TV-B-Gone V1.2 PNP transistor question

[Kyleperson]

I have a question about the TV B Gone v1.2 shown here:
https://github.com/adafruit/TV-B-Gone-kit/raw/master/pcb/tvbgone3sch.png

I had planned on building this circuit with only one IR LED, and omitting all transistors...until I realized that the first transistor out of the MCU was a PNP. Given that a PNP shuts off(per say) when voltage is applied, is it required that I keep the PNP to invert the high and low signal?

[EasternStarGeek]

No, it isn't.

If you want to drive the LED directly from the microprocessor, connect the anode of the LED to the positive supply through a 100 ohm resistor and the cathode to the digital output. In this case, a Logic LOW at the output will turn on the LED.

Keep in mind that the LED will not light nearly as brightly, and will drastically reduce the range.

[Kyleperson]

No, it isn't.

If you want to drive the LED directly from the microprocessor, connect the anode of the LED to the positive supply through a 100 ohm resistor and the cathode to the digital output. In this case, a Logic LOW at the output will turn on the LED.

Keep in mind that the LED will not light nearly as brightly, and will drastically reduce the range.

Something about that doesn't really sit right in my head, but I'll give it a try anyway. If you could explain, that would be great, otherwise, thanks for your help.

[philba]

The reason there are no current limiting resistors for the LEDs in that design is because IR LEDs need to be driven at a much higher current than visible LEDs. Well, they don't HAVE to but to get any kind of distance, you need to pulse them pretty hard. That's why there are transistors in that design - the AVR is limited to 40 mA on an output pin which really isn't enough current. I'd shoot for 100 mA pulse current or higher. Note also that since it runs off of 2 alkaline cells, the 4 LEDs won't see super high current. Guessing from the design docs, the IR LEDs each get around a max of 250 mA pulses with 2 AAs, driving them out of spec. This definitely shortens their life but, given that the duty cycle is miniscule, probably doesn't matter.

So, I would use the transistor design of the original plan.