Model Railroad LED Locomotive direction indicator.

[Sierra9093]

Hi folks,

I am working on a small project to build some LED lights to place in signal bridges on my model railroad layout. The purpose of these light is to indicate the train direction on a given track by looking at the color on the signal bridge. Red would indicate one direction and green would mean another.

So far I have worked out that I would want to take a tiny amount of current/voltage from the track to trip a larger current/voltage via a pair of transistors. I have enclosed a picture of the circuit as it now stands on paper and from what I can tell, it is probably no functional. But this is as close to a schematic that I could think up to base this upon. The maximum track voltage is around 15VDC. And from that voltage I would like to tap a small current to trip the transistors. I want to keep the base voltage/current at a minimum because being a model railroad, I would like to conserve as much voltage/current as possible for the operation of the locomotives.

I am an electronic novice, so any suggestions from anyone familiar with this type circuit or can figure out what I am trying to do would be very helpful and greatly appreciated. I have two DC power sources that represent the track to switch up the polarities. The 20V power supply is the supply voltage for the signal lights. Thanks!

 

[mbarazeen]

you can directly connect the LEDs from rail with propper current limiting resistors, no need to connect through transistors as shown in your schematic. if you use transsistors the resistance value can be in the order of kohms or you can use FETs instead of transsistors.

 

[LTX71CM]

you can directly connect the LEDs from rail with propper current limiting resistors, no need to connect through transistors as shown in your schematic. if you use transsistors the resistance value can be in the order of kohms or you can use FETs instead of transsistors.

True, but the LEDs would change in brightness as the locomotive's speed is varied.

Sierra9093 can you give us the lower and upper current and voltage limits expected at the rails? What's your controller capable of?

 

[Sierra9093]

Hi Everybody,

And thanks for the quick reply! I have already installed LED's with a current limiting resistor and one of the problems is the gradual brightening and dimming of the LED's via the throttle. The other problem and more importantly is the current/voltage draw from the operation of the locomotives on the railroad. I have a pretty large layout and though a couple of indicators here and there probably would not be a problem, but if I put as many as I would like the drain could be significant. I figure if I could build a circuit with a transistor that would draw the bare minimum in terms of current and or voltage, to switch on a curcuit with a much larger current/voltage draw I could overcome both of these encomberances. Currently, I do not have a meter that could accuratly measure the amperage, but the transformer is rated at 2.8 amps. I scratch-built my own power supply using that transformer (36 VCT) as the power source. After tapping for the DC throttle curcuit, I have an 18-20 VAC accessory curcuit that I want to use to power the indicator circuit, after converting it to DC, via a simple rectifier circuit. The top voltage at the rails is about 15-17 VDC. I assume that the current could go from 0 to the 2.8 amp capacity of the transformer. What I would like to use in the directional indicator circuit is a bare minimum draw off the rails to switch on the transistor that would then allow the 7-8 VDC through to power the LED's. So, what is the smallest (and cheapest) transistor that I could use for the LED module, each of which would have 2 LED's, one red and one green. What transistor would I use that could control 7 volts through the emitter upon being provided the smallest possible current/voltage at the base. Thanks again!

 

[Reloadron]

If you are going to want a bunch of these things a circuit like this is popular. You would need a dual supply but a single dual supply could likely power over a dozen of them. Another popular option is a circuit like this which does rely on your track power. Personally I would buy a handful of 741 op amps and run with the first. Yes, you need to build a power supply but the rest is cake. The latter circuit shows up all over the web.

Ron

 

[Sierra9093]

Ron, thanks again for your reply. I have decided to go with the 741 project. A question that I have is that i have found a LM741CN by NSC. Is this the same as the 741 in the polarity detector. They seem to have different pin set ups. It may not mean anything but before I buy twenty of them at .22 cents, I want to be sure. I have included a drawing of the pin differences. Thanks again!

 

[Reloadron]

You got me, I have never seen that pin out for any variation. NSC? Would that be National Semiconductor Corporation? All the data sheets I looked at for National show the pin out just like in the initial drawing for an 8 pin 741CN.

Ron

 

[mbarazeen]

whats the thickness of the rails you are using? if the rails are having thick base then voltage drop will not become an issue, you can directly connect the LEDs via limiting resistors.

edit: Provided that you have a good regulated power supply

 

[Sierra9093]

Ron, I guess it is not that different after all. Pin 2 and pin 3 are switched and pin 1 is in a different location. Then my diagram has a pin 8 which seems to be inactive, which is consistant. Actually I think that they are the same, but I was looking for a perfect match up. The circuit that I have designed still uses all of the same pins. Pin 2 is still negative and 3 is still positive. One question that I do have is is it possible to replace that 10K potentiometer with say, a 5K resistor to keep me from having to buy a ton of potentiometers?

Mbarazeen, As for tapping straight off the rails, right now I have two setups doing just that now for a total of 8 LED's, and it seems that just these 8 are affecting the layout if just a little bit. If I do as I plan and put more signal bridges around the layout, a total of 10 (X4) planned so far, the current draw could add up (I think). What would be a decent resistor for a 3mm LED, given 17VDC and a max current of close to 3 amp?

Thanks again guys!

 

[Reloadron]

The pot is used as a null. You might be able to omit it all together. I am not sure on that note but would try a proto type less the pot.

As to LEDs in general. A single LED might draw about 10 mA, really not much but when you start lighting 50 or 40 of them off your rail power then you start looking at 400 to 500 mA or 0.5 Amp so I can see where the train may begin to suffer. As to your 3 mm LED question, it depends on the forward voltage and current of the LED. It would look like this:

V supply - V led / current so if we say a 1.5 volt LED you would have 17 - 1.5 = 15.5 / .010 (assume 10 mA led) = about a 1.5 K resistor. Just as an example.

Also, I can appreciate you want red and green LEDs but try to buy LEDs with similar forward voltage & currents if possible when the time comes.

Ron

 

[crutschow]

The pot is to null op amp offset voltage (less than 10mV max. for the 741). This affects the exact switch point when going from the red to green (or vice-versa). Since the designed circuit switch-point hysteresis, as determined by R2 and R3, is over 100mV, then the <10mV of op amp offset is negligible compared to that, and the pot can be safely eliminated.

 

[k7elp60]

A simple circuit with a pair of LM317LZ's as constant current sources will also work. Schematic attached

 

[Sierra9093]

LED Curcuits

Ron, the LED's that I have picked are 3mm round with 35mA current draw at 2.0 forward voltage for both red and green LED's from FutureLec (LED3R & LED3G) They both also had this spcification (Forward Voltage Vcc=Min,Ii=-12mA), which I cannot quite decipher but might have some relevance to you guys. Ccrutschow seems certian that I can do without the pot and though his explination seems complicated, it sounds right so I will go with that, especially since you concur. Thanks to both you Ron and Ccrutschow! I am including a drawing of both schematics. Before removing the pot and after, where now it shows that pin 4 connects only to the negative of the power source.

k7elp60, I like your schematic because it is so simple, but here I run into the same problem of drawing current directly off the rails to power the LED's, but now, not only am I powereing the LED'd off the rail, but the IC's and other components as well. The way I want to power the total LED curcuit is from a seperate power source from the 18-20V AC of course rectified to 7 VDC. If you or anybody has an idea of how to do that, simply and cheaply, I would certianly appreciate it. Thanks a lot too, buddy!

Here are the schematics, they may kind of small. If they are not readable let me know.

 

[Reloadron]

Our pleasure.

Yes, that pot is not needed in your application. I didn't want to overly get into null & offset so sort of glassed over it. Carl covered it well. The schematic you have should work fine for your application. I was happy Carl weighed in on it also as you hate to tell someone to build something you think will work or should work. Then find out it doesn't after someone is sitting on 50 or so 741 op amps.

One more note is the resistor in series with your LEDs. You may want to make that about 750 Ohms. That is what this is about:

(Forward Voltage Vcc=Min,Ii=-12mA)

You have Vsupply 12 Volts - Vled 3 volts = 9 Volts / LED Ifwd .012 mA = 750 Ohms. The resistor limits the current through the LEDs so you don't toast them. Using lower value resistors will allow more current reducing LED life or destroying the LEDs.

Ron

 

[Sierra9093]

Guys,

I think that I have the circuit basically set. Now I am working on tapping the 18-20VAC to convert to 12VDC to power the LED's. I have a schematic in mind and will have it in a day or so that you all might look at it. Thanks again for all the help and thanks in advance!

 

[Sierra9093]

Final Designs

OK, here are the final designs for my little project. I was going to order the LM741CN from a company called Futurlec, they are very cheap, but their communication leave a lot to be desired. So instead I am going to order them from Newark Electronics, that is if their S/H isn't crazy. Instead of going with the LM741CN I am going with a UA741CN, I don't know what the difference is, but if there is a critical difference, please let me know. I compared the parameters and specs, which seem to be in line with the LM741CN, maybe even an improvement, because it takes a wider supply voltage range.

Here is the link for the UA741CN; **broken link removed**Thanks again!

Ray

 

[Reloadron]

You will need a +/- 12 volt supply. I mentioned earlier I would just build a separate supply to power all the rail polarity sensors. That can be done cheaply. I wish your drawings were just a little larger too. Bit hard for me to read them.

Ron

 

[Sierra9093]

Ron, Thanks again for your interest and most welcome advice. As I do these project, not only do I want to make these gadgets and devices, but in doing so, I hope to educate myself on the workings of electronics which has always captured my imagination since I was a child. The advice that I get from these forums is gold, and when I can understand that advice, it's platinum. I am sorry that the drawings were not as clear as they should be. I have put the drawing in another format that I hope is more clear. In it, unlike the previous drawing I have tried to clearly distinguish the power supply from the indicator unit. Ergo, there is a power supply for the indicator, and it is 12 Volts DC. On the previous drawing, it might have been a little jumbled with no clear separation of the directional unit from the power supply. I am using a 12 volt regulator from Mouser, it is at this link. I hope that this is what you were thinking of. Thanks again!

Ray

 

[4pyros]

Wow seems way over kill!!! I would just use CL2s and LEDs. Even with 20 sets its only .4 amps or run them off the common rail with a separate power supply. Its still only one part and an LED. Andy

 

[Sierra9093]

What are "CL2's"?

Hi,

I would be glad to look at an alternate method. Just what are CL2's?

True it might seem like overkill, but I anticipate that the layout will grow and I could use the extra power for other needs like uncouplers and machinery that might need a DC power source.


Thanks!