Installing lighting in buildings

[ColinK]

One of my friends has asked me to help install lighting in a large scale model house. While I know how to do this using miniature bulbs, I would like to use LED's for this to reduce current draw and because LED's rarely blow (unless the current is sent the wrong way).

 

We are looking at installing 8 lights, probably wired in parallel, single on/off switch and battery powered. The LED's will need to be fairly bright. I'm hoping that by having them battery powered I can avoid having to install resistors. However, I think I should include a 'normal' diode in the circuit to avoid blowing all the LED's by reverse polarity.

 

Any advice would be appreciated, especially on what type of LED's and what diode to use. Many thanks.

[Ray H]

Have you thought of the LED strips of lighting? They come with adhesive tape backing and some can be broken down into a specific number of LEDs rather than having a fixed length strip.

[cpman46]

Most LED strips are sold for 12V dc operation and as Ray H says some can be cut down into smaller numbers. The strips contain resistors to limit the current through the LEDs. If you are proposing to use battery power and individual LEDs in parallel then you will need to fit current limiting resistors to each LED otherwise you will find one LED may hog most of the current resulting in different brightnesses. Without any current limiting resistors you will most certainly blow your LEDs.

 

You do not say what battery voltage you are proposing to use. The idea of a rectifier diode e.g. 1N4002 (1A) in series with the positive supply lead to guard against reverse polarity is a good one.

 

The following LED series /parallel array wizard may help if you are using individual LEDs. http://led.linear1.org/led.wiz

[vulcanbomber]

Have a look at these http://www.maplin.co.uk/12vdc-led-strip-218015

 

They can be rewired as single LEDS if you want 

[Crosland]

You must always use a resistor and LEDs in parallel should have individual resistors.

 

It's far more efficient in this kind of application to wire the LEDs in series. If you can't provide a high enough voltage for all 8, then consider two parallel strings of four LEDs .

 

Andrew

[corax67]

We have had a lot of success recently fitting lighting to all of the houses & shops on our church layout using a reel of SMD LED strip I purchased off eBay of the princely sum of £6.

 

It is a five metre strip containing 300 LEDs which can be cut into multiples of 3 LEDs all of which work on 6-18v and at the end of each cutting point are solder pads for easy wiring.

 

We are running ours off the 12v loop we have running under the layout for this exact purpose connected to a variable power supply (I think it's an OnTracks unit)

 

The strips are also fully waterproof and run really nicely off a little 4-AA cell battery box I use as a test rig so should be perfect for your purpose.

[ColinK]

Many thanks for your replies. I hadn't heard of LED strips before, so I'll check them out. Thay might also be handy for providing some light in my partially covered fiddle yard where there is a 12v supply I could use.

[Emmo]

What value resistor is required if wiring LEDs in parallel? I will be using 12v DC (via mains) to illuminate them.

[corax67]

The resistor value depends entirely on what LED you use, 3v LEDs use an entirely different value to 12v ones and the strips need no resistor at all.

 

If we can have some details we can give you a definitive answer

[Emmo]

12v

[naugytrax]

Try 1k ohms. If too bright, use 2.2k ohms instead.

[corax67]

If too dark substitute a 470 ohm resistor

[TrevorGCB&W]

Have a look at my post in "Help with platform lighting"

 

Trevor

 

An (old) engineer who does for a tanner what anyone else does for a bob.

[DCB]

Interesting stuff, but going back to the original post DO NOT USE RECHARGABLE BATTERIES, you could melt a plastic building or start a fire in a card one as they have virtually no internal resistance and will literally produce flames or make connecting wires glow red  if shorted out when fully charged..  I melted a deep groove in my Lima 37 when its 2XAA NiMH Power pack shorted.  Ordinary non rechargable batteries are fine, even pairs of flat ones from a old torch will make couple of 5mm yellow LEDs glow for several hours at a time

 

Oddly enough speaking as  as someone who habitually uses LEDs without resistors, (and creates great annoyance by so doing) you will need resistors to get bright yellow LEDs from batteries,  2X non rechargable  AAs in series will not  provide enough voltage for really bright yellow LEDs, my 2.9 volt power pack certainly does not with Maplin 3mm and 5 mm Yellow LEDs and 3 X AA will be far too much voltage. so for bright Yellow you will need resistor or 8.

 

Note a 3 mm Red LED.will draw far more current for the same voltage than a 3mm Yellow LED so don't mix .  Even on 2 flat AA cells my Maplin 3mm Red LEDs are too bright for tail lights.