Hobby Lighting

To All,
My son built a replica of an arena (about 2' x 3'). We wanted to add lights to it and really couldn't find anything small and that would work so I took
a strand of Christmas lights and since there were too many, I cut it to get about 20 lights and rewired it. They worked but then the next day they didn't. It looked like many had burnt out. Is it because there were less lights that the power was then too strong? Any ideas if I can get around this or better, does anyone know of any lighting that has small bulbs (kind of for hobbies) that I could use?
Thanks, Mike
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Mike wrote:

I am not sure what an "arena" is, in this context.
However, if it is not too girly for the purpose, there is a wide range of light fixtures and fittings available for dolls' houses.
You are quite correct about having too few lights will cause premature failure of a string of conventional Christmas lights. Each lamp is specially designed to short itself out when it fails - reducing the string still further and shortening the life of the remaining lamps. But meaning that the string will be tolerant to one or two failed lamps. Normally there is a special "fuse lamp" that protects what remains when the remaining number of working lamps has fallen too low. You may have chopped this one out as well - so be glad that your arena didn't turn into a recreation of Towering Inferno.
However, you can get low voltage (sometimes, in the UK anyway, referred to as outdoor) Christmas tree light sets. These have a transformer and sometimes even an electronic controller to change the lights in purty patterns. You can also get LED Christmas lights. Which do much the same thing, but the LEDs should last longer. They may be a better bet for pruning down to the number of lights that you want.
Now is a good time to buy. My local shop has a box of 480 LED lamps and controller on sale, reduced to 9.99GBP.. Quite a cheap way of buying LEDs..
--

Sue



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Last time I was in a hobby shop (which I admit was a long time ago) they use to sell "pea" lamps for model railroaders. At the time, it was the smallest light you could get and they were pretty bright too.
Rather than cut up a Christmas Tree light string, if you have just a bit of soldering skill I would recommend purchasing individual LED's. Radio Shack has them fairly cheap. Companies like Jameco Electronics and MCM Electronics have them for even cheaper.
You can run them off of standard DC voltages (from a plug in DC power adapter) like 12 Volts, 9 volts, or 6 volts, but you do need to put a dropping resistor in series with the LED to keep it from getting too high a voltage and burning out.
The value of this resistor is fairly easy to calculate. Maybe someone here can show you how to do it.
The advantage of LED's is that they run cool, last for tens of thousands of hours (if you limit the current properly), come in just about every color available, use very little current, and are available in high-brightness versions, if desired.
Beachcomber
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Beachcomber wrote:

Take a look at this: stock # 15101 OP at MPJA http://www.mpja.com /
100 LEDs for $1.95
Ed
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Check for lighting at doll house sites and stores; lots of them around. I was amazed at what I found - even working light switches!
Unfortunately I can only send yuou to Google though; can't find the darned links!
Caution: Getting too realistic and "authentic" can be addicting AND expensive!
Pop
wrote: : : >Mike wrote: : >> To All, : >> : >> : >> : >> My son built a replica of an arena (about 2' x 3'). We wanted to add lights : >> to it and really couldn't find anything small and that would work so I took : >> a strand of Christmas lights and since there were too many, I cut it to get : >> about 20 lights and rewired it. They worked but then the next day they didn't. : >> It looked like many had burnt out. Is it because there were less lights : >> that the power was then too strong? Any ideas if I can get around this or : >> better, does anyone know of any lighting that has small bulbs (kind of for : >> hobbies) that I could use? : >> : >> : >I am not sure what an "arena" is, in this context. : > : >However, if it is not too girly for the purpose, there is a wide range : >of light fixtures and fittings available for dolls' houses. : > : >You are quite correct about having too few lights will cause premature : >failure of a string of conventional Christmas lights. Each lamp is : >specially designed to short itself out when it fails - reducing the : >string still further and shortening the life of the remaining lamps. But : >meaning that the string will be tolerant to one or two failed lamps. : >Normally there is a special "fuse lamp" that protects what remains when : >the remaining number of working lamps has fallen too low. You may have : >chopped this one out as well - so be glad that your arena didn't turn : >into a recreation of Towering Inferno. : > : >However, you can get low voltage (sometimes, in the UK anyway, referred : >to as outdoor) Christmas tree light sets. These have a transformer and : >sometimes even an electronic controller to change the lights in purty : >patterns. You can also get LED Christmas lights. Which do much the same : >thing, but the LEDs should last longer. They may be a better bet for : >pruning down to the number of lights that you want. : > : >Now is a good time to buy. My local shop has a box of 480 LED lamps and : >controller on sale, reduced to 9.99GBP.. Quite a cheap way of buying LEDs.. : > : >-- : : Last time I was in a hobby shop (which I admit was a long time ago) : they use to sell "pea" lamps for model railroaders. At the time, it : was the smallest light you could get and they were pretty bright too. : : Rather than cut up a Christmas Tree light string, if you have just a : bit of soldering skill I would recommend purchasing individual LED's. : Radio Shack has them fairly cheap. Companies like Jameco Electronics : and MCM Electronics have them for even cheaper. : : You can run them off of standard DC voltages (from a plug in DC power : adapter) like 12 Volts, 9 volts, or 6 volts, but you do need to put a : dropping resistor in series with the LED to keep it from getting too : high a voltage and burning out. : : The value of this resistor is fairly easy to calculate. Maybe someone : here can show you how to do it. : : The advantage of LED's is that they run cool, last for tens of : thousands of hours (if you limit the current properly), come in just : about every color available, use very little current, and are : available in high-brightness versions, if desired. : : Beachcomber : :
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(pardon for the repost, can't seem to respond and have to post as new)
Thanks for your help. I took a quick look at the doll house lights but don't see a type of light that I'd want. The LEDs sound good and I can solder but don't know enough about what to purchase and how to configure it. I'll explain what we want.
My son built the Coninental Airlines Arena (where the NJ Devils play). It looks great and has see through panels so you can see inside but it is dark. It is about 2' x 3'. We'd like to have some lighting underneath the roof and also some lighting around the outside of the arena. These need to be 2 separate runs since the entire roof (and lighting) can come off to really look inside. How many LEDs do you think would be needed for this inside portion? Can they be setup with a on/off switch and batteries so that we would not need to plug it in? I would need to know also about any other issues and where to put what (i.e. resistors as you mentioned). If this is too much to ask, do you think I could get the info at the Radio Shack?
Thanks again.
wrote:

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Calculating the dropping resistor for an LED circuit is really not complicated.
I found a site that may help you. The circuit is just a battery (or DC output plug in supply), a resistor, a LED, and maybe a switch, all in series with each other.
http://www.theledlight.com/resistancecalculator.html
LED's are polarized (+ -) so if you hook them up backwards, nothing bad happens... they just don't light up.
Most LED's require 0.20 ma and anywhere from 2.0 to 3.5 volts. The packaging should have this information.
Then you look at your power supply voltage. Let's say you have an LED that needs 2.0 volts and a power supply battery that puts out 6 volts. That means your resistor needs to drop 4 volts (6 - 2) = 4 volts.
By ohms law, your resistor should be 4 / 0.20 = 200 ohms
The power dissipated by the resistor will be I^2 x R or 0.20 x 0.20 x 200 = .08 watts so a common 1/4 watt resistor (0.25 watt) will work just fine.
Beachcomber
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Beachcomber wrote:

This requires current entries in mA. May be confusing!

That should be 20 mA or O.020 A.

NO! Ohm law requires R in ohms, I, (current), in Amperes, V in Volts. This would be: 4/0.020 = 200 ohms (Right answer, wrong math!)

Actually, 0.20 x 0.20 x 200 = 8W (But the current is 0.02 A so: 0.02 x 0.02 x 200 = 0.08)
so a common 1/4 watt resistor (0.25 watt) will

Right answer, wrong math.
Checking by: W = E x I = 4 x 0.020 = .08 W
--
Virg Wall, P.E.

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Mike wrote:

There are dozens of web pages devoted to LEDs and how they work and how to use them, for example:
http://www.theledlight.com/LED101.html
A Google search will find far more.
An alternative for generating many pin-points of lights (eg Starship Andromeda style) is to use fibre optics. Buy a cheap "mare's tail" fibre optic desk lamp and redeploy a number of the fibres, spreading them to where the lights are needed but using a single light source to "power" them.
They will run for many hours off a battery. Even if an incandescent lamp is used instead of an LED.
For area illumination, rather than point sources of light, you may want to check out cold-cathode kits. They are fitting them in (well, mostly under) cars and computers. You would need a bigger battery though.
For low levels of "atmospheric" lighting, you might want to check out electroluminescent cable. It uses very little power and can be very effective.
--
Sue












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it.
as you can see there are a lot of options. let me throw out another idea. each of the xmas light bulbs probably is 2.5 volts ( see http://people.howstuffworks.com/christmas-lights.htm )
you simply obtain a power supply, either AC or DC at some voltage... the easiest to come by is probably 12 to 13.8. next cut sections of bulbs say 5 or 6 depending on how bright you want them to be. and then keep adding 'zones'
the advantage here is you have a "safe" 12 volts running through the project instead of 120.
a rough estimate of .5W @ 2.5V = gives .2 amp per 'zone'
a fuse for each zone might come in handy (see the shunt thingy in xmas lights) i would pick 1/2 A as a nice common value.
good luck and post a pic somewhere when its done :)
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I would go for the LED arrangement, you can put some in series and although still need a current limiting resistor, the battery drain will be less. Lantern type 6V batteries are good for this type of project. Another poster suggested cutting down an outdoor christmas light set with a dimmer. I would not trust this because the dimmer might still pulse full voltage on the string resulting on a safety hazard.
With the LED's you have to be careful of polarity and be sure to adjust the resistor value properly. They will burn out if the voltage is too high. Once done, they are foolproof.
Joe
Mike wrote:

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Joe Leikhim K4SAT
"The RFI-EMI-GUY"
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Thanks for all the help from everyone.
I'm trying to calculate exactly what I need. I'm not sure how many LEDs but let's say I had 10 and did the following.
batteries (3 x 9volt) on/off switch resistor LEDs 10
Would that be correct? As far as the resistor, I came up with the following (trying to learn). If each LED is 20 mA and 2V, then 10 is 200 mA and 20V. If I have a 27V power supply, then I need to remove 7V. 7V/.2A5 Ohms Then I'm lost. What resistor would I need?
As far as the wire, any special type? Should it be coated?
Thanks again. Mike
PS. My son also built Shea Stadium (NY Mets, about 3' in diameter) and we put false lights but are thinking to go back and put real ones. Would the LEDs give off a nice glow to the field?
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Mike wrote:

Yes - but you might want to use a 24V DC wall wart supply instead. Something like DCTX-2450 at $4.50 from Allelectronics http://www.allelectronics.com/ would supply power to your ten leds in series with a 470 ohm 1/2 watt resistor, and you wouldn't have to worry about running the batteries down. And - with that supply, you could run a number of 10 LED, 470 ohm resistor strings in parallel, so you would not be limited to 10 LEDs.

No - with the LEDs in series, the current stays the same, it does not add. The voltage does add. So it would be 20 volts at 20 mA.
If I have a 27V

You have the right formula, but the current is 20 mA, not 200 mA, so it's 7/.02 or 350 ohms.

You can use a standard value 330 ohms, at 1/2 watt resistor. Your LEDs would glow very brightly. Personally, I would use a 470 ohm resistor. The LEDS won't glow as brightly (but they will still be bright) and the batteries will last longer.
Someone said LEDs require 20 mA. That is *not* the best way to say it. It would be better to give a range, or say something like "figure about 20 mA". An LED will glow nicely at well under 20 mA - it's just that 20 mA seems like a good design goal for this project. I mention this because you may wonder about the two different value of resistance above.
Understand that we are talking about LEDs that are not white. They are red, amber, yellow, green - etc, but not white. White LEDs require higher voltage (figure 3.6v per LED) and are more expensive. Also, some of them have a blue-ish tint to the light.

No special type needed. Just use some insulated wire that is easy to work with.

No, unless you use white LEDs that don't have a blue-ish tint to them. Even then, "nice glow" is subjective, so its hard to say if you'd like it or not. Perhaps the blue tint would be fine.
Ed

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Not quite. If you connect them in series, in one, out to the next, etc, then the current remains at 20 mA, and the total LED voltage is 20 volts. The resistor needs to drop 7 volts at 20 mA, so its value is 7/.0250 ohms (390 is a standard value, so the current will be slightly lower). The power in the resistor will be 7*.02 or .14 watt, so a 1/4 watt resistor will be fine.
Ben Miller
--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
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Thanks. It's getting clearer.
How did you know to use the 1/2 watt resistor and if my formula changes (more LEDs), how do I come up with that? And when you bumpbed the resistor from 330 to 470, how did you know that? Is it that they don't make any other ohm resistor in the middle?
At this point, I'm guessing about the 10 LEDs. Is this a trial by error situation? Setup 10, see how bright it is, add/subtract? I think we would want the white LEDs to try and emulate the arena lighting.

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Mike wrote:

Here's the computation I used: 10 LEDS at ~1.8V per is 18 volts. Your supply is 27 volts. The difference is 9 volts which must be dropped in the resistor. With a 330 ohm resistor, current will be over 27 mA to drop 9 V, ( current = 9/330 = .02727 ) and the power dissipated will be > .245 W ( power = 9*.0272 = .24545 ) That's just a bit under a 1/4 watt. A safety margin in heat dissipation is always advisable, so you go to the next higher standard wattage, which is 1/2 watt. The formulas: I = (SupplyV-Total_Vf)/R where Total_Vf is the forward voltage drop of one LED times the number of LEDs in the string; R is the resistor value in ohms, and I is the current in amperes. P = E*I where P is the power to be dissipated in the resistor; E is the voltage across the resistor; and I is the current determined in the first formula.
If you increase the number of LEDS and do not change the supply voltage, just use 1/2 as the wattage figure and you'll be fine on that account. If you change the supply voltage, you must do the computation.
And when you bumpbed the resistor

You can get resistors between 330 ohms and 470 ohms. But those two - 330 and 470 - are the most commonly used standard sizes in that range, and are available at Radio Shack if that's where it's convenient for you. Sizes between those values are not available there.
In a general project, a good target for current through a LED is usually considered to be 20 mA. The 470 ohm resistor will give you 19.1 mA, while the 330 ohm will give you 27.27 mA. 470 therefore gets you closer to the target and provides a larger margin between the operating current and the maximum current the LED can stand.

Determining the resistor size is by formula - but for figuring out the most pleasing light, I suppose it is trial and error. If you use white LEDs you need to find out what their Vf figure is. It will *not* be ~1.8 volts. If you do not have a data sheet on the white LEDs you plan to use, then figure the Vf at 3.6 volts per LED.
Ed

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