How to power IR LEDs with arduino?

Question

I need a circuit or something to power up IR leds as arduino only give 5v and IR leds needs 1.2V. Is there a way to make one arduino pin a power supply of 1.2 V code or something ?

Answer 1

Typically, rational control of an LED sets the amount of current that goes through the LED, rather than setting its voltage. You could add series resistors so that when the desired current is flowing through the LED, about 1.2 volts is across the LED and the rest (say 3.6–3.8V) is across the resistor. (See Google images for typical circuits, and see various LED resistor calculator, eg the LED resistor calculator at hacktronics.com.) It also is possible to use PWM to control LED brightness.

Note that if you have 3.6V across a resistor and 1.2V across an LED, you are wasting 3/4 of the power expended. When LEDs are low voltage (ie, like 1.2V for your LEDs, vs. 1.6 for typical red or orange LEDs, vs 3+ V for white or blue) you can put several of them in series. For example, put three of your IR LEDs in series, and calculate R so that 1.2 V drops across a resistor and 3.6 across the series of LEDs. Also try four LEDs in series, and calculate R to drop 0.1 to 0.3 V.

Edit 1: Ignacio Vazquez-Abrams says, a little obscurely, “Getting warmer. But this is a IR LED” and also “Pull up the datasheet for a IR LED. Read it carefully. The AVR can't handle one of the numbers”, perhaps in reference to IR LED current requirements.

Typical 5mm IR LEDs can handle several times more current than typical 5mm visible light LEDs. For example, a typical green LED might be rated 30 mA max sustained current and 20 mA recommended, while a typical IR LED might be rated 100 mA max and 50 mA recommended. Also note that 50 mA is 10 mA more than the 40 mA that most Arduino pins list as absolute max. A sustained current of 40 mA or more in or out of an Arduino i/o pin may damage the processor. If you want to drive a high-current IR LED, use an external driver (eg a transistor). See eg the answer to How can I efficiently drive an LED? in Electronics SE, which shows dropping resistor calculations, constant current sources, etc. Also see links given in JYelton's answer to Infra Red LEDs - Voltage Advice.

Treesrule14 points out “IR leds ... don't emit in the visible light range”, which is true; they emit infrared radiation, or heat. Note, using an IR LED in a remote control usually is incompatible with controlling its average brightness with PWM; the pulses used in PWM would conflict with the pulse train in typical 36, 38, or 40 KHz IR control protocols.