I'm new to electronics, my background is mostly on the software development side… so go easy on me, as this might be a really stupid question. I just want to be sure what is safe to assume and what not.
My project consists on emulating the IR signal sent by the RC controller of a toy helicopter so I can then command it with my hand using a LEAP motion device.
The IR signal works perfectly, but I was not being able to match the range the RC controller was capable of getting. Anything like a computer monitor in between was enough for the helicopter to fall to the ground.
I purchased the brightest IR leds I could get (TSAL6100, 1.35V@100mA) and I was definitely getting some better range than my previous IR led but still nowhere near as much as the RC controller.
Since the IR is always blinking (and not constantly on), I assumed "it wouldn't hurt much" if I started reducing the resistors and try to get more brightness (that I've been checking with a cellphone camera). I was still not being able to match that of the RC controller…. so then I started increasing voltage with a separate power source, since arduino max output is 5V and RC controller using 6 AA batteries must be somewhere around 7V
I know the dangers of doing this, but I went ahead anyway and had a 12V power source without any resistor, making those 3 IR leds blink and now I get the same and even farther range than the RC controller….. so far none has burned, however, how crazy am I for doing this ?
What would be the correct way of achieving the exact same performance as the RC controller…
Will the IR leds blinking at 38kHz ever burn out under these conditions ?
Only under these conditions I am able to see through my camera that the led brightness matches that of the RC controller….. so how are they doing it "properly" ?
Thanks in advance.
Best Answer
These LEDs are specialized on short pulses of high current, so pretty much what you're doing. The datasheet suggests that 100µs pulses can be driven by currents as high as 1A (pulse peak, see "basic characteristics" -> "Radiant Intensity").
With 38kHz your pulses are on the order of 15µs only, so you might be getting away with even higher currents. But note that the datasheet doesn't give any info above 1A (see figure 3, which might be of interest to you).
Your way of reaching those currents is dangerous for the life of the LED though. Don't drive them with 12V without a current limiting resistor, even if the resistor limits the current to something high like 1A. That's just how LEDs work, their differential resistance isn't linear. What's probably saving you right now is the batteries' inability to deliver such high currents in so short pulses. You're effectively not driving them with 12V, because your batteries' voltage drops temporarily. That's not so good for the batteries either, depending on their chemistry, or anything connected to the batteries (might brown-out). You could add a fat 47µF ceramic capacitor to buffer the energy in front of your output stage. That might help burn those LEDs :).
The reason you are getting worse results than the controller might also have to do with the angular distribution of the emitted light. The LEDs you're using are pretty focussed (see fig. 9), their half-intensity angle is just plus/minus 10°. Maybe you'd be better off with an LED that has more spread?