Question about IR LED current and range - Page 1
Forum section: electronics / beginners

author: kornel.f
author_rank: Newbie
timestamp: November 09, 2021, 01:27:04 pm
content: Hello!Currently I'm working on a project in which an ESP8266 (NodeMCU v0.9) will control a TV using an IR LED (a Universal IR Remote project).The LED I bought from the online store has no name, so I can't provide the datasheet. I've read online that the typical IR LED maximum current is about 100mA and that some people suggested just below 100mA should be enough for hobbyist projects.So I designed the following circuit [see image 1]. Which gives just about the right output current from ESP (~11 mA) and current on IR LED (~80 mA) according to simulation below [see image 2].But when I tested it, the IR LED just won't light up enough to be able to control my TV even from a distance of 1 meter (I checked using my smartphone camera and the light is very faint).So I decided to remove the 20 Ohm resistor so that the circuit looks like this [see image 3]. And now the remote works perfectly fine up to 3 meters. But according to simulation, the circuit would produce a current of about ~500 mA on the IR LED [see image 4].Curious, I decided to measure the IR LED current with a DMM using the DC Ampere setting. It only shows a current of about 7.5 mA.I have a few questions regarding this (sorry if some of them are stupid):1. Is 80 mA not enough for an IR remote from 1 meter range? What's the adequate amperage?2. Is it okay to remove the 20 Ohm resistance? As far as I know LEDs have very little resistance and I'm surprised that it works and nothing burns.3. Why does my DMM measure a current of only 7.5 mA? I know that this has something to do with duty cycles and averaging of currents. But then how can I verify the circuits I designed and simulated?Thank you very much. This is my first post here

author: Terry Bites
author_rank: Super Contributor
timestamp: November 09, 2021, 02:05:57 pm
content: In the Sim your BC547 is being driven into hard saturation with a base current of ~10mA, the GPIO pin is limited to 12mA, far too close for comfort. I've not seen a graph of Io vs Vo but I would imagine the 3.3 V had dropped a fair bit under that load. Check the voltage at GPIO pin when flashingYour Sim model uses a drive source with no defined output impedance (unlike your GPIO) allowing it to source infinite current! (I know, infinity is not a number)

author: kornel.f
author_rank: Newbie
timestamp: November 09, 2021, 02:30:23 pm
content: Quote from: Terry Bites on November 09, 2021, 02:05:57 pmSo what you're saying is:1. The input voltage from ESP8266 wouldn't be 3.3V anymore due to the close-to-maximum current sourcing.2. The 3.3V Vcc may have some resistance, so that effectively, that resistance limits the current on the LED.And due to both of those points, my simulation will be incorrect. Thank you!Do you have some suggestions on how I can model circuits closer to reality? Specifically in this case and in general if you don't mind. Thanks again.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 09, 2021, 02:43:50 pm
content: For IR remote control signal the LED is no all the time but moulated with an off ratio of usually less than 20% often less than 10%. For the sort time the current for the usual IR LEDs can be quite a bit higher. Some 400 mA or 500 mA is OK than.The IR LEDs come with different focussing. The ones with smaller opening angle get more power to the center, but need some aiming.If one has enough voltage, one can sometimes used 2 LEDs in series.The maximum distance about scales with the square root of the LED power. So it takes considerably more current to increase the maximum distance.The IO pins have a kind of effective series resistance at the output. Especially with older chips the resistance was often higher for the high ouput level.I would model the IO pin with some 100 Ohms of series resistor to the signal.For the supply one can also use some resistance for the power source (e.g. as parameter in the LTspice voltage source). For a real world ciruit there should be capacitors to buffer the supply.

author: kornel.f
author_rank: Newbie
timestamp: November 09, 2021, 03:01:28 pm
content: Quote from: Kleinstein on November 09, 2021, 02:43:50 pmAhh, I see. That answers my questions. Many thanks!

author: Renate
author_rank: Super Contributor
timestamp: November 09, 2021, 06:32:21 pm
content: What Kleinstein said.They whale on those LEDs, it seems sometimes that the current limit comes from the battery, not the circuit.I think that I've seen up to an Ampere.