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I made this circuit for a 5v logic. I plan to use it as a remote control. So pulses. i hope that with this circuit i achieve more range as with the default remote. I already made it but now i have some doubts.

IR LED Datasheet

NPN Transistor (2N3904) Datasheet

As there is a voltage drop of 0.7v because of thee transistor i simply used 4.3v to do the math

For the signal/base resistor i used 330ohm 1/4Watt just to be sure that it saturates the transistor properly.(should be around 13mA, arduino handles 20mA)

The max forvard voltage of the ir led is 1.6v it and should work with 100mA.

As the ir led resistor needs to be 1/2 Watt i used 2 resistors 68ohm+56ohm in paralell which i measured and give me 31ohm (math suggests 27ohm 1/2).

1. Would you change something?

schematic

simulate this circuit – Schematic created using CircuitLab

2. Can i use this circuit with a 3.3v logic as signal but feed the led with 5v?

schematic

simulate this circuit

I'm new to electronics and the more i learn the more doubts i have.

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  • \$\begingroup\$ Your two resistors should be in parallel, not series, to make 31 Ohms. \$\endgroup\$ – Peter Bennett Jan 10 '16 at 5:31
  • \$\begingroup\$ Yeah typo , sorry ... i corrected the question and replaced with "paralell" \$\endgroup\$ – cocco Jan 10 '16 at 5:38
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You probably can, but you may need to change the base resistor. Remember the 4.3 volt calculation you made? Do you think you ought to redo it with (3.3 - 0.7)?

Actually, what exactly was your calculation for the base resistor? The usual rule of thumb for calculating this sort of circuit is to set the base drive at 1/10 to 1/20 the collector current, but you have about 1/8.

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    \$\begingroup\$ also there are some other circuits that use a 330-470 ohm resistor. sparkfun.com/products/retired/10732 this one uses 330ohm at the base. \$\endgroup\$ – cocco Jan 10 '16 at 5:47
  • \$\begingroup\$ So what base resistor would be appropriate for both 3.3 & 5v? \$\endgroup\$ – cocco Jan 10 '16 at 5:49
  • \$\begingroup\$ it's the second diagram that shows how i would use it with a 3.3v logic \$\endgroup\$ – cocco Jan 10 '16 at 6:04
  • \$\begingroup\$ And also in that case it is powered with 5v. only the base is at 3.3v \$\endgroup\$ – cocco Jan 10 '16 at 6:23
  • \$\begingroup\$ cdn.sparkfun.com/datasheets/Kits/PowerIR-v11.pdf here is the shematic for something similar sold on sparkfun... it's for 5v ... it uses 330ohm ... my circuit just allows more milliamps to flow trough the IR led.but there is no mention if it works with 3.3v on the base of the transistor.In any case i would always power the led with 5v. \$\endgroup\$ – cocco Jan 10 '16 at 6:42

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