One resistor on two LEDs

Question

This question is a little bit stupid and silly.

As I am chemist, I don't remember electricity physics very good. As I remember it shouldn't harm anything, but I need to be sure.

LEDs will be blinking sometimes together. But not longer than 50ms.

They will be parallel for only short time. Because they supposed to blink reporting 2 modes 1st - normal, 2nd - error. And switching between them can be no longer than 50ms.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

I use resistor for both LEDs, but it just curiosity question.

@IgnacioVazquez-Abrams it is similar, but they won't work together. And the only one simultaneous blink is the real parallel. — XuMuK, Jul 25 '15 at 21:53
It is still the same question. As long as you understand the implications of using a single resistor and those implications are acceptable, you can do so. — Ignacio Vazquez-Abrams, Jul 25 '15 at 21:54
@IgnacioVazquez-Abrams Thank you!!! This is exactly what I am thinking. — XuMuK, Jul 25 '15 at 22:00
If you want them both to light at the same time, drive Pin1 and Pin2 alternately, faster than the eye can see (100Hz or more) — , Jul 26 '15 at 10:16
@BrianDrummond No, they are going to blink for 50 ms every 2-3 seconds. And the only time the are lighted together is not longer than 50 ms. 1st indicates normal work, 2nd - error. — XuMuK, Jul 26 '15 at 13:28

score 2 · Accepted Answer · answered Jul 25 '15 at 22:00

2

Yes, it will work, but I don't recommend it. The above circuit is Ok, as long as only one LED is on, it'll work fine. If both leds are on, that's alright, they won't get damaged, but their behaviour are unsure; depending on their characteristics, probably they will glow with a different brightness, but thats all.

But I still highly suggest that you use separate resistors for each led, because that's the right way to do it. It's never a good design to wire diodes in parallel.

answered Jul 25 '15 at 22:00

lszabi

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It is a question about situation: if I have only one resistor and I am in hurry. Also I won't use this for a long time. – XuMuK Jul 25 '15 at 22:04
As I said, it's okay then, if you don't have any other choice, and the design is not really important. (eg. improvised prototype or sth.) Just don't use it in "serious" circuits. – lszabi Jul 25 '15 at 22:06
This is not really true. This is NOT like the two parallel LEDs with one resistor situation. The two LEDs will interact BUT you will not get the 'current hogging' situation you see with a single series circuit with two LEDs in parallel. – Russell McMahon Jul 26 '15 at 07:10

score 2 · Answer 2 · answered Jul 25 '15 at 22:10

they are never in "parallel", since they have different anode connections. they do share their cathode connection though.

Yes it will work, with the following drawback:

When pin # 1 and pin # 2 are both High (~5V), the LED with the Lowest forward voltage will light up much more than the other one. Possibly so much more that it seems like only one LED is on.

There will always be a slightly difference in the forward voltage drop of the LEDs, even if they are from the same production batch.

The resistor will always ensure safe operation, but you might see only one LED light up because the LEDs share their cathode.

A 220 ohm resistor is cheap and small it would be best if you used it, but you won't damage anything by using this configuration.

Completely agree - parallel working will be once and not longer than 50ms. And if there is only one resistor and 2 AM - no chance to get another one. — XuMuK, Jul 25 '15 at 22:14

score 0 · Answer 3 · answered Jul 25 '15 at 22:17

If the two LEDs are the same colour, using a single resistor will probably work, but the LEDs will be dimmer when both are on than when only one is on.

If the LEDs are different colours, only the one with the lower forward voltage will light when you expect both to light. For example, red LEDs typically have a forward voltage of about 1.8 volts, while green LEDs are 2.2 volts. If you have a red LED and a green LED in your application, only the red LED will light when you expect both to be on, if the LEDs share a single resistor.

One resistor on two LEDs

3 Answers3