculations again, when I make the next light bar, and see how that goes.. not panicing about it now ! ..

cheers Wink

 

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Hero
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Re: L.E.D.`s - series or parallel?

@shutter 

It sounds as if you are seriously over driving the LEDs. Maybe thet are not very bright devices when driven properly? If you have an ammeter see how bright they are at 20 mA.

I seem to have read something in the past relating to things to do with a dead laptop. Strikes me that the screen backlight from one would do what you want.

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Re: L.E.D.`s - series or parallel?

@Baldrick1  OK.. will have a look at that tomorrow.. but they seemed to be very bright to me... as mentioned... probably as bright with no resistor..

 

Point of interest... they seem to "throw a round beam"  ( that widens out with distance from the led.)..so I have mounted them "horizontally" so that the sides of the beam are the main source of light under the screen... I have to work out how to make a reflective surface (space and location make it difficult! ) ....  to throw the beam back across the screen and slightly bend it upwards..( sort of like dipped headlight in reverse)  If I used the leds "facing upwards" I would have four round bright circles of light on the screen surface, which is not what I envisaged.

Yes. understand the idea of a duff laptop... !   but i don`t have one to mess with.. and it would probably be more hassle, to figure out how to get that to do what I want..

cheers

 

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Hero
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Re: L.E.D.`s - series or parallel?


@shutter wrote:

 

.....but could not get enough light in the right places under the "screen"... so I am going to have a rethink, about how many led`s I need... possibly another 4 in a row opposite side ...so that is the mk 2 version for sometime in the next week or so.. !


@shutter 

ISTR it was you who posted a YouTube video of things to do with an old LCD screen, which involved removing various parts of it.

Couldn't you cobble something up using one of those?

If it was LED illuminated they would be around the edge firing inwards to give nice even and bright effect.

But you probably haven't got one knocking about!

Edit:  Oops, I hadn't read to the end of the thread and @Baldrick1  beat me to it!  Embarrassed

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Re: L.E.D.`s - series or parallel?


@RobPN wrote:

But you probably haven't got one knocking about!

Edit:  Oops, I hadn't read to the end of the thread and @Baldrick1  beat me to it!  Embarrassed


Hah Hah ! ...

My next trial will involve usiing 5mm internal dia plastic tubing in a series of cross screen strips. LED in one end.. and the light should diffuse along the plastic tube...

 

It worked when I made my "flashing Morse key"...

 

https://nemosphotography.blogspot.com/2019/05/illuminated-morse-key.html

 

so hopefully it will work for this project..

 

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Hero
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Re: L.E.D.`s - series or parallel?


@shutter wrote:

 

My next trial will involve usiing 5mm internal dia plastic tubing in a series of cross screen strips. LED in one end.. and the light should diffuse along the plastic tube...

 


@shutter 

So how large and how bright does this portable 'light-box' need to be?

Couldn't you use a laptop or tablet 'as is' by just displaying a white image at full brightness?

Portable, battery powered and rechargeable! Thumbs_Up

Edit: and ready-made.  If it ain't broke don't fix it. Wink

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Community Veteran
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Re: L.E.D.`s - series or parallel?

The lightbox, is "kinda" wedge shaped... it is about 2" high ( at far end )  15" wide X 21" in length.but the actual area that will be lit up, is only A5 paper size...

The reason for the size and shape, is that I want to use it as a "writing" surface... where the A5 lighted area will show light underneath a sheet of lined paper, to give me guide lines, to write on the upper sheet, without using pencil to mark the paper up.

With "ordinary" black lines, either produced by pen & ink, fibre pens or printed from the printer. do not show sufficiently well for me to use without a lighted undersurface area.

Originally , I thought about having a full size A4 lit area, but thought that I would only need the smaller ( half size A5) size due to the fact that I would be writing, or drawing, only on a smaller area at a time.