WIRING TWO LEDS FOR INDICATION


Ed
 

Need some help here. I am trying to wire a pair of 2mm red/green, 3 lead, common cathode LEDs together to indicate turnout position.  To prepare the LED sets,  I have soldered the anode of two 1N4148 diodes to the common cathode and the cathode of each diode to the anodes on each side of the LED.  I then checked these assemblies out using the diode test function of my multimeter and the green lights when the polarity is one way and the red when the polarity is the other way.  I then take 2 of these LED sets and solder them together so that the polarity of one set is the opposite of the other.  In this way, I should get the green of one to light when the red of the other lights and vice versa.  The problem is that only the red lights up on each LED.  When I check each individual LED set, the green lights one way and the red the other just as intended but when assembled, only the red lights. What am I doing wrong here?

Ed Robinson



dvollrath@...
 

From your description, Ed I'm not sure what you did with the still 3-leaded device other than add a diode in anti-parallel with each Red and Green LED....Or how you are connecting what voltage to the sets... Or limiting current through the LEDs. If you ended up connecting the two sets directly in parallel the Red LED has a lower voltage drop than the Green LED, preventing the Green LEDs from lighting up. One way to fix that is to use a separate current limiting resistor at the cathode for each R/G LED, then connect the anode of a Red and Green LED from each set together and the other end of the current limiting resistors together. Now current will pass through both a Red and Green LED with the difference in voltage drops taken up by the resistors. You should not need the extra diodes if you are energizing the signal lights from an on/off switched DC source, but keep them in if you power source is DCC or other AC voltage. There is no way to connect the 3-leaded R/G parts in series to do what you want.


DonV


Ed
 

Thanks Don.  I am using these in conjunction with a Tortoise to provide panel indication. I have used this method with success before but I used 5mm LEDs in the previous application. I thought about the difference in voltage drops and will try your solution.  I will report back.

Ed Robinson


dvollrath@...
 

Ed, there is  way to put the LEDs and protective diodes in a series string all in series with the Tortoise motor, but the combined voltage drop might limit the Tortoise motor speed way too much. Take a look in the files section for a file called 3 wire LEDs to see how to hook 'em up in parallel across the tortoise motor. The second set of the LEDs (not shown) with its own resistor should be connected opposite in polarity as the first to get one signal showing Green while the other signal shows Red. If both signals show the same color, you have one of them connected backwards.


DonV


Ed
 

Don,

OK, I figured out the issue - the voltage from the diode tester on the multimeter was not enough to drive the green led when hooked up in my multi-LED setup.  When I used 12vdc it works fine.  The red LED is brighter but I can adjust that with a resistor in the red leg but that is adding a complication I am not ready to take on.  By the way, I have uploaded a picture of the circuit I am using in the files section entitled "Tortoise Indication".

Ed Robinson


dvollrath@...
 

Good call, Ed. Most diode testers / VOMs don't put out enough voltage to light up non-Red LEDs.


In the circuit you show the two common cathode R/G LED sets, each with protective diodes to accommodate reversing current flow to determine which color will light up. You have wired them in series with the tortoise motor to control and limit current. However you have wired the two LED sets in anti-parallel to yield Red on one circuit and Green in the other, and reversing their colors when voltage to the Tortoise is reversed. However, the voltage drop to illuminate the Green LED cells is greater than that of the Red. So in one case the Red LED plus the diode across its Green counterpart in the same package is conducting current with a certain combined voltage drop leaving less than desired voltage for an equal amount of current to flow through the other parallel wired set which has 1 diode plus a Green LED. Hence, the Green LEDs will have weaker current flow and may not even light up at all. [LEDs in parallel will not necessarily share current.] Yes, adding an experimental  low ohms resistor in series with the Red LED lead may resolve the brightness issue.


Additional ways to improve the usefulness:

1. You could try reconnecting the two LED sets, with their diodes, in series but opposite in polarity with each other and the Tortoise motor. But be aware that the Tortoise motor will slow down.

2. .You could use the LED connection of #1 but use an additional 1K resistor in series with the LEDs, etc. and wire that string in parallel with the Tortoise motor.

3. You could also rewire to connect each one of the LED sets separately, with its own 1K current limiting resistor in series, in parallel with the Tortoise motor.


DonV