FreeTrack Forum

I received my three sfh206k leds by Osram and if I connect one to 1.5V battery  it doesn't work(shine). All my remotes are detected by FT, so it's just a led problem. I tried reversing it, but no success.

Manual says it's 1.3V led. I also tried 1.2V battery and it doesn't work

I need this to work so i can move on, making a cap.

Thank you for suggestion.

Don't forget to put resistor before to test with 1,5v battery .... (because you can burn your led ...)

Well, i see you make quite a common mistake with led voltage here.
One thing to remember is that a LED is not a lightbulb, which are rated for different voltages, like a 3V lightbulb means that you'll need 2 1.5V batteries for it.
What decides about LED brightness is the amount of current flowing trough it, not the voltage - so the same LED can as well work powered from a 2V battery with 20Ohms resistor, as with 100V battery with a about 2 KiloOhms resistor (explanation below).

Actually, the only thing you need to know in order to understand how the LEDs should be powered is Ohm's law:
http://en.wikipedia.org/wiki/Ohm%27s_law
I=V/R: the current flowing trough an element equals to voltage across it divided by its resistance.
In short, causes the current to flow is voltage (the bigger the voltage, the bigger the current) and what inhibits current flow is resistance (the bigger the resistance, the smaller the current flow). So it's all about finding a proper resistance value for given voltage, since the current is known: most IR LEDs have maximum current 100mA (any more can damage them), but they work ok at anything from 30mA (this will be enough only with IR filter removed) to 80mA (should be enough even with IR filter present, but will cause more battery drain). About  50mA is usually the safest starting point.

Another thing to remember is that the current that can be supplied by a battery is INFINITE (as opposed to a stabilized power supplies), and it's limited only by resistance of connections between battery and other parts, and any resistors in the circuit - that's why the battery heats up when its leads are shorted - extremely high current flows trough them, causing them to heat up.

And the forward voltage given in the LED data sheet is not the voltage required by LED to work, but voltage drop across the LED. That means that when you connect a LED with 1.3 forward voltage to a 4.5V battery (it's just an example, don't do it - you'll burn the led if you use it without a resistor) and measure voltage between its legs, you won't get 4.5V, but 3.2V (4.5V - 1.3V: battery voltage - voltage drop) - and this is the voltage that will cause the current flow, and will be used for resistor calculations.
If you connect few LEDs in parallel, the voltage will drop across each one by 1.3V, so the voltage drop across 3 LEDs would be 3.9V. So if you tried to power these LEDs from 3V battery, they won't work at all, because the voltage drop across LEDs is greater than power supply voltage, meaning that there'll be no more voltage left to cause the current flow.

Connecting your LED with 1.3V forward voltage to a 1.2V battery is exactly the same case - voltage drop is simply bigger than battery voltage and no current is flowing trough the LED. Similar situation happens with 1.5V battery - even if the battery really has exactly 1.5V (they usually have less, and you'd have exactly the same situation as above), voltage across the LED is only very low, and even the little resistance of connections between LED and battery is enough to limit the current flowing trough it to a value at which the LED won't shine.

So, it you want your LEDs working properly, they need some voltage overhead above their voltage drop - at least 1V, but a bit more would be even better, plus a resistor to limit the current flowing trough it to a safe level.
Here's an example: if you try to connect your LED directly to a 4.5V battery, the voltage across the LED would be 3.2V. Resistance of a typical connection between a LED and battery is something below 0.1 Ohms, but i'll use this value from calculations. So current flowing trough the LED is:
I=V/R = 3.2V/0.1 Ohm = 32 Amps (that's 32000mA!!!) - that's 320 times more than the LED can actually handle - this amount of current will burn the LED before you'll even notice it shining.

So knowing all that calculating proper resistor value for any given setup is easy. Let's say you want to build a 3-point setup, with 5V battery, and your 1.3V LEDs with 50mA current powering them:
if you calculate the voltage drop across 3 LEDs connected in parallel, you'll get 3*1.3V=3.9V
So voltage actually causing current flow across them is:
battery voltage - voltage drop = 5V - 3.9V = 1.1V.
Using this value and required current value of 50mA, you can write Ohm's law as:
R = V/I = 1.1V/50mA = 22 Ohms

If you put these values to led assembly wizard on freetrack website: 3LEDs, 5V batteries, 1.3V forward voltage, 50mA forward current, you'll get exactly the same result - 22 Ohms resistor in serial circuit.

That's really not that hard - just some basic maths. And if you still don't know what it's all about, just use the led assembly wizard and build a circuit based on it
http://www.free-track.net/english/hardware/calcled/

And remember - never connect you LEDs directly to a battery - not even for a short time.

[EDIT]
One more thing i noticed - the led calculator on the site requires a period as decimal separator, not comma or it won't work properly, so inputing 1,3 as forward voltage will cause wrong results, you'll have to use 1.3
[/EDIT]

I finnaly put this thing together. The trick was that the cathode(bigger part) side of led is on a + side of power supply. Don't know how's that possible, but it works for me.

And i'm using 5v power supply and 22 ohm resist. So tommorow i'll start with some testing&tweaking in FSX.