The resistor is there to turn a constant voltage source into a current source. Ideally, you want the value of the resistor to be as high as possible and still have the LED visible in all viewing conditions.

The resistor value can be calculated from:

(Vs - Vdiode) / diode current.

Standard resistors of the appropriate wattage are used.

Edit: Personally. I find 20mA is too high for an LED.
For example, if I want the drive the LED with 2mA from 5V supply,

R = (5V - 2V) / 2mA = 1.5kΩ

So what is the difference of calculating a "regular" resistor for an LED versus a current limiting resistor. I know the difference is numbers but overall how so?

 

An LED with a proper series current limiting resistor driven at moderate levels has about a zero percent risk of failure.

That's pretty demonstrative. That's good enough for me. However if I wanted to hookup a status light to it could you?

 

Sorry, basically a indicator LED that comes on when the main LED has power, and is dark when it doesn't/or it is damaged.

Why do you need one LED to tell you that another LED is on? Why can't you just look at the other LED? Why can't you put them in series so that they are either both on or both off?

It might be a bit different if you wanted the second LED to be on any time the other LED was SUPPOSED to be on, regardless of whether it actually was or not. But that is not what you are asking for.

 

Why do you need one LED to tell you that another LED is on? Why can't you just look at the other LED? Why can't you put them in series so that they are either both on or both off?

It might be a bit different if you wanted the second LED to be on any time the other LED was SUPPOSED to be on, regardless of whether it actually was or not. But that is not what you are asking for.

Because the second one is IR. I could use an IR camera but its simpler this way (to me).

Parallel: I'll be driving 8 IR LEDs and you can attach many LEDs in parallel. The series you can't. And for that reason I want to be able to remove whatever module that I deem as being faulty or destroyed. The status LED I know right away instead of using a cam. The whole point is to only replace faulty modules instead of having the whole series fail. I can get away with missing 1 or 2 until fixed but not all. Then, which one was it.

 

Parallel: I'll be driving 8 IR LEDs and you can attach many LEDs in parallel.

Each with their own current limiting resistor?

You don't need to see the light to measure the voltage on the anode of the LED. Are you looking for an indication of one LED being burned out? Or all of them?

Still don't see where line voltage is coming in to play.

 

You're not showing an LED connected to line voltage.

Given its current schematic config I would find it hard to do that.

 

Because the second one is IR. I could use an IR camera but its simpler this way (to me).

Parallel: I'll be driving 8 IR LEDs and you can attach many LEDs in parallel. The series you can't. And for that reason I want to be able to remove whatever module that I deem as being faulty or destroyed. The status LED I know right away instead of using a cam. The whole point is to only replace faulty modules instead of having the whole series fail. I can get away with missing 1 or 2 until fixed but not all. Then, which one was it.

That still doesn't explain why you can't simply put the indicator LED in series with the IR LED that it is monitoring?

If you are going to be driving 8 IR LEDs, does each one have it's own current limiting resistor? If not, if they are truly in parallel (generally a bad idea), then how are you going to determine which ones are not working?

You might mount an IR photodiode such that it only sees one of the IR LEDs and then use those to determine which ones actually are or are not working.

 

Each with their own current limiting resistor?

Yes

You don't need to see the light to measure the voltage on the anode of the LED. Are you looking for an indication of one LED being burned out? Or all of them?

Yes - Indication of one LED being burned out.

 

Are you looking for an indication of one LED being burned out?

Yes

 

That still doesn't explain why you can't simply put the indicator LED in series with the IR LED that it is monitoring?

If you are going to be driving 8 IR LEDs, does each one have it's own current limiting resistor? If not, if they are truly in parallel (generally a bad idea), then how are you going to determine which ones are not working?

You might mount an IR photodiode such that it only sees one of the IR LEDs and then use those to determine which ones actually are or are not working.

does each one have it's own current limiting resistor?
Yes

That still doesn't explain why you can't simply put the indicator LED in series with the IR LED that it is monitoring?
No, I could do that. I think that's what I need.

 

Are you looking for an indication of one LED being burned out?

Yes

If you have multiple LEDs to check, you'll need a comparator per LED. You connect the anode of each LED to a comparator and check for a voltage higher than ground (or some voltage lower than the forward voltage of the LED).

 

If you have multiple LEDs to check, you'll need a comparator per LED. You connect the anode of each LED to a comparator and check for a voltage higher than ground (or some voltage lower than the forward voltage of the LED).

Which one would you use?

 

Which one would you use?

Which what? Comparator? LM393 or LM339, depending on how many you need.

 

Which what? Comparator? LM393 or LM339, depending on how many you need.

How about putting a 1.8mm colored LED in series with the current limiting resister and LED. The indicator LED would stay Lit as long as the primary LED does.

 

How about putting a 1.8mm colored LED in series with the current limiting resister and LED. The indicator LED would stay Lit as long as the primary LED does.

That would require as many indicator LEDs as you have IR LEDs. With comparators, you could wire-OR all of the outputs and only need one indicator LED.

Not aware of any 1.8mm LEDs; 3 and 5mm, but not 1.8. Is there a new through hole package I'm not aware of?

 

How about putting a 1.8mm colored LED in series with the current limiting resister and LED. The indicator LED would stay Lit as long as the primary LED does.

That would require as many indicator LEDs as you have IR LEDs. With comparators, you could wire-OR all of the outputs and only need one indicator LED.

Not aware of any 1.8mm LEDs. 3 and 5mm, but not 1.8. Is there a new through hole package I'm not aware of?

Mouser has a lot of them. Lot of through-hole choices.
1.8mm LEDs

 

How about putting a 1.8mm colored LED in series with the current limiting resister and LED. The indicator LED would stay Lit as long as the primary LED does.

With the proper resistor value the RED LED can have 3 states.
Dim indicates IR is OK.
Bright indicates IR shorted
OFF indicates IR open.

 

Mouser has a lot of them. Lot of through-hole choices.
1.8mm LEDs

Did not know that... Learned something new.

 

With the proper resistor value the RED LED can have 3 states.
Dim indicates IR is OK.
Bright indicates IR shorted
OFF indicates IR open.
View attachment 285818

Why does this work?

What is the resistor value over the RED Led? (Vs - Vr)/.020A
R=5v-1.25v/.02A = 182.5ohms.
Nearest = 187ohms.

 

The voltage across the 200 ohm resistor is appx 3. 8 volts when the IR diode is good. That's enough voltage to light the red LED + resistor combo. If the IR LED shorts you now have 5 volts across the red LED which will be brighter and if the IR diode is open then no current can flow through either LED.