# LED Resistor calculation

#### Hoof Hearted

Joined Mar 12, 2022
6
I'm building a circuit to give me a long range IR transmitter. I got the diagramn here:

Long Range/Distance Infrared Transmitter - The Circuit (easycircuit012.blogspot.com)

This shows 3 Leds in series with a 10 Ohm resistor. My Leds have a voltage drop of 1.3V each. I don't know the current rating. Let's assume 10mA. So on that circuit, the 10 Ohm circuit seems to be ridiculously small.

I'm hoping to run this circuit with 5V to be compatible with my other bits. So 5V - (3 x 1.3V) = 1.1V across the resistor. I think I need a 110 Ohm resistor. This is very different from the circuit diagram. So i've come in here asking for comments. Go easy on me please. My first time here .

#### crutschow

Joined Mar 14, 2008
30,090
To make a long range transmitter, the LEDs need to be driven with their maximum rated current, so you need to know the rating of the LEDs you are using.
10mA will likely not give you the range you are looking for.

#### Audioguru again

Joined Oct 21, 2019
4,794
Amazon sells clothes ands shoes. They know nothing about LEDs and probably don't even know who made them.
Buy electronic parts from a real local electronic parts distributor like Digikey.

#### Hoof Hearted

Joined Mar 12, 2022
6
I'm a novice. I didn't know about digikey. I'll shop there next time.

#### crutschow

Joined Mar 14, 2008
30,090
Since they don't have a current rating, you will have to guess.
LEDs that size often have a maximum rating of 20mA, but for short duty-cycle signals you can likely go higher than that.

#### Hoof Hearted

Joined Mar 12, 2022
6
I've registered an account at digikey. I'll order some 'proper' ones today. But going back to that circuit diagram than i'm working from.... It shows a 9V power supply. So if the 3 diodes have a combined voltage drop of 3.6V, then there will be 5.4V across that 10 Ohm resistor. Unless I'm mistaken this will mean that the resistor is dissapating 5.4 Watts. This can't be right because the resistor is shown as being rated at 0.5 Watts. So now i'm wondering if the Mosfet transistor limit the current? I did a lot of electronics when I was younger. I haven't done much in the past 25 years. It's still coming back to me.

#### BobTPH

Joined Jun 5, 2013
5,220
IR emitters like those are usually pulsed at 50 to 100 mA. They cannot be operated continuously at that current.

With 9V and 3 in series I calculate 500mA. So that circuit is designed fir higher power LEDs.

Bob

#### Hoof Hearted

Joined Mar 12, 2022
6
Thanks Bob, but what about that 0.5 Watt resistor? Will that be ok dissapating 5 Watts intermittently?

#### bertus

Joined Apr 5, 2008
21,940
Hello,

Bertus

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#### BobTPH

Joined Jun 5, 2013
5,220
Possibly, but why are you still talking about that design when it is wrong for you parts?

I think your power calculation is wrong.

P = V^2 / R = 5.4^2 / 10 = 2.9W.

And what are you trying to do, anyway? Do you know how to use an IR LED to transmit data?

Bob

#### Hoof Hearted

Joined Mar 12, 2022
6
Yes you are correct. I was calculating wrongly. I just wanted to understand the way that circuit works so I can make alterations to suit my purposes. Yes I do know how to transmit IR data. In the office at work, we have two TVs on the wall which show real-time information from the production database system. The trouble is..... People forget to switch the TVs on in the morning, and they also forget to switch them off at night. So i'm using an Arduino to monitor the light levels in there, and transmit IR signals to switch the TVs on/off when the office lights are switched on/off. I've mocked up a prototype already. It works sort of, but doesn't quite have the range to reach the TVs. Hence the need for a long range solution. The TVs are not together, so i'm using a servo motor to turn the LED cluster to face each TV when a signal needs to be sent to it. It's a lot of effort to fix a small problem. I just wanted a useful project to get me into Arduino development. So it's interesting for me. I don't care how long it takes.

Ian

#### Audioguru again

Joined Oct 21, 2019
4,794
The circuit you copied does not turn on the LEDs continuously, instead it pulses them on and off with a CD4047 IC.
The pin 10 output of the CD4047 produces a perfect squarewave with its off time exactly the same as its on time, then the LEDs and resistor have an average of half the calculated current. They used high current LEDs and the average current is 255mA.

You will not be driving the LEDs with a CD4047 IC, instead you will be sending the transistors an on and off code done for each TV to be controlled so you do not know the actual on and off ratio or the average current in the LEDs and resistor.