# Choosing Resistors for IR LED

#### domii77

Joined Nov 12, 2021
4
Hi guys, I hope I'm in the right place for asking this question.

Note: I'm a complete amateur in regards to Electronics so apologies if this might be a stupid question..

I've recently built a headtracking device for use with flight simulators. The circuit is as simple as it gets: A 3V Battery-Pack (2xAA Batteries), 3 IR LED's connected in paralel. The LED's are rated for 1.35V and IF of 100mA and peak IF of 200mA.
Now i calculated that i would need 16ohm resistors infront of each LED's for this to work, and it does. But the LED's are a bit to dim for it to work very well with the program i use. Now i was thinking that if I drop the Resistor values, the LED's will be brighter. I know that 200mA can only be sustained for a short time.

So heres my question: How low can I go with the Resisto Values for this to still work safely over longer periods of Time? Or should I approach this differently?

#### Alec_t

Joined Sep 17, 2013
12,473
Welcome to AAC!
If I'm reading the datasheet correctly, 200mA can only be drawn for 100 microseconds, with a duty cycle of no more than 50%.
How long a pulse does your program provide?

#### domii77

Joined Nov 12, 2021
4
Welcome to AAC!
If I'm reading the datasheet correctly, 200mA can only be drawn for 100 microseconds, with a duty cycle of no more than 50%.
How long a pulse does your program provide?
So I'm not entirely sure what pulse means in this, but as far as the circuit is concerned, theres no code involved here.
Just a battery pack attached to some LED's. All they do is provide light at the side of my head, that is pointed at a camera with an IR-Pass Filter. From here I can use software that determines the position of my head. And heres the Issue with brightness. The LED's aren't always bright enough for the program to recognize them, especially when I turn my head. And boosting camera signal doesn't work since there would be too much backround noise in the image. I'll attach the data sheet of the battery pack, just incase I misunderstood and pulse has something to do with that.

#### ericgibbs

Joined Jan 29, 2010
14,701
hi domii,
What the data sheet is stating is that 100mA is only allowed as a 20mSec on time [pulse].

What actual current do you want to drive the Emitter, if you use a high current, the AA batteries will not last long before being discharged.
E

#### domii77

Joined Nov 12, 2021
4
hi domii,
What the data sheet is stating is that 100mA is only allowed as a 20mSec on time [pulse].

What actual current do you want to drive the Emitter, if you use a high current, the AA batteries will not last long before being discharged.
E
View attachment 252505
Okay if I unterstand correctly: Currently in the Circuit I have 3V Powersupply, the Voltage drop over the LED is 1.35 and I'm using 16ohm resistors. So according to the formula (that I found online) I=V/R, the current would be I=(3V-1.35)/16=0.103A. So is that allready too much if I have this thing on continuously? I mean I allready used it a bunch and it doesn't seem like the battery goes out too quickly. I'm wondering if I can increase current to make the LED brighter, or would that wreck the LED?

#### BobTPH

Joined Jun 5, 2013
4,284
brightness. The LED's aren't always bright enough for the program to recognize them, especially when I turn my head
I think your problem is that these LEDs are highly directional, and you are trying to detect them at too large an angle. These are rated for an angle of only ±17°. You want ones that are wide angle, like ±60°.

Bob

#### Alec_t

Joined Sep 17, 2013
12,473
I mean I allready used it a bunch and it doesn't seem like the battery goes out too quickly.
Perhaps the battery's internal resistance kept the current below 100mA. Otherwise, you're lucky the LEDs have survived.

#### domii77

Joined Nov 12, 2021
4
I think your problem is that these LEDs are highly directional, and you are trying to detect them at too large an angle. These are rated for an angle of only ±17°. You want ones that are wide angle, like ±60°.

Bob
Riiight i didn't even know that was a thing. I think this might just be the solution to my problem. Thanks for the tip!

#### Audioguru again

Joined Oct 21, 2019
4,127
You calculated resistors when using a power supply but the batteries are probably 2V instead of 3V because they are worn out.

#### bassbindevil

Joined Jan 23, 2014
237
You could sand the convex tip of the LEDs flat to make them less directional. Use progressively finer grades of sandpaper (and if necessary polishing compound) depending on what sort of finish works best in your application. Or use Big Clive's "cracked ice" technique of just clipping off the ends:
http://www.bigclive.com/ice.htm

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#### kaindub

Joined Oct 28, 2019
92
You mention an IR filter
I suspect that the circuit actually calls for IR LEDs and you are using normal (red ) LEDs.
Change to IR LEDs and use the normal bias and you will find that the sensor is sensitive enough.
The Red LEds do not have much content at IR frequencies.