SEQV4 : Adding leds to gate out

5 posts in this topic


I continue here to get about my problems.

So, i made a separate eurorack panel with a TPD, and the CV outputs ( CV/Gate, / clocks / triggers)

I want to add leds for the gate outputs.

I checked some schematics about how it is done. (Especialy Hexinverter midi2cv, Yusynth, and Altitude euro Aout here on midibox)

i understand how it works, but i am not able to get it working.

Some data :

Gate Voltage mesured : 4.8V

Leds : 20mA 1.85V ( so 470R resistor)

Transistor : 2N3904


If i understand this is a NPN, so closed by default, and it will opens only when a bit of current is flowing, (current provided by the gate)


But the led is always lit.


Should i ad a resistor between to leg to create a kind of fake load ?



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This is still a bit of guesswork. Did you make a custom PCB for your panel? Or do you have separate boards for each component/panel-wired jacks? Is the DOUT shift register chain on a regular DOUTX4 board, or something different? 

How have you tested the setup? If you have a breadboard/stripboard prototype then let's see a photo with all of the wiring, especially the transistor pinout. Are you certain that the gates are really firing on and off? With a SEQ or another way?

shows the B-E resistor. It provides a current path to 0V/ground so the transistor can turn off rather than leaking through the junctions. It's not 100% necessary, but something to try. Another option is to use an octal transistor array e.g. ULN2803 (inverted) or UDN2981.


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Thanks for your answer ! I resolved my problems, and it is now working, but i don't understand what i did, led was always lit, i shuted down, and re power on my SEQ, and it was working perfectly ? Really strange behavior.


But it was really interresting to test it. i realise how i loved to build kits, but how it is bad to build something you don't understand.

I read a lot yesterday about simple components, and decided to buy a book to learn deeply what i am doing, i suppose this is the next DIY step. but after 4 years, i really need to understand. i also realise how, i hated the electricity at the school, and haven't remembered anything !


So i restart from nothing, and i feel happy to learn htings !

Sorry for the philosophical post.

To explain, i have a DOUT on a seperate PCB, i will build the gates led on a separate breadboard, and attach it on the back of my eurorack panel, with the AOUT _NG, and the TPD.


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Okay, I'm glad it works now. Turning it off and on again is usually a good thing to try :). Often CMOS needs powering down because they're in "latch up" -- a fault condition that can only be resolved this way. Or just resetting the SRIO data etc. 

Happy building and learning!

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Ok, all is working like it should be i haven't connected everything yet.

One questions.

I use a TPD and the CV/GATE/CLOCK/TRIGGER side by side.

TPD is connected and powered by the SEQ PSU, which is a PSU with no connection to ground. (i will have to connect the CORE_STM32F4 ground to it)

I use my eurorack power supply to power the AOUT_NG, Only the +12V/GND/-12V

If i understand correctly, on the AOUT, the GND will be different from 5V and +12/-12V.


Can it be a source of problems like ground loops ?

Is there a problem about removing the +5V and GND from the J1 connector on the AOUT_NG and power it with the eurorack PSU ?


By writing this expect another problem, GATE/CLOCK/TRIGGER, come from a DOUT, which have is proper ground so anywat i will get a ground loop ?


Thanks for your asnwer !

EDIT : (another thinking)

I have a SEQV4 PSU which provide +12/-12V so i can power everything from it, but, same problem, i will have ground loops between my eurorack PSU,adn the SEQ PSU, when i will connect a jack between CV/GATE/CLOCK/TRIGGER jack's and an input of my modular ?


Edited by Lamouette

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