NPN BC547 SOURCING + SINKING

Other Hardware LEDs and Multiplexing
1

Hey Guys.. im trying to build a 8x8 matrix using Arduino + 2x 74H595 to make the control.
To properly supply the matrix im trying to use BC547 like this: NPN Sourcing (5v Collector) + LED - NPN Sinking (GND Emitter).
It works, but im getting some strange randon flickering during the refresh..
Im wondering if there is something wrong with this scheme or maybe its the breadboard fault due to the high speed switching..

Here is my scheme... im using 10k resistors between 74H595 and the base of BC547.
I tried to put a "pull down" resistor between base and ground of the transistor but no changes at all..

Any ideas?

I know there are some "dedicated" ics to source and sink that i could use, but should this config work?

Thanks!

2

but should this config work?

Not very well, if you source current with a NPN transistor you will only ever get a voltage that is 0.7V below the logic high which could be as low as 4V.
However

but im getting some strange randon flickering during the refresh..

is due to either lack of decoupling:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
or a software problem.

3

As Mike says on both counts, plus by using BC547 transistors rated at only 100mA you are limiting the current you can safely run with to 12.5mA per LED.

4

Thank you all, guys! Ill try a more appropriate circuit! :slight_smile: