Blue LEDs for Power and Blinkenlights

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Ammer66
Posts: 4
Joined: 20 May 2018, 12:41

Blue LEDs for Power and Blinkenlights

Post by Ammer66 » 20 May 2018, 13:05

Hi there,

my kit has not arrived yet, but i would like to replace the red LEDs for Power an Blinkenlights with blue ones. :oops:
Don't throw stones at me, i can't help myself, im addicted to blue LEDs :idea:

I am just an electronics noob, but i know that blue LEDs need different resistors than red ones.
So my question is: Can i get away with just replacing the LEDs, oder is there more to change :?:

Thx in advance,
Ammer66

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marcelk
Posts: 377
Joined: 13 May 2018, 08:26

Re: Blue LEDs for Power and Blinkenlights

Post by marcelk » 20 May 2018, 19:45

I expect you need different resistors for R3 (single resister) and R8 (array of four). The red LEDs in the kit are 2mA/1.8V and are protected by 2.2kΩ series resistors.

HGMuller
Posts: 20
Joined: 14 May 2018, 05:46

Re: Blue LEDs for Power and Blinkenlights

Post by HGMuller » 21 May 2018, 09:09

I would not expect the resistor value to be very critical: it controls the current through the LED, and thus how brightly it emits. As long as you don't exceed some maximum current, having a 'wrong' value would just result in a different brightess. 2mA doesn't sound like a lot to start with (power dissipation at 1.8 V just 3.6 mW), and I would be surprised if it could damage the blue LEDs (which probably require a larger voltage, and would thus get a somewhat lower current from the same resistor.)

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marcelk
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Re: Blue LEDs for Power and Blinkenlights

Post by marcelk » 21 May 2018, 13:41

In the Ben Eater series you can see the he doesn't use resistors. That is a bit of a poor practice, but he gets away with it because he has older 74LS chips that don't deliver much current in the first place. And probably his LEDs need a lot and he uses a juicy USB charger.

With the chips in the kit and without resistors, the LEDs are going to get all the current they ask for and more, so you must calculate the minimum resistor value or they might burn out. We selected low amperage LEDs for the kit, and then calculated the minimum resistor value as follows:

(5V - 1.8V) / 0.002A = 1600Ω

So we use the same 2.2kΩ as used in all other places.

HGMuller
Posts: 20
Joined: 14 May 2018, 05:46

Re: Blue LEDs for Power and Blinkenlights

Post by HGMuller » 21 May 2018, 18:06

Well, it seems that blue LEDs run at ~3.3V, so only ~1.5V left over the resistor, instead of ~3.0V. So if you don't change the resistors, they would get half the current of the red LEDs. But that just means they burn a little less bright than they could. To the eye a factor 2 in light yield is barely observable. And you would have to factor in the relative sensitivity of the eye for red vs blue as well.

What would happen if you would run 16mA through the red LEDs? Would they really burn out? I always used to worry more about not burning out the driver IC than the LED.

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marcelk
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Re: Blue LEDs for Power and Blinkenlights

Post by marcelk » 21 May 2018, 19:59

I've blown a few that way judging from my damaged parts jar.
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