In the past, I have wired front and rear LED lights on a diesel by reversing the wiring for the rear light. Not exactly a constant lighting circuit, but it works to light either LED forward or in reverse. I have recently built a constant lighting circuit that also worked for the particular application that required it, but bear with me as I ask this (perhaps) foolish question:
Assume that I installed wheel wiper phosphor bronze contacts and 2 wires (each rail pickup) from one-side insulated IM metal wheels and ran the wires to a single LED, say- situated inside a caboose shell. (also assume either 470 ohm or 1000 ohm resistor wired to the LED) There would be 2 wires (P & N and 2 wires N & P) connected to the LED leads (and resistor on the P lead).
Then, would the LED lit up in a forward direction, and then also when the direction of travel (and current) was in a backward direction?
In other words, would the LED light up both going forward and in reverse?
I am trying for a very simple solution, rather than building another constant-lighting circuit.
Is this possible?
Wiser heads, please advise....
Thanks,
Cedarwoodron
cedarwoodron
If I understand your circuit correctly it will not work. What it will cause is a direct short because you will have connected both sides of the track together at the LED.
What you need is a rectifier. It will convert the changing polarity of the track power to constant polarity which you then connect to the LED/resistor.
This is one example that will work:
http://www.allelectronics.com/make-a-store/item/FWB-11/1-AMP-100-PIV-MINI-DIP/1.html
On the surface you will see '+' and '-' pins and two pins with '~'. Connect the + and - to the LED observing the polarity, and connect all the leads from one side of the track to one ~ pin and all the leads from the other side of the track to the other ~. It doesn't matter which ~ pin. If you only have pick up from one wheel on each rail you will get a lot of flickering. Either put pick ups on all wheels or add a capacitor to the circuit.
You don't need to use a socket. You can solder directly to the pins. Just do it quickly so you don't cook the components.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
cedarwoodronAssume that I installed wheel wiper phosphor bronze contacts and 2 wires (each rail pickup) from one-side insulated IM metal wheels and ran the wires to a single LED, say- situated inside a caboose shell. (also assume either 470 ohm or 1000 ohm resistor wired to the LED) There would be 2 wires (P & N and 2 wires N & P) connected to the LED leads (and resistor on the P lead). Then, would the LED lit up in a forward direction, and then also when the direction of travel (and current) was in a backward direction?
not sure i understand. sounds like you're trying to each leg of a single LED to both trucks/rails, which would cause a short.
why not try wiring two LEDs in paralle with reversed polarity? one or the other will turn on.
greg - Philadelphia & Reading / Reading
I know exactly where you are going, but if you draw it out, you will see you are hardwiring both rails together creating a direct short (trace the wiring in your diagram).
A simple correction would be to add a bridge recifier to the input from the track. Connect the two track wires to the AC terminals and your LED to the correct + and - terminals.
The downside to this is that there will be a voltage drop across two diodes regardless of direction. That voltage drop is about 1.4 volts. If you are using white LEDs, they require about 3.4 volts to light, so you will need around 4.8 volts on the rails before the LED will come up to full brightness. In most cases, your train would have to be moving at a good clip before the light is up to full power.
Constant lighting has always been a stumbling block for DC users. I remember back when I used DC, I installed bridge rectifiers in series with the motors in my engine to reduce the start voltage. This would allow the lights to come up to full brightness before the train moved.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Mark & Dave:
I have a 100 V dc/ 1500 mA silicon bridge rectifier diode left over from my other "constant lighting circuit" which is Radio Shack #276-1152. It is a small round item with the + marked at top(north), the - opposite at bottom (south) and the AC leads on each east & west side. So, I just use one wire from each wheel pickup (2 wires only, then) to the two side AC leads, and direct wire the single LED P lead to the + rectifier lead and the LED - lead to the - LED lead, right.
This is just for running a single LED in a work caboose cabin.
My rail pickups are phosphor bronze strips wrapped around each wheel of each truck, so there are 4 wheels delivering current.
Does this sound right, now?
Correct .... don't forget your resistor as well on one leg of the LED. A 390 ohm resistor (or close) would work fine for the lower DC voltages you would be using. You could probably go as low as 100 ohms without much worry.
Or you can just buy LED's with the rectifier/resistor from Evan Designs:
http://www.modeltrainsoftware.com/bl-212.html
I use these for all of my engines - No messing around with polarity!
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
Jim:
That's just way too easy! What the heck am I going to do with all the resistors and LEDs I have accumulated (hundreds of each!)?
"Or you can just buy LED's with the rectifier/resistor from Evan Designs"
But Jim, Dave, et. al.:
That would take all the fun out of ordering 100 3mm LEDs, 50- 1K and 50- 470 ohm resistors from China with free shipping for under $5.00 each a batch! As well as the fun you guys get to have watching my baby steps in wiring stuff and answering all my crazy questions!
PS: Suffice it to say, the project I am presently working on is but one more learning experience. After this, having built a successful constant lighting circuit and this other less intense one for this project, I will only need to improve my component "neatness" skills.
jrbernier Or you can just buy LED's with the rectifier/resistor from Evan Designs: http://www.modeltrainsoftware.com/bl-212.html I use these for all of my engines - No messing around with polarity! Jim
But those need 7 to 19 volts to even function ! That's even higher than the 4.8 volts needed if you build your own .... what's the difference ?
Bigger resistor since they can go up to 19 volts and not run the LED at maximum. 100 ohms would WAY exceed the 20ma of the LED at 19V, 300ohm would more than double it still. 330 ohm is too small even if the voltage only gets up to 14V actually. (as I keep doing calculations) 330 ohms is too small even for 12V maximum. That's the problem with driving LEDs from a variable DC source, if you plan for the maximum voltage, the minimum required before it lights up goes up. Say you go with 560 ohms, which then is under 14ma at 12V (with the bridge rectifier too), then at say 5V you have less then 1ma. It's a big jump too- a 330 ohm resistor at 12V is almost 23ma.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Mark,
They will fire off at under 4v - If you are using them with DCC, they there is no problem as you have about 14v on the rails all the time.
Most DC constant lighting units require 3-5 volts.....
Here's a couple DIY pages that might be of some help ....
http://www.trainelectronics.com/LED_Articles_2007/LED_104/index.htm
and
http://www.pollensoftware.com/railroad/index.html
I haven't built either of these, but they both look promising.
The CL2N3 in the first page is the simplest. You still need a bridge rectifier feeding it, AC terminals to the track pickups, and then the CL2N3 in series witht he LED connected tot he DC terminals. No resistors needed.
The LM334Z on the second page is more complex, but it is also configurable, so you can feed the LED with less than 20ma based on the resistor value. It too would need to be hooked to the DC terminals of the bridge rectifier, unless you want the light to be directional.