# PN2222A LED Circuit



## Tennywang (Mar 5, 2021)

I'm going to add an IR LED to my Arduino Uno, and I'm struggling to wrap my head around the math involved in determining the exact resistor values required to drive the LED using a PN2222A transistor.

I'm aware that my LED has a voltage drop of 1.35V, that I intend to run it at 100mA, and that I'll be using 5V from the Arduino to power it. What I don't get is the arithmetic for calculating the transistor's exact voltage drop between the collector and emitter. I'm also trying to figure out the math behind calculating the milliamps that must travel through the transistor's base in order for it to fully turn on (but not waste extra electricity).

I understand that I can use a wide range of resistor values and the circuit would still work, but I'm hoping to figure out the math so that I can go as near to utilizing the exact right resistor values as feasible.


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## Corday (Mar 3, 2010)

From TSF Rules:
You may not ask for assistance with homework assignments, projects or book reports for school college or university


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## TonyDi (Oct 28, 2015)

Are you sure of the 1.35V drop across the LED? I'm nowhere near being an expert on this and I haven't figured things like this out for about 40 yrs now. I would look at how much current you need thru the resistor. It's the same as the current going thru the LED. so I'd figure another 0.6v drop across the transistor. So you have 5V - 1.35V - 0.6V = 3V. You want 100 mA. E=IR. So you're looking at 30 Ohms. Use a 1/2W resistor.


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## johnwill (Sep 26, 2002)

It might be useful to know what LED operates at 1.35V and 100ma. The only LED's I know of with that kind of operating voltage are IR LED's.

How about the easy way? Use a FET to drive it, that will remove the issue of power dissipation in the transistor as well as the voltage drop.


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