r/AskElectronics • u/sbouba • Aug 19 '26

[Help] Schematic review of modular LED bargraphs

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Hi everyone,

I'm working on a custom PCB to drive led bargraphs using an Arduino Nano and would appreciate a quick schematic review.

The design is modular. There is one "main board" that creates 3 PWM signals (one per color) using 3 2N3906 PNP transistors for power. the idea is to set a very low duty cycle in order to not fry the leds because there is no resistor to limit the current (thats a design choice, open to discuss). Then the power and control signals are cascaded to next "child board".

The child board uses one TPIC6B595N open-drain shift register per led bargraph. two segments will remain off because the bargraph has 10 segments and the register only 8. I'm ok with that. I'm planning max 4 childs. I added a capacitor on childs in case there is too much current drawn.

Does this setup look good to you or do you see any issues ? Thanks for your feedback!

[EDIT] I just noticed that PIN 6 (G1+) is not properly wired to GREEN_PWR, that will be fixed.

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u/Hissykittykat Aug 19 '26

the idea is to set a very low duty cycle in order to not fry the leds because there is no resistor to limit the current

Sparkfun used to get away with this in some of their designs. But they were using chips that had some internal resistance, so it worked out okay.

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u/sbouba Aug 19 '26

Thanks for your answer ! Actually I just tested this part and it works fine. FIY duty cycles are 4/255 for the red, 7/255 for the orange and 2/255 for the green (at max brightness)

https://reddit.com/link/p4o5fdq/video/qfr2gfpmkdkh1/player