Night Lighting Structures: Where to Start?

[NittanyLion]

I'm planning a few little dioramas and I want to rig them for lighting.  Street lights, interiors, etc.  I want them to be completely self-contained, so battery power is a must (as opposed to a wall wart or powerpack).  Unfortunately for me, I have no idea what I'm getting myself in to.  I've made several assumptions:

1) the total voltage of the total number of LEDs I want to use must be a lower number than the total voltage of the battery pack powering the diorama.

2) resistors on one (but which?) wire going into each LED.  But how do I know what sort of resistor to use?

3) the power bus for the LEDs can't draw directly from the batteries themselves.  I have no idea why I think this.  

This is a total "explain it to me like I'm five" situation.  Heck, I'm not even sure where I'd acquire most of the components I need (the batteries have to go IN something!).  I guess a good starter would be that I have two of those Walthers streetlights with the plug base.  I hooked one up to a powerpack and my multimeter told me that it was drawing 4.5v DC at the light level I liked.  So my basic assumption is that two of them would be drawing 9v.  Which leads me to believe I need: some sort of thing to hold the batteries, an on/off switch, resistors (to ensure that each light is only drawing 4.5?  would they go on a + or - wire?  how would I tell them apart anyhow?  how do I know what resistors to use?), and the lights.  

Some of my plans are fairly ambitious in the long run, so I figured I should start small.

[richg1998]

Sounds like you need to do some research and stay away from assumptions.

http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm

http://www.moreleds.com/railroad.htm

Generally you cannot change light intensity like you can will light bulbs which you may not want to do anyway.

Many LED's are point source. Inverted cone LED's can work quite nicely as they give 360 lighting.

Rich

[Mark R.]

When doing calculations for what resistors to use and what type of power supply to use, you need to know not only the voltage, but the current rating of the LED or bulb, which is usually in milli-amps (ma = thousandths of an amp).

1) Your LEDs will be wired in parallel, so the total voltage of all the LEDs isn't your calculation - it would be if you are wiring them in series .... but in this case you wouldn't. If you are using white LEDs, they run on about 3.4 volts. So not matter how many 3.4 volt LEDs you have in parallel, the buss only needs 3.4 volts on it, BUT, you add up all the current ratings as a total current requirement for all the LEDs. Simply put ....

Series - Add up the total voltage, total current is the same as a single unit

Parallel - Add up the total current, total voltage is the same as a single unit 

2) It doesn't matter which leg you attach the resistor to. The "best practices" method says you should have a resistor on each LED. You can use the online calculator to figure out what resistor to use ....

http://ledcalculator.net/

Using the above calculator with a 9-volt battery, a 3.4 volt LED and an average industry amp rating of 20ma, it shows you would need a 300 ohm resistor on each LED. That being said, you CAN wire all your LEDs in parallel and just use a single resistor on the input to the LED buss. Use this calculator ....

http://www.hobby-hour.com/electronics/ledcalc.php

Using the same figures as before, but calculating for ten LEDs in parallel with a single resistor on the buss, the resistor would only be 30 ohms .... BUT, you would need at least a 2 watt rated resistor, which is much bigger than the typical 1/4 or 1/2 watt resistors we usually use.

3) Your buss runs directly from your battery source (although I'd suggest running it through a switch so you won't have to remove the battery(s) to turn it off). Then attach your resistors as mentioned above.

4) I'm add my own number 4 .... battery life. At the bare basics, a standard 9 volt battery is rated at about 300ma/hrs (will provide 300ma of power for one hour). A single white LED is rated at 20ma of current draw. 300 divided by 20 equals 15 hours of run time until the battery is dead. If you are running ten LEDs, the battery will be dead in 1-1/2 hours. Granted, you can buy much higher rated batteries - some rechargables with ratings as high as 1200ma/hrs ! These would run ten LEDs for 6 hours.

Mark. 

[hon30critter]

Nittanylion

A complete answer to your question would involve writing a small book so I will only make a few suggestions.

First, any plastic structure you want to light needs to have something on it (inside or outside - it doesn't matter) to prevent light from leaking through the walls and joints. I spray my plastic structures flat black before the colour coat and it does the trick. A decent coat of grey or red primer will also work.

Second, if you are going to use batteries then resistors are not necessary. Most white LEDs operate around three volts, so two 1.5 volt batteries in series should give you the right power. The question is how many LEDs you want to operate. If there are only a half dozen or so then you can get away with just two AAs. If you want to run a lot of LEDs then you will have to use larger batteries in a "2 in series, 'x' in parallel" set up. As far as battery holders go, here is one great source:

http://www.digikey.ca/scripts/dksearch/dksus.dll

To answer your third question, you need to figure out how to set up a "buss" system. Basically that means you will have a positive wire and a negative wire running through your diorama to which you will connect all of your LEDs. The end of the buss will be connected to your battery pack through an off/on switch. I use phosphor bronze wire for the buss lines because it is quite stiff and is easy to solder to. Here is an example of the buss wiring in my engine workshop:

 

 

This is set up for 12 volts hence the resistors but if you study the wiring you will be able to identify the buss wires and how they are connected across the three parts of the structure. (Note: I am still working on the wiring at this stage. I will remove the wiring to paint the structure black first, and then I will paint it with the appropriate colours).

LEDs are polarity sensitive, but keeping them all lined up the same way is as simple as looking at the length of the leads. One is longer than the other. Wire them all the same way. A simple test will tell you which is negative and which is positive so you can hook it up to the battery pack properly. If you wire it all up and nothing lights then either you have your positive and negative leads reversed on the battery pack or, heaven forbid, you have installed an LED backwards! To avoid that disaster, test as you go!!!!

The street lights you tested are likely not LEDs if they operated at 4.5 volts and you were able to vary the light output a lot. A previous poster suggested that you can't vary the output of an LED much but I politely disagree. LEDs are quite capable of being adjusted by changing the input voltage.

Hope this helps.

Don't hesitate to ask more questions.

Dave

 

 

[NittanyLion]

Not quite sure where I got the idea that they were LEDs in those streetlights.  They're pretty clearly little regular bulbs in there.

I guess, having completely over estimated the complexity of the question, I will return to this at some later date with a more specific question in mind!

[rrinker]

 Well, you are really in luck because the current issue of MR has a bunch of information on lighting things up. Definitely worth a look.

                    --Randy

 

[MisterBeasley]

Leds draw very little power, but small incandescents use about 30 ma each.  That's for the Miniatronics bulbs I use, 16v.  By comparison, I'd guess that the Walthers ones used in their street lights are about the same.

If you had 10 of them, that would be 300 milliamps.  I looked up the output rating of Duracell D Cells, and found it to be 14.7 amp hours.  Of course, that's only a 1.5 volt battery.  I run my 16 volt bulbs at 12 volts, so you would need 8 batteries.  The good news, though, is that those batteries should run 10 300 milliamp bulbs for 49 hours.