Passenger car lighting with LED strips, EDIT:photos added

[craigolio1]

Hey everyone. 

I'm going to experiment with lighting passenger cars using adhesive strips of LEDs, as seen here...

http://www.ebay.com/itm/130910873142?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

The strips have cut points every 3 LEDs.  In a standard N scale passenger car a strip 9 LEDs long fits perfectly.  For the price you just can't beat it.

They have resistors in the strip already and are designed to be run at 12 volts but are noted as being used for car accent lighting, 14.4v, and are able to be dimmed as well so less than 12v would be fine too.

I have used them to light cabinets for the Home Theaters I install and they are very bright so I think the full 12 volts would be too much in an n scale passenger car.  I'll experiment to find the voltage that I want to use.

 I'm using a Digitrax Zephyr.  My passenger cars have Kato trucks which can get power from the rails.  I would like to drop the voltage to the level I choose after some experimentation and also include a capacitor to prevent flickers.

Here is what I'm thinking.  I don't actually know the voltage on my track.  I do know that it's a form of "AC".  I know how to calculate the resister value I need to use for whatever voltage I decide so when I experiment I'll figure out those values.

Questions: 

Are the LEDs ok to run right off the track AC (no rectifier) or is something else needed in there to prevent messing with the DCC signal on the bus?

How do I chose a capacitor?  Do I need only a capacitor (I think I likely need a resistor or something to set a charge/discharge rate?  How do I put the cap into the circuit?

I would appreciate it if someone would show me a simple circuit design for this. 

Thanks, Craig

[peteski]

Craig, 9 white LEDs per car is *A LOT* of LEDs!  They will be wasted but if that's what you want... the features you are asking for are feasible.

First I need to the value of the resistors already installed in the strip. Between every 3 LEDs there is a dark colored rectangular component. That is the current limiting resistor for every trio of series-connected LEDs. There should be some numbers on that component.  Can you read them off and let me know what they are?  If you have a voltmeter (multimeter) in your arsenal of hobby tools, power one of those strips of 9 LEDs using a 9V battery and measure the voltage across that resistor.

To dim the LEDs you'll need to add another series-connected resistor to limit the LED current (not their voltage). We'll figure out the value once I know the value of those built-in resistors.  One possible problem is that when you drive white LEDs using low current, their color temperature shifts somewhat towards blue.  I guess you'll have to decide if that is acceptable to you.

As you suspected, it will be beneficial to convert DCC signal to DC (using a bridge rectifier) and possibly filter it using a large value capacitor (to lessen the flicker).  Your DCC booster most likely supplies between 12 and 14 volts to the track.  Once you provided the info I asked for, I can draw you a circuit diagram.

[TiVoPrince]

Used
a similar product for my Kato Phase3 Superliners.  Wired them direct to the DCC rails.  Skipped the Kato lighting kit opting for Atlas SD60 pickup strips cut in half as a less expensive option that created no waste.  Adding a rectifier and cap may may happen in the future but they seldom flicker on my layout. 

What I really need now is a simple, cheap and effective way to add 100+ passengers to my set of cars.  Bright lighting makes the lack of patrons especially on the upper deck pretty obvious to me.

Bright?  It looks like there is a miniature sun inside the cars.  They are way too bright with three segments of three LEDs inside.  In a normally lit room they are intensely bright and that makes TiVoPrincess happy.  Adding a strip of metal 'Nashua' tape to the underside of the roof prevents shine though.  Initially I put in some Kato LED lighting kits, but the uneven nature of the LED at one end and diffuser is unsatisfactory, to me anyway.  If the LEDs are too bright a coating (or three) of Tamiya Smoke or Canopy Blue can create exactly the brightness you want with the distributed light that fine passenger cars deserve.

You will find a lot of uses for the leftover LEDs.  Stucture lighting becomes so simple with peel and stick on solution for lighting you may decide to model night rather than daytime.  Placing the lighting flat against the ceiling really helps the shadows look 'right'.  Get the warm white strip to simulate regular incandescent lighting, because a light coat of Tamiya Canopy Blue changes them to a nice flourescent look with minimal effort...

[Mike C]

Craig, 9 white LEDs per car is *A LOT* of LEDs!  They will be wasted but if that's what you want... the features you are asking for are feasible.

First I need to the value of the resistors already installed in the strip. Between every 3 LEDs there is a dark colored rectangular component. That is the current limiting resistor for every trio of series-connected LEDs. There should be some numbers on that component.  Can you read them off and let me know what they are?  If you have a voltmeter (multimeter) in your arsenal of hobby tools, power one of those strips of 9 LEDs using a 9V battery and measure the voltage across that resistor.

To dim the LEDs you'll need to add another series-connected resistor to limit the LED current (not their voltage). We'll figure out the value once I know the value of those built-in resistors.  One possible problem is that when you drive white LEDs using low current, their color temperature shifts somewhat towards blue.  I guess you'll have to decide if that is acceptable to you.
As you suspected, it will be beneficial to convert DCC signal to DC (using a bridge rectifier) and possibly filter it using a large value capacitor (to lessen the flicker).  Your DCC booster most likely supplies between 12 and 14 volts to the track.  Once you provided the info I asked for, I can draw you a circuit diagram.

What about a simple circuit using a small round battery and maybe a reed switch. Something that would all mount in the ceiling and not need track power???.....Mike

[peteski]

Mike, it is against my religion to even consider using batteries for passenger car lighting!   I refuse to even consider such a circuit! 

Batteries have short life and have to be replaced fairly often.
Batteries can and will leak their caustic electrolyte. That can damage the metal parts of the circuit.
Battery powered lighting has to be manually turned on and off (another hassle).

Even ignoring the above mentioned negative points, the strip lights being considered here are not a good candidate for a battery powered circuit.  The LEDs are hooked up three in series. That would require at least 9V battery to illuminate them.  If you were to use those standard 1.5V watch button cells, you would need to stack 6 of them to get to 9 volts.  Also, these strips (with 9 LEDs per car length) will be "power hungry" and will drain the battery rather quickly.  The commercially available battery-powered illuminating circuits usually use one or two LEDs (which use much less power).

[DKS]

^^^This.

Utilizing track power really is the best approach. The circuit can be designed to eliminate flicker using a large cap which, if large enough, can act like a very short life rechargeable battery. And if you want to get really fancy, you can install an accessory DCC decoder that would allow you to turn the lights on and off with your throttle.

[craigolio1]

Hey guys,

    Thanks a lot for the replies.  I'm happy to hear that someone else has done this.  I was pretty sure I wasn't the first to think of it.  Using the Tamiya clear smoke to tone down the brightness is a good idea.

    I agree that nine LEDs is way over kill for an N scale passenger car, but I also agree that the effect the Kato lighting kits have is not desirable. 

    I chose track power over batteries as a lot of my passenger cars are kitbashed and I don't think they would take repeated disassembling to change batteries.  That I just don't want to waste batteries.  I have dozens of passenger cars and I just want them to work.  I do plan to add a reed switch so that I can turn them on and off.  I don't want to spend the money, effort, or space to put accessory decoders in each car.

For under $20 I have enough LED strips to do the whole fleet and then some so once we nail down the simple circuit I'm good to go.

Peteski, thanks for offering your help.  Below is the info you asked for plus a little more.

Resistor value:  the number on it is 151.  150 ohms?

Voltage across the resistor:  .487v with my power supply outputting 9.04 volts.

Current draw of 3 LEDs at 9.04 volts = .003A

Craig

[peteski]

Ah, you have a digital multimeter and you know how to use it!    Good, that'll make things easier.  Yes, 151 is 150 ohms.  Your measurements show me that the LEDs have forward voltage drop of 2.84V @ 3mA  each. That's what I was after.

I already have a diagram started but it is at work and I won't be back there until Sunday night.  I'll post it then along with a parts list.

Can you work with the tiny surface mount (SMD) components or you prefer larger components with leads?

Is the LED brightness you get at 9V (and 3mA per 3 LEDs) ok, or do you want them even dimmer?  If you want them dimmer then do some experimeting:  If you have a variable power supply, hook the 3-LED strip to it and adjust the voltage until you get the desired brightness. Then tell me what was the current going through the LEDs (just like you did before).  Or, go to the Radio Shack and pick up some 47 ohm resistors (any wattage) and start adding them in series with the 3-LED strip until the desired brightness. Again, note the current through the LEDs at that point.

[craigolio1]

Peteski,

thanks for your efforts.  Yes I have a mulit-meter! Haha. Actually I took electronics in high school and have a lot of knowledge ( installing home theaters and car stereos) so I understand the basics, ohms law etc, and can assemble and trouble shoot a lot of stuff, but I've never been very good at selecting components to use in a design. 

Surface mount is no problem.  The smaller components are preferred inside my passenger cars, especially the dome cars, etc.

I'll do some tests tonight and give you some direction.

Thanks again for your help.

Craig

[craigolio1]

Peteski, I'm at working fiddling with this tonight, and I've discovered that with all nine LEDs burning, a 15k resisitor brings the brightness to a level that I feel is realistic.  It looks noticable but not to bright from outside. 

Regretfully I forgot my multi-meter so I can't measure the current draw tonight.

Craig

[craigolio1]

Peteski,

I measured the current through all three LEDs...  .011A

With the 15k resistor my multimeter was flashing .000, then .001 and so forth.  I switched to teh mili and micro A settings and nothing happened.  It would even light the car.  My meter must have an issue (I'm sure I moved the leads to the correct holes for each setting).  Anyhow, I hope that helps.

Craig

[TiVoPrince]

Does
the 15K resistor introduce much heat?  I had given some thought to dimming via resistor but did not want a hot resistor inside my cars.  Perhaps a personal preference but worth some consideration...

[craigolio1]

I only had it hooked up long enough to see the brightness and measure the current.  I used a 1/4 watt.  I let it sit for a while tonight and let you know!

Craig

[peteski]

Was the 15K resistor used with a 9V or 12V power source?  Either way, it makes sense that the current was *VERY* low.

At that low current the forward voltage of the LEDs is probably around 2.4V per LED. So 3 LEDs in series drop 7.2V.  With 9V supply the voltage across the 15K resistor will be 1.8V.  So the current going through the resistor (and through 3 sets of three series connected LEDs) will be 1.8/15000=0.00012A (or 0.12mA)!  That is why your meter had a hard time showing the current.  Divide that by 3, and each 3-LED set was only using 0.00004A (or 0.04mA)! The micro amp ranges on your multimeter have a higher internal resistance (in order to measure the current), that might be why the LEDs wouldn't light up when you used those ranges.  I'm surprised that those LEDs are bright enough with such a low current.

As far as power dissipation goes, with a high value resistor, like 15K, it is not a problem. 0.00012 * 1.8 = 0.000216 Watts (0.216mW). Even the smallest 0201 SMD resistor would be usable. A 1/4W or even 0603 or 0805  (which IIRC are 1/10 and 1/8W) resistor won't even get warm.

Time allowing, I'll post a schematic diagram and a parts list tonight.

[craigolio1]

Thanks for the detailed explanation.  You are correct.  I was using a 9v battery in this case.  There was zero heat.  With the room lights on, you can't even tell the lights are on in the car.  With the lights off, it is very noticable, but not over powering.  The effect I'm after is that which I remember from a view as a kid, when a VIA rail train was moving parrallel to the highway we were driving on.  It was probably 300m away from us.  It was late in the evening and the sun had just gone down so we had a bit of dusk.  I remember being struck by the train's sollouette and the cars lighting clearly visible. To me, these cars look just like that.  Since on my layout, I would be a sizable scale distance away, they should give just the right effect. 

Craig