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Resistor rating for IR LED for WiFi IoT remote

ntewinkel

Hello!

I was pondering this, and searching into the webs and on the lines, and then realized I have this awesome community of smart people here that probably already know the answers!

Short version:
Are 1/4 watt resistors enough to pulse an LED at 1 amp for an IR remote?
The LED is rated for 100mA continuous, 1.5 amp peak, and 1.6v forward voltage max.
I used an online calculator to figure out that with a 5v supply, a 6.8 ohm resistor will give it a 500mA current, and 3.4 ohms gives a 1amp current.

I'm not sure how to calculate the current on that LED - is it 1.6 volts x 1 amp? or 5 volts x 1 amp?

Either way though - even just the 1.6 watts is 6 times the current rating of the resistors I have. Is that ok for pulsed use for remote control like that?

Should I be using a set of parallel resistors instead (knowing I'll have to up the resistor values to compensate) ?

Project details:
My heat pump is kinda noisy, so at night we set it to "night mode" which is the quietest fan setting it has. We can't keep it at that setting because it won't heat the house well enough, so in the morning we turn the temperature up a few degrees and set the fan to "automatic".

I've been trying to automate this process, because the heat pump remote is kind of annoying in that you have to step through all 5 fan speeds to get back to Automatic.

I bought a little WiFi IR remote off Amazon a few years ago, which is ok for turning the heat up in the morning, but it only supports 3 basic fan speeds, so it doesn't properly turn the fan down at night. I recently bought another similar newer unit, hoping that after a few years my Daikin heat pump would now be fully supported, but it's still the same. Frowning2

But there's hope! I found a Github repository that actually fully supports my Daikin heat pump codes! And for my favourite IoT micro-controller even - the Wemos D1 Mini.

Using a basic IR LED I had scavenged from something in the past, I was able to get it to work, but the IR LED range was just -barely- able to reach the heat pump when aimed just right (I realize now that I was using far to big a resistor value).

So I got a few new "high power" IR LEDs (the clear QED234 ones) from element14's shop, which I hope will give me the range I need. Then I did the aforementioned calculations to find the ideal resistor values to get some good oompf out of the LED to reach further.

I was just about to start soldering things up when I realized that 1amp of current is higher than the 1/4 watt rating of the resistors I have.

I'm hoping you'll have some good advice for this software-biased hobbyist Laughing

Also - I'm guessing a resistor failure would generally not cause a short? (ie, losing a resistor is ok, I have plenty more - losing an IR LED is tougher as I only have a few!)

Thanks!
-Nico

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  • 0

    Hello Nico,

    some comments:

    1) Remotes usually send a modulated signal (38 kHz is a very common frequency, but anything between 34 and 56 kHz could be possible). This is good for your use case, as it cuts the duty cycle of your LED on time basically in half already. Unfortunatelly the datasheet is missing a standard diagram that shows if 50% or 25% duty cycle is OK at 1 A. The data might be UART or with a custom protocol encoded which again reduces the duty cycle, but to be on the safe side let's assume 50%.

    2) Thanks for sending the LED part number, that always helps to trigger better comments. The 1.6 V max forward voltage is only valid for a current of 100 mA. Figure 5 on page 3 shows a forward voltage of ~2.95V at 1000 mA. So your resistor to limit the current would be (5V - 2.95V)/1A = 2.05 Ohm. This would be 2.05 W through the resistor, with the duty cycle 1W. As Frank already said, there must be a switching transistor, which also has a resistance, even if it is a MOSFET. Tolerances of all components are also important, because small changes with low resistor values like this change the current a lot.

    image

    3) IR wavelength also might be important, they also come in 850 nm and 1130 nm.

    A little drawing how you plan to solder things together also might help for some more advise.

    Cheers,

     - Wolfgang.

Reply
  • 0

    Hello Nico,

    some comments:

    1) Remotes usually send a modulated signal (38 kHz is a very common frequency, but anything between 34 and 56 kHz could be possible). This is good for your use case, as it cuts the duty cycle of your LED on time basically in half already. Unfortunatelly the datasheet is missing a standard diagram that shows if 50% or 25% duty cycle is OK at 1 A. The data might be UART or with a custom protocol encoded which again reduces the duty cycle, but to be on the safe side let's assume 50%.

    2) Thanks for sending the LED part number, that always helps to trigger better comments. The 1.6 V max forward voltage is only valid for a current of 100 mA. Figure 5 on page 3 shows a forward voltage of ~2.95V at 1000 mA. So your resistor to limit the current would be (5V - 2.95V)/1A = 2.05 Ohm. This would be 2.05 W through the resistor, with the duty cycle 1W. As Frank already said, there must be a switching transistor, which also has a resistance, even if it is a MOSFET. Tolerances of all components are also important, because small changes with low resistor values like this change the current a lot.

    image

    3) IR wavelength also might be important, they also come in 850 nm and 1130 nm.

    A little drawing how you plan to solder things together also might help for some more advise.

    Cheers,

     - Wolfgang.

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  • 0 in reply to wolfgangfriedrich

    Thanks Wolfgang! I really appreciate all of your help!

    That’s a lot of learning for me Laughing 

    I wasn’t aware of the change of forward voltage with changing current, that’s interesting.

    I replied to Frank’s comment with more details, but in a nutshell yes a transistor in the most basic Arduino-101 circuit. I might be able to reduce the output to a more manageable level while still getting enough light to reach the heat pump.

    I’ll do some more math on the Daikin IR packet to see about the duty cycle.