Need a Primer for LEDS in Model Railroading

Ed

From your last post you mentioned that you were using 3VDC Hankscraft stall switch motors.  Here is a circuit from a similar stall motor from Switchmaster:

So the LEDs could be wired between the last DPDT switch and the Hankscraft motor (ignore the 2 diodes and 2 incandescent lamps on the right).  This eliminates the need for the separate cherry switch.  Depending on the polarity of the power to the motor, one or the other LED lights since they are wired opposite of each other.  If they are reversed (eg. green when you want red), reverse both LEDs.  

Assuming your supply voltage is 3VDC, for the green LED I would use a 22 Ohm resistor, and for the red LED I would use a 47 Ohm resistor.

LED basics
1.  LEDs are polarity sensitive devices.  They only light up when the current flows in the one correct direction.  
2.   Here is how to identify the anode (+) and cathode (-) leads
3.  LED must have their current limited.  This is commonly done with a current limiting resistor in series with the LED.  The resistor value is calculated based on the supply voltage, the forward voltage of the LED (usually 2V for red, and 2.6V for green LEDs), and the intended current to be supplies (usually 20ma).  You can use this online LED calculator, or use the "50 rule" which says the resistor value is 50 x (supply voltage - LED forward voltage).

Red LED:  50 x (3V - 2V) = 50 Ohms
Green LED:  50 x (3V -2.6V) = 20 Ohms

You can move up or down a little to match common resistor sizes which is what I did.  The LED Calculator also tells you the minimum resistor wattage which in your case is only 1/8 watt, and you can use the commonly available 1/4 or 1/2 watt resistors.  Note that will higher supply voltages 12V, 18V etc, you will need a larger value resistor.

If you still want to use a cherry switch, let me know and I will diagram it for you.

Bob

Bob,

Thanks for the circuit but my electrical knowledge has dwindled over the years.   I would prefer to keep it as simple as possible so the spdt cherry switch.  That's a yes to your offer.

Regards,

Ed

If you prefer a single LED, use a 2-pin bi-color red/green LE, a couple less parts and the same functionality.

GRJ,

I need 4 leds, a green and a red at each location.

Thanks,

Ed

@Ed Kelly posted:

Bob,

Thanks for the circuit but my electrical knowledge has dwindled over the years.   I would prefer to keep it as simple as possible so the spdt cherry switch.  That's a yes to your offer.

Regards,

Ed

OK.  Let me know the DC voltage of your power supply and I will give you a visual circuit.

Bob

I grew up knowing about right and left hand threads and which way they went, and I was in my 50's before I ever heard the term, "righty tighty, lefty loosey", and after a lifetime in electronics, I have never heard of the term 'Cherry switch'.  What is that?  I did search it and found a company named 'Cherry' and pictures of microswitches, so is that what the term cherry refers to?

@CALNNC posted:

I did search it and found a company named 'Cherry' and pictures of microswitches, so is that what the term cherry refers to?

Yes, Cherry was the maker of the switches that Lionel used for years, and they started naming them that.  The industry term for them is micro switch.

Bob,

I have a functioning HO power pack so any voltage up to 12v would work for me.

Thanks,

Ed

I did the circuit using the full 12VDC output of your hobby transformer.  You should be able to get by with the one resistor.  There shouldn't be a big difference in the brightness of the red vs. green LEDs.  If there is, let me know.  Make sure the polarity of the 2 LEDs is oriented the same.  Your DC transformer screw terminals are probably not labeled + and -, so you can either test them with a meter or just flip the direction switch if they don't light up.

Bob

Actually, typically there is a significant difference in the brightness of red and green LED's at similar current levels.  It's simple to adjust with separate current limiting resistors for each color.

Also note that red and green LED's run at different voltages, so putting them directly in parallel will usually fail as well, you'll only have red lit as a rule.

@gunrunnerjohn posted:

Actually, typically there is a significant difference in the brightness of red and green LED's at similar current levels.  It's simple to adjust with separate current limiting resistors for each color.

Also note that red and green LED's run at different voltages, so putting them directly in parallel will usually fail as well, you'll only have red lit as a rule.

John is correct that the brightness differed significantly between red and green.  Here is a revised schematic with separate resistors for each LED.  After starting with 680 ohms for each, I found the green to be too bright and the red to be too dim.  These values below evened them out pretty well.

R1 1.2K, 1/8 or 1/4 watt
R2 330 Ohm 1/2 watt

The calculated current draw of the red LED is 30ma, and the green one is 8ma.  Here is a picture of the the lit LEDs FWIW.

These are both 3mm LEDs.  The green ones are labeled "Ultra Bright."  Brightness will vary depending on the LED selected, so you may need to adjust the resistor values (smaller value resistor = brighter), but I would not go below 330 ohms.  

Bob

You're actually overdriving the red LED, common LED's have a max average operating current of 20 milliamps, that will greatly shorten it's life.  If you have 12VDC feeding the LED's the minimum resistor value for the red LED should be around 500 ohms as the red LED typically operates at around 2 volts.  That leaves 10 volts to be dropped across the resistor, making the value 500 ohms at 20 milliamps.

John, I appreciate the correction.  I had thought that while 20ma was typical, you could drive them up to 30ma safely. The little 3mm red LEDs I had laying around were surprisingly dim at 20ma.  I have some 5mm ones that are much brighter.

For the OP, here is the corrected schematic.  Also I should mention that you can use an old DC wall-wart type power supply in place of the toy train transformer.  That's what I used for the tests.

R1 1.2K, 1/8 or 1/4 watt (you may want to use a 2K resistor to bring down the green brightness further)
R2 510 Ohm 1/4 watt

Bob

Bob and John,

Thank you both very much.  Two questions remain: 1. What would the circuit be if I used the 6 volts from the switch motor power supply (common wire and plus 3 and minus 3) instead of the 12 volts from the HO power pack? and 2. Where do I get all these parts?

Thanks again,

Ed  

PLEASE NOTE: THIS A CORRECTION FROM MY ORIGINAL VERSION OF THIS PARTICULAR POST

Ed

With a 6VDC power supply, use these values:

220 Ohm 1/4 or 1/8 watt for the red LED
470 Ohm 1/4 or 1/8 watt for the green LED (or use 2 in series if you want it dimmer)

Options for getting the components include:

Digikey (OK price but shipping will be more than the components)
Eba* (dirt cheap shipped from China but you will wait 3-6 weeks, US sellers are more expensive but ship quickly)
Aliexpress (dirt cheap Asian online retailer)
Amaz** (reasonable prices, often free shipping if you are a Prime member)

I typically get components from Amaz** if I'm in a hurry, or Eba* China reseller if I need a larger quantity and can wait.

I would suggest getting a resistor assortment from Amazon** 10 each of 50 different values for $8.99 and free 2 day shipping for Prime.  With this kit you can swap out resistor sizes to get the brightness you need.  Don't go below 220 Ohms with a 6V supply.

3mm and 5mm LEDs are available from the same sources.  You can get 50 or 100 for a few dollars.

Bob

If you're not in a hurry, Aliexpress has huge resistor assortments for peanuts.

As for Digikey, for small orders you can specify USPS shipping for $4.99, not that expensive.

Bob and John,

Thanks again.  I could not have done it without you.  I will be ordering today or tomorrow.

Ed