I would like make a lot of small projects using ATmega328P (and other ATmega family) based Arduinos (UNO R3, old Nano and similar). That means USB power (5V, 0.5A max, but typically way less, like 100mA) and frequencies up to 16MHz. I want to use lot of BJT transistors for LED switching, sniffing all signals, amplifying weak signals etc. etc. Mainly because transistors are cool and I want to get some practise with using it. I want to buy fistfull of PNP and another fistfull of NPN transistors and start experimenting, but I do not know, which types are best suited. There is too much of available types and I cannot understand which I should buy. At beginnig I will NOT be driving motors or solenoids, I want to drive LEDS, resistors and singal other transistors and MCU. (Also try open collector NOR to detect when any of many signal goes high and similar techniques, mainly for the sake of doing it). If I would like some powerfull work, I will buy some power transistors later. The question is - What transistors I should buy? They should be cheap (as I want buy like 50 or 100 of each type, maybe on AliExpress), common (so I could use the same type after many years, when the old ones are destroyed or lost) and good enought (fast, so I will not need to solve frequencies, 0-0.8V would be considered no signal and 2.5V-5V would be considered full signal and should survive some accidental missuse, which happens on breadboards (like accindental contacts to something else, full capacitor or bad polarity) without burning too often :sweat_smile:) They do not need to be excelent for HiFi audio, GHz frequencies or driving more power than 0.5W for more than split of second. What types of PNP and NPN you use for your projects and why? What types you recomend me? Thanks for all and each opinions :slight_smile:

[image] gilhad: What transistors I should buy? Any one that fits your needs. But why BJT and not MOSFET? My needs were <= 0.03$ and 15V, 100mA, 100mW at least. Best available in dual case (SMD and PTH) and handy pinout (E-B-C)..

Good question :slight_smile: Basically I had seen mainly BJT schemas around me, I started play with BJT 40 years ago (and no MOSFET was aviable for me at the time) and I remember when I got some informations about MOSFETs, it said they are expensive, hard to obtain, need big voltage and are easy destroyeable, so I was not interested. But times are exchanging and I was stuck in my old minset. If you found what fit your needs, it would fit my too and I will love to learn about it :slight_smile: (What type you suggest?)

Hi Last year I bit the bullet and purchased a pack of assorted BJT from Ali. I tested a select few and they appear ok. I made the top label and the data label on the inside of the lid. Great for just opening and going ummm.. yes... no... that will do... when the protoboard is on the bench. [20251014_172710resize] [20251014_172917resize] Tom.... :smiley: :+1: :coffee: :australia:

I actually only have BC547 & BC557 as general purpose transistors in my box.

2N3904 and 2N3906 are my stock general purpose devices for controlling low power relays etc. Readily available in the UK.

[image] gilhad: 0-0.8V would be considered no signal and 2.5V-5V would be considered full signal I think you should first learn how a BJT works. They are current controlled current devices not voltage controlled. Maybe you are thinking about MOSFETs and not BJTs

Yes, you are right. I do somehow know it, but not actively. LEDs are also current controlled, but there is some threshold, where they do not operate (I mean shine) under - something like 2V for red, 2.5V for many other and 3V for blue (approx. ), so I can use (with some resistor) 2 red LEDs in serie on one pin, but only 1 blue LED. And none of LEDs would shine for even highest values of LOW signal (Vol =4.1V). So that is what I meant with the statement (which was bad formulated). And yes, MOSFETs within such range will be better for my needs. (I just did not consider them by mistake.)

[image] gilhad: I started play with BJT 40 years ago I started with electronic tubes 60 years ago :slight_smile: BJT are fine for analogue circuits while MOSFET are better suited for binary (ON/OFF, PWM...) output. MOSFETs should be logic level compatible (IRL...) and support the output voltage. [image] gilhad: none of LEDs would shine for even highest values of LOW signal (Vol <= 1V) Digital outputs can only feed LEDs in one direction, while in the other direction they operate in parallel to the LED and can not output any current. The worst case signal levels come in 2 sets for guaranteed output (provided under acceptable load) and recognized input states.

[image] gilhad: And none of LEDs would shine for even highest values of LOW signal (Vol =4.1V). Your pin voltage is not really relevant since you want to use transistor to drive the LEDs. Pick some BC337/BC327 from AE and start experimenting how they work. Or pick $2 assortment with practical plastic box. aliexpress.com/item/1005008077649425.html Don't forget to get some resistor assortment kit as well!

[image] gilhad: MOSFETs within such range will be better for my needs For the needs described, forget about MOSFETs — regular BJT transistors already listed are perfectly fine. Consider MOSFETs only when driving high currents and when you can control the gate with voltages above 5 V (logic-level IRL types help a bit, but they’re not the best choice if you only have 3.3 V; they perform best when the gate gets at least 9–10 V). To avoid excessive power dissipation in MOSFETs, a practical rule of thumb is to choose ones rated for 10–15 times the current they’ll actually handle (in that case, a heatsink usually isn’t needed).

Universal PNP (and PNP) transistors (BJT) for ATmega328 based Arduinos - what to buy?

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LarryD October 14, 2025, 9:18pm 23

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Universal PNP (and PNP) transistors (BJT) for ATmega328 based Arduinos - what to buy?

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