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Darlington transistor ULN2803A problem
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kuroguro



Joined: Apr 06, 2009
Posts: 1
Location: Denmark

PostPosted: Wed Apr 08, 2009 2:30 am    Post subject: Darlington transistor ULN2803A problem Reply with quote  Mark this post and the followings unread

Hi all.

I'm trying to make a midi board for a Solina stringer, and I've run into a problem.

My idea was to use Octal Darlington transistors (ULN2803A) and use each transistor to trigger a note.
My problem is that when a key on the keyboard is pressed - a negative voltage (-17V) is coming through.

How on earth can I achieve that when using digital logic (0 and 5V) to control the darlington transistors ?
I thought of connecting the -17V to the last collector of the darlingtons, but then I need to convert the logic to corresponding negative voltages (as a base-collector voltage difference 1.4V will turn the darlington on)
Otherwise I could leave the base "floating" when it's supposed to be off.

I intended to use 74HC595 shift registers to control the 2803's, but even though they are "tri-state" - there's no way to replace logic oFF with "floating".
At least not a way I know of.

Does anyone know of a shift register, where I can output a negative voltage or "floating" ? Or have some other idea of how I can convert digital to -17V for 49 notes ?

I hope someone can help me - thanks.
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stewpye



Joined: Apr 30, 2009
Posts: 49
Location: Brisbane, Australia

PostPosted: Thu Apr 30, 2009 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

I'm new to this forum. I've been on synth-diy and the stompbox forum for years.

You can do it with just a shift register, 1 NPN and 1 PNP transistor (and 2 resistors). It would be best with the 74LS596 open collector shift register. I did a spice simulation to check it works. I've attached a PDF schematic. Let me know if you want the spice file.

R3 would be the pullup resistor for your open collector output. when it is pulled low (as shown) the -17V is switched on. Let me know if you need any further explanation.

Cheers,
Stew.


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stewpye



Joined: Apr 30, 2009
Posts: 49
Location: Brisbane, Australia

PostPosted: Thu Apr 30, 2009 1:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Actually, if it's a 0V - 17V switched signal you need you could arrange it a bit differently, but I have to go to work now...

Stew.
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