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Questions about infrared LEDs for remotes
Peabody:
I've built my own Android version of the IR remote for Roku devices, but with a bit different functionality. It's working, but the IR seems a bit more flaky than a real Roku remote, and the Roku remote is brighter on my phone's camera.
I borrowed the LED from an old universal remote, so I want to assume it's 940nM but don't know for sure. My Arduino is currently powered by a regulated 3.3V, A GPIO pin drives an NPN transistor, which in turn provides collector current from the 3.3V supply through a 33R resistor to the LED, with the emitter at ground. The base resistor is 2.2K. The voltage drop of the LED is 1.2V, and it's kinda blue. The Roku remote is powered by two AAAs.
So I'm getting more than 60ma through my remote's LED, which if the relative brightness thing is meaningful, makes me wonder how much more current the Roku remote LED could be getting. But specifically I have these questions:
1. Is there any reason to believe that the Roku IR system uses anything other than 940nM or anything other than 38K? Rokus seem to work fine with universal learning remotes, which I would think all use 940nM LEDs.
2. Is there any reasonable way to determine whether an LED is 940nM or 850nM?
3. Do IR LEDs come in high-efficiency super-bright versions like red LEDs do? What spec would tell you that an IR LED is one of those? Does anybody have a favorite that's very "bright" on very few milliamps?
In case it matters, and it might, the Arduino is 3.3V, 8MHz, and the IRremote library is driving the LED, I think this library was originally written for 16MHz, and I'm not completely sure the shift to 8MHz, which it appears to do ok, is actually being done correctly. When the battery holder for it arrives, I plan to run this on a single 18650 with no regulator.
Thanks for any words of wisdom.
oPossum:
Roku is 38 kHz. Your LED current is lower than what is usually used in remotes. Use the unregulated supply and current of 500 mA to 1A. A darlington config will be required. IR LEDs do vary in efficiency and also beam width. Vishay makes some excellent IR LEDs in the TSAL6000 and TSAL7000 series. 940/950 nm is the right wavelength for consumer IR.
Peabody:
--- Quote from: oPossum on September 16, 2019, 03:44:48 am ---Roku is 38 kHz. Your LED current is lower than what is usually used in remotes. Use the unregulated supply and current of 500 mA to 1A. A darlington config will be required. IR LEDs do vary in efficiency and also beam width. Vishay makes some excellent IR LEDs in the TSAL6000 and TSAL7000 series. 940/950 nm is the right wavelength for consumer IR.
--- End quote ---
Thanks very much for the information. But I'm confused about the current needed. The Roku remote and all the others I've seen are powered by two alkaline cells, so, essentially 3V. To get 1A of current through an LED with a voltage drop of 1.2V, the resistor would be 1.8R. Or even at 500mA, it would be 3.6R. Is that right? Or do the designers rely on internal battery resistance and not even bother to use a resistor for the LED?
Anyway, I will look at the Vishay LEDs. Thanks very much for that lead.
oPossum:
A typical remote will have a resistor of a few ohms. Sometimes less than an ohm. I usually just rely on the internal resistance of AA batteries and the PN2222A to limit current to about 1A.
Peabody:
Ok, this makes sense now.
I looked at the Vishay LEDs, and the intensity rating for the TSAL6100 is twice that of the 6200, which in turn is twice that of the 6400, all at 100mA, and all at the same price. Is there a reason why I wouldn't pick the 6100? It seems you simply get more IR output per mA, and it's hard to see how that could be bad. Is there a tradeoff I'm missing here?
I'll be using an 18650 to power this. So I think I will include a resistor. Also because these TSALs are rated at 100mA, 200ma peak.
I found an old MPSA13 Darlington in the junque box. Rated at 500mA, wich would work with the Vishay LEDs, but not if I really need 1A.
A question about your circuit. If you put the LED in the path from Vcc to both PN2222s, instead of in the path to only the second one. would that also work, and eliminate the need for the 47R resistor. If I use the MPSA13, it would have to be done that way.
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