driving two different color leds with 2 wires only

Thread Starter

bug13

Joined Feb 13, 2012
2,002
Hi guys

I need to drive two different color leds with 2 wires only over ~40M away. I am planing to do this as below, is there a better way to do it? Or any feedback/suggestion/critics etc...

Thanks guys!

Capture2.PNG
 

Papabravo

Joined Feb 24, 2006
21,159
What is the voltage drop budget for the 80M of cable?
What is the Vf of the LEDs?
How fast do you need to turn the Transistors ON and OFF?
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
I see, forgot to say they are just low power white leds.

ILed is ~10mA max, that’s a lot, only need 5mA.

Vf is ~3V

I haven’t measure the cable the cable resistant, but I expect no more than 10ohm total resistance, 10mA max current. So I expect 0.1V drop.

Driving them 100Hz max, that’s plenty to fool our human eyes :)
 

Tonyr1084

Joined Sep 24, 2015
7,852
Not certain how to answer your question. But here's what I'm seeing: Two LED's and two controlling inputs. Assume inputs are labeled A & B. If A is high and B is low then one LED (lets call it LED1) will light up. If B is high and A is low then LED2 will light up. If both A & B are high then neither LED will light up. If both A & B are LOW then neither LED will light up.

Is that what you want?

And clarify please: You said you have two white LED's of differing colors ? ? ?
 

ebeowulf17

Joined Aug 12, 2014
3,307
What were you planning for resistor values? I haven't simmed or calculated anything for real, but off the top of my head it seems like you're going to be dumping a LOT of current through whichever top resistor (R1 or R2) is above the active transistor.

Maybe there's a combination of resistance values that balances better than I'm imagining, or maybe the wasted current doesn't bother you, but it certainly looks strange to me.

I would've expected one of two likely solutions:
  • H-bridge arrangement
  • One pin controlling on/off function (bjt, MOSFET, relay, whatever) and the other pin controlling polarity reversal through a DPDT relay.
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
Not certain how to answer your question. But here's what I'm seeing: Two LED's and two controlling inputs. Assume inputs are labeled A & B. If A is high and B is low then one LED (lets call it LED1) will light up. If B is high and A is low then LED2 will light up. If both A & B are high then neither LED will light up. If both A & B are LOW then neither LED will light up.

Is that what you want?

And clarify please: You said you have two white LED's of differing colors ? ? ?
Yes, that's what I want.
And sorry for the confusion, they are both white LEDs, but with different color cover/dome.
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
What were you planning for resistor values? I haven't simmed or calculated anything for real, but off the top of my head it seems like you're going to be dumping a LOT of current through whichever top resistor (R1 or R2) is above the active transistor.

Maybe there's a combination of resistance values that balances better than I'm imagining, or maybe the wasted current doesn't bother you, but it certainly looks strange to me.

I would've expected one of two likely solutions:
  • H-bridge arrangement
  • One pin controlling on/off function (bjt, MOSFET, relay, whatever) and the other pin controlling polarity reversal through a DPDT relay.
Current consumption is not a main concern, but I do agree it's a bit high. I do want to keep the cost to minimum, so I don't really want to use a h-bridge, relay etc. I deally, I would want to control it with 3.3V or 5V logic.
 

Tonyr1084

Joined Sep 24, 2015
7,852
Ideally, I would want to control it with 3.3V or 5V logic.
Kind of low on the voltage for that long a run. May work. Might have to get a "Good" wire. Something like a good ethernet like wire, one that is rated for long runs of data transmission. You may even need a power source at the lights just so you can switch them on and off. 3.3 volts is already close to the forward voltage of a white LED; depending on the exact LED you have. And I don't know if you're going to be able to turn on a regular transistor with just 5 volts. Might have to go with a logic level MOSFET. Of course, then the solution becomes even more simple. I'd bang out a drawing but it's close to my bed time and I have a few things to do before I hit the hay.
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
Kind of low on the voltage for that long a run. May work. Might have to get a "Good" wire. Something like a good ethernet like wire, one that is rated for long runs of data transmission. You may even need a power source at the lights just so you can switch them on and off. 3.3 volts is already close to the forward voltage of a white LED; depending on the exact LED you have. And I don't know if you're going to be able to turn on a regular transistor with just 5 volts. Might have to go with a logic level MOSFET. Of course, then the solution becomes even more simple. I'd bang out a drawing but it's close to my bed time and I have a few things to do before I hit the hay.
Sorry I mean to say the output voltage of my MCU to the transistor inputs are 3.3V or 5V. The 40M wires are referring to the two wire between the two LEDs and my transistors.
 

dendad

Joined Feb 20, 2016
4,451
You could get rid of R3 and R4 then select R1 and R2 to set the currents.
Try R1 = R2 = 1Kohm.
The cable should not amount to vert much and it is not critical anyway.
 

ebeowulf17

Joined Aug 12, 2014
3,307
Because when anyone of the led is turn on and the other one is turn off, but the polarity is reversed, that will causes the led damaged anytime.
I thought that each LED allowed enough current to flow that it limited the voltage across it to its forward voltage rating (Vf.) The Vf of each LED should be much lower than the reverse voltage it would take to a damage an LED, so effectively each LED is protecting the other LED from dangerous reverse-voltage levels.
 

ScottWang

Joined Aug 23, 2012
7,397
I thought that each LED allowed enough current to flow that it limited the voltage across it to its forward voltage rating (Vf.) The Vf of each LED should be much lower than the reverse voltage it would take to a damage an LED, so effectively each LED is protecting the other LED from dangerous reverse-voltage levels.
If we considering two situations that the first is pin to pin and one of led reverse the polarity and the second is like the first posted, now we discuss the first posted, what do you think that how many volts will be cross on the led which is turn off and comparing to the led which turn on ?
 

ebeowulf17

Joined Aug 12, 2014
3,307
If we considering two situations that the first is pin to pin and one of led reverse the polarity and the second is like the first posted, now we discuss the first posted, what do you think that how many volts will be cross on the led which is turn off and comparing to the led which turn on ?
Sorry, I get it now. @dendad set me straight with that last post. I apologize.
 

DickCappels

Joined Aug 21, 2008
10,153
Some LEDs cannot tolerate any reverse bias without suffering long term degredation. Check the datasheets for the LEDs you intend to use before you settle on a design.
 
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