NPN Transistor selection...so many to choose from!

I am looking for a SOT-23 packaged transistor.

The device will run off a 3V CR2032 cell.

The base of the transistor is on an ATTiny84V pin, so should output at ~2V to 3V during use.

The pin will be pulsing a square wave 38Khz to the transistor base (to modulate the IR LED).

Thank you for any help!

|500x324

Googles your friend: https://electronics.stackexchange.com/questions/29029/choosing-the-right-transistor-for-a-switching-circuit

Is the schematic shown what you intend to build? If so, the MMBT3904 shown would probably work. What part number IR diode are you using?

hammy: Googles your friend: https://electronics.stackexchange.com/questions/29029/choosing-the-right-transistor-for-a-switching-circuit

Yep, thanks for letting me know that Google knows the links to electronics distributors. I was more concerned about a more generalised "holistic" selection of a component, and if anyone has more in-depth knowledge about part selection for my intended use...say if noise/rise time/capacitance/gain/voltage drop/sag etc. would effect my preferred outcome. Google is great for generalised knowledge, but when it comes to specifics, sometimes a human who has experienced a similar set of circumstances tends to more of help.

If you wanted to know about the sub uS electron transfer in the c-559 chain and its implications for synthetic modelling of biological energy systems, you'd be best talking to someone with a doctorate in the area of study, who will give you ideas/places to locate information...they usually wouldn't respond "Google".

Anyway...

avr_fred: Is the schematic shown what you intend to build? If so, the MMBT3904 shown would probably work. What part number IR diode are you using?

Thanks, I felt they seemed a decent choice (price, availability and they seem to have the correct specs).

I am using TSK5400S side looking LEDs: Datasheet for Vishay TSK5400S LEDs

I was more concerned about a more generalised "holistic" selection of a component,

My technique is to look at the distributors catalogue and order them by price and if relevant collector current. Then choose the cheapest. Worked for me the last 50 years.

Grumpy_Mike: My technique is to look at the distributors catalogue and order them by price and if relevant collector current. Then choose the cheapest. Worked for me the last 50 years.

Thank you. I guess it only becomes an issue with higher frequencies? Gain bandwidth becomes a bottleneck in a way?

Yes unless you have specific requirements a general purpose transistor will do in the vast majority of Arduino related applications.

As shown, the IR switching transistor is being overdriven. The accepted base current for driving a transistor into saturation is IC/IB=10 and this is confirmed on the MMBT3904 datasheet. That would raise the base resistor to from 100 to approximately 240 ohms, dropping the base current more than 50% for the same collector current of 100ma.

If you’re looking for battery life, a ~10ma savings should be considered. Sure, the duty cycle is low but as the battery weakens, the voltage drop will be higher than necessary with that increased current.

avr_fred:
As shown, the IR switching transistor is being overdriven. The accepted base current for driving a transistor into saturation is IC/IB=10 and this is confirmed on the MMBT3904 datasheet. That would raise the base resistor to from 100 to approximately 240 ohms, dropping the base current more than 50% for the same collector current of 100ma.

If you’re looking for battery life, a ~10ma savings should be considered. Sure, the duty cycle is low but as the battery weakens, the voltage drop will be higher than necessary with that increased current.

Thank you! The base resistor was something I was going to look up later. I have “upgraded” to the new ATtiny841 over the old ATtiny84s which will save me maybe 10-20% juice while active.