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I've done some research but can't seem to find an answer to my specific problem.

I'm using an Arduino as a beam-break sensor. In order to do that it modulates the IR LED at 38kHz by code and listens for this signal on the sensor pin. Now, I want to increase the range of this system to 15m using a retroreflector on the other side. In order to do that I need more power on the LEDs than the 20mA the Arduino can output.

As I understand it I will need a transistor to do this.

I have ordered these LEDs

And these TIP120 transistors

Will I be able to accomplish having 4 LEDs running through this transistor at 100mAh each and then have the Arduino modulate the transistor at 38kHz so the combined output of all LEDs is modulated?

I already have a 12v power supply in place so that will be the supply voltage for the system.

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  • \$\begingroup\$ IR LEDs usually only drop about 1.5V. So you could just wire 4 in series, and select an appropriate current limiting resostor. \$\endgroup\$ – Simon B Dec 3 '18 at 16:22
  • \$\begingroup\$ I was thinking that as well. My problem, however, is if I understand the use of the transistor correctly and if I need anything else for this to work \$\endgroup\$ – BlueSails Dec 3 '18 at 19:01
  • \$\begingroup\$ You will also need a couple of resistors. Driving a transistor from an Arduino is such a common thing, you can just Google it. Find an example of "low side switching". \$\endgroup\$ – Simon B Dec 3 '18 at 23:43
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In addition to the transistor and LEDS, you will need a couple of resistors. It will be easiest to wire the diodes in series, with one resistor for current limiting. you should also add a resistor before the base of the transistor.

See this Stack Exchange answer for an example. Circuit A - "low side switching" - is almost certainly better.

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