Lighting Bi-Color LED from Single Power supply

ADWSystems · Jan 7, 2012

I'd like to use a bicolor LED powered from a single power supply (either 0-5V or 0-12V). I have two applications. One driven by a comparator (LM393 most likely) and the other driven by a simple transistor with both emitter and collector available. But for some reason the solution elludes me and I can't figure out how on paper to make it work. Specifically, I'm looking at All Electronics Parts LED-6 (2 pin, reverse polarity to change color), and LED-141 (3 pin).

Sceadwian · Jan 7, 2012

You just use a single resistor depending on the voltage you're using... How do you intend to reverse the polarity of the LED using only a comparator or a transistor?

audioguru · Jan 7, 2012

An H-bridge circuit (look at it in Google) reverses the voltage to a DC motor. It can also reverse the voltage to your bi-color LED.
The H-bridge has 2 inputs. One input will light one color and the other input will light the other color.
Think about what happens when you activate both inputs.

Diver300 · Jan 7, 2012

If you have a push-pull driver, that pulls up or down, connect one side of the 2 pin LED to that.

Connect two resistors, one to ground and one to the supply, from the other side of the LED. Select the resistor value for the LED current required.

It wastes more current than the H bridge, especially on low voltage supplies, but it is simple.

ADWSystems · Jan 7, 2012

Sceadwian said:
You just use a single resistor depending on the voltage you're using... How do you intend to reverse the polarity of the LED using only a comparator or a transistor?

I don't. Hence the problem/question.

ADWSystems · Jan 7, 2012

audioguru said:
An H-bridge circuit (look at it in Google) reverses the voltage to a DC motor. It can also reverse the voltage to your bi-color LED.
The H-bridge has 2 inputs. One input will light one color and the other input will light the other color.
Think about what happens when you activate both inputs.

Almost following you. The 3 pin LED has two inputs, but I only have 1 output.

ADWSystems · Jan 7, 2012

Diver300 said:
If you have a push-pull driver, that pulls up or down, connect one side of the 2 pin LED to that.

Connect two resistors, one to ground and one to the supply, from the other side of the LED. Select the resistor value for the LED current required.

It wastes more current than the H bridge, especially on low voltage supplies, but it is simple.

I get you Diver300. Make 2.5V with two resistors, and having an output that can sink and source connect the led to the output and to 2.5V.

I may have an option for an output that can only sink current. I'll need to draw it out to post. I treid to type it, but even I couldn't follow it

audioguru · Jan 7, 2012

It is easy to light the LEDs separately in a 3-pins bi-color LED. A 2-pins LED needs to have the polarity reversed to light the other LED.

ADWSystems · Jan 7, 2012

audioguru said:
It is easy to light the LEDs separately in a 3-pins bi-color LED. A 2-pins LED needs to have the polarity reversed to light the other LED.

Not having a chance to draw it out, lighting the 3 pin led from a single output has become easier said than done. As for the 2 pin led, I can follow Diver300's idea; it may even work on an open collector output selecting the right pull up resistor.

ADWSystems · Jan 7, 2012

Update: I just spent the past 2 hours trying to figure out how to drive the 3 pin (common cathode) LED from a single output. Is this even possible?

audioguru · Jan 7, 2012

ADWSystems said:
Update: I just spent the past 2 hours trying to figure out how to drive the 3 pin (common cathode) LED from a single output. Is this even possible?

It is extremely simple if you look at the schematic of how the LEDs are connected on the website of the manufacturer.

Many people here have said that the cheap Chinese "common-cathode" LEDs they bought off E-Bay are actually common-anode. Since you bought a cheap Chinese LED from E-Bay then I am sorry for you.

ADWSystems · Jan 8, 2012

audioguru said:
It is extremely simple if you look at the schematic of how the LEDs are connected on the website of the manufacturer.

Many people here have said that the cheap Chinese "common-cathode" LEDs they bought off E-Bay are actually common-anode. Since you bought a cheap Chinese LED from E-Bay then I am sorry for you.

I've tried simulating any number combinations in Protel, but can't come up with a circuit to light both LEDs. I can light one but not the other. Do you have a sketch you could post? I could light both if they were in series Anode1-Cathode1/Anode2-Cathode2. But that's not the part I am looking at from ALL ELECTRONICS. (where did eBay enter into this? but thanks for the warning)

ericgibbs · Jan 8, 2012

ADWSystems said:
I've tried simulating any number combinations in Protel, but can't come up with a circuit to light both LEDs. I can light one but not the other. Do you have a sketch you could post? I could light both if they were in series Anode1-Cathode1/Anode2-Cathode2. But that's not the part I am looking at from ALL ELECTRONICS. (where did eBay enter into this? but thanks for the warning)

According to All Electronics, its a HP device, Common Cathode.

If you could post at least a diagram showing the active devices you have available on the project to drive the LED, it would help.

ADWSystems · Jan 8, 2012

ericgibbs said:
According to All Electronics, its a HP device, Common Cathode.

Yep knew that. But I didn't have a Dual LED symbol and when I put two in to make one, I put them head to tail. I could get that to work. Unfortunately, I realized after a few minutes of relishing in my success that my schematic didn't match my part and haven't been able to devise a circuit to do so. I'm think it's writer's block. This can't be that hard.

ericgibbs said:
If you could post at least a diagram showing the active devices you have available on the project to drive the LED, it would help.

I have an open collector output (LM393) and a NPN transistor output (from an opto-coupler, both collector and emitter are available). All powered from a 0/5V power supply.

ericgibbs · Jan 8, 2012

ADWSystems said:
I have an open collector output (LM393) and a NPN transistor output (from an opto-coupler, both collector and emitter are available). All powered from a 0/5V power supply.

You are not being very clear in your requirement.

Is the LM393 AND the NPN being used in combination to drive the dual LED.???

Which combinations of LED's states do you want.

ie:
Both Off, Both On, alternate LED on/off.???

ADWSystems · Jan 8, 2012

Sorry. I was trying to be succinct from my first post. I have two scenerios, one has an open collector output, the other a transistor (easily configured as open collector, but the emitter is available if necessary). I'm looking to use one LED or the other, but would like to know what options there are make either work (trying to improve my knowledge base). Very simple states; on steady one color, off steady the other color.

ADWSystems · Jan 8, 2012

Diver300 said:
If you have a push-pull driver, that pulls up or down, connect one side of the 2 pin LED to that.

Connect two resistors, one to ground and one to the supply, from the other side of the LED. Select the resistor value for the LED current required.

It wastes more current than the H bridge, especially on low voltage supplies, but it is simple.

ADWSystems said:
I get you Diver300. Make 2.5V with two resistors, and having an output that can sink and source connect the led to the output and to 2.5V.

I may have an option for an output that can only sink current. I'll need to draw it out to post. I tried to type it, but even I couldn't follow it

I was thinking about the 2-pin circuit for a moment and tried to simulate it (see attached sketch NOT from the simulation)

When the ouput is off, current flow is through the pullup through the LED through R2 to ground (upper left to lower right). When the output is on, the current flow is through R1 through the LED then the transistor to ground (upper right to lower left). When the output is off there is less current than when it is on.

R1 must equal R2 to produce 2.5V. Then mathematically the pull-up resistor must be 0 ohms in order to have the same current through both resistors in the on and off states. BUT if the pull-up is 0 ohms, then (A) 1 led is tied to +5V and will never light and (B) when the output turns on it will short the power supply and blow the output. Is it possible to get the same current through both leds in a circuit close to this?

4pyros · Jan 8, 2012

What if you added a resistor between the transistor and ground the same size as the pull up?
I think you also need a current limiter in series with the LED

alec_t · Jan 8, 2012

how to drive the 3 pin (common cathode) LED from a single output. Is this even possible?

Yes, it's possible! See attached.

4pyros · Jan 8, 2012

alec_t said:
Yes, it's possible! See attached.

Looks like D2 would stay on all the time?

Lighting Bi-Color LED from Single Power supply

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