November 13, 2012 AT 7:39 am

ASK AN EDUCATOR! – “How can I extend the range of an IR LED transmitter?”

Alessandro asks:

I’d like to know the best circuit to drive some IR LED and get maximum distance. I watched TV B gone circuitry, but i use an arduino so I may use up to 12V..

I don’t understand why using more than one transistor I would get more current

Although your Arduino can be powered by a 12V supply, the I/O is regulated to 5V or 3.3V depending on you model. LEDs commonly operate around 1.5V (2.6V in the case of the TV-B-Gone), so some level of series resistor is required.

Let’s start by taking a look at the TV-B-Gone schematic. Specifically v1.2:

The schematic tells us that each IR-LED is driven by a companion 2N3904 NPN transistor. According to Adafruit’s datasheet on the Super-bright IR LEDs used in the kit, each LED has a forward voltage of 2.6V (max 4.0V) and a forward current of 1A max. You don’t need to current limit the LED’s with a resistor as each 2N3904 is only capable of switching up to 200mA and our supply voltage is <=3V.

We use the following to calculate the necessary current to switch the NPNs:

Ib = Ic / Hfe
Ic = 200mA //Assuming a max collector current of 200mA
Hfe = ~20 //Also assumed from the datasheet as their Hfe chart only goes to 100mA

Ib = 10mA

As the TV-B-Gone uses 4 LEDs (4 x 10mA = 40mA), you hit the 40mA limit the AVR can safely provide. This accounts for the use of the 2N2907 PNP transistor connected to each NPN's base. This transistor is then switched by setting PB0 low and the LEDs fire up!

So you see, the transistors are used to allow the AVR to safely switch on and off the high current loads required by the LEDs. Although the LEDs are capable of up to 1A of current, the 200mA allowed by the NPNs provides a significant amount of distance while ensuring a long lifespan.

If you are interested in reading more about transistors, the University of Nevada happens to have good writeup.

Good luck with your circuit and don’t blow out your LEDs!

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“Ask an Educator” questions are answered by Adam Kemp, a high school teacher who has been teaching courses in Energy Systems, Systems Engineering, Robotics and Prototyping since 2005.

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