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Simple mulitplex question

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Pepsiiuk

New Member
Hey

I'm designing a circuit that uses a pic16F876 to mutiplex some red LEDs. In the data sheet it says the pic can only sink/source a maximum of 25mA per IO pin. Is this a limit caused by the design of the chip or is it a case of it can sink/source more but it would damage it?

If it is a case of 25mA being a limit, if I wanted to drive a LED with 25mA from an IO pin, would a resistor still be required? Or do i still need one to create the desired voltage drop?

Thanks.
 

Pepsiiuk

New Member
Use resistors, it's not a safe limit on the chip, just a maximum recommended current.

You might check my PIC multiplexing tutorials?.
Thanks for that.

I might if i get stuck again. I just needed to get that cleared up so i could start prototyping.

Thanks again.
 

colin55

Well-Known Member
Don't forget, when you multiplex 4 LEDs, each LED will only get 5-6mA.

That's why you need drivers.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Don't forget, when you multiplex 4 LEDs, each LED will only get 5-6mA.

That's why you need drivers.
Actually, my tutorial multiplexing 64 LED's in a 8x8 matrix is surprisingly bright, even without drivers.

Obviously if you're trying to make an outdoor display drivers are essential, but for indoors you may be OK.
 

Pepsiiuk

New Member
It’s an indoors only project, well for the time being anyway.

I've already thought of that. If I do require drives I’m planning on using a single transistor per display. Would this do or would I need a specific driver IC? (I don’t know of any off the top of my head)

If i can just use transistors would it be better to have the transistors connected to the 5v and have the pic sinking the current or the transistor connected to 0v and have the pic sourcing the current?

Wouldn't this exceed the pics max sink/source limits?

Thanks
 

colin55

Well-Known Member
I have produced a simple 15 x 7 matrixed display using PIC16F628 and 4017.

See 15 x 7 project on: talkingelectronics.com website
 
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