Matej Lajcik's build log


Hi, I’m Matej (don’t worry about butchering my name) and I’m a hobbyist for couple of years.
I’m self-taught and I don’t like breadboarding.
Just waiting for ATtiny841 breakout boards to come from China and am currrently in the middle of a more fun project -7-segment driver with a discrete diode ROM storing digits. No secret squirell here.
There are few questions I have regarding transistors as switches.

edit: link


Hi Matej,

Tell us more about the questions you have on transistors! :slight_smile:



Hi Niels,

From what I’ve seen people are mostly just arguing about driving LEDs with transistors. What is the right way. The way I did it was putting signal to the base, VCC to collector (of NPN) and from emitter to two LEDs in series and then to ground.
But I’ve never really seen anyone doing it that way. Sometimes I’ve seen people having commonned emitter and connect base and collector and throwing resistors here and there and everywhere and it’s just all so confusing. I tried to understand it from The Art of Electronics but apart from making my head ache I didn’t get much. They only seem to work with transistors as amplifiers.


This is essentially what I’m working with.

EDIT: It works just fine. I just bodged this together and it works like a treat. Taking less than 10mA at 5V.


Yes, that works but only because you’re using two LEDs with 5V. You’re right in the forward voltage sweet spot.

The traditional way of driving LEDs is to set a constant current through them, say 10…20mA, depending on the LED. That’s what those circuits you described are probably doing.



Yep, I’d recommend to stick a resistor in line at the bottom of that stack, even if it ends up being a zero ohm resistor (as it effectively was in this circuit). It’s good to always think about current flow in the circuit by inserting a linear element.


Yes, that is why I choose two serial diodes and not, say three in parallel and dropping the rest with a resistor.

The zero ohm resistor, I always thought of it as a jumper. Didn’t know it can work as a fuse. But yes it would be the weakest point of the circuit. I might want to order a few.


Great exercise - a 7 segment display with your “diode ROM”, well done! Will you be interfacing this to your ATTiny841?


Thank you, honestly I didn’t really think of it any more than “that would be a fun to do.” So far the display/ROM is standalone. But adding switch for direct control and a pin header for the digits really seems like the way to go. The good thing about being a hobbyist is that I don’t need to worry about starving to death for not having marketable ideas.
The possible future for the board would be Charlieplexing - adding another timer/counter circuit with a trimmer and grounding transistors to the segments and let it cycle one at a time. People might appreciate it for the educational value of it. Who knows!