LED on pin A4 ?

horizontech · Joined: 09 Nov 2003 Posts: 13

Hi !

I try to use pin A4 on pic 16f84a to drive a Led true a 330ohm resistor

Not work ?

I have try 4.7K pull up resistor the led stay at 25% on !!

I need some idea ?

Thanks

Alain Tanguay

newguy · Joined: 24 Jun 2004 Posts: 1909

Not too sure about a 16F84A, but pin A4 on a lot of PICs is open drain.

Mattr0 · Joined: 27 Mar 2005 Posts: 30

you are going to have todrive the LED through a transistor to get the full brightness

If you think about it you are taking the 4.7k + the 330 Ohm resistor to drive the LED. which would probably get you in the range of 1 mA The transistor takes a very small amount of current to turn it on to accomplish this task

the Dreaded a4 had me working for hours until I found that out a few years back

Humberto · Posted: Sat Apr 23, 2005 2:49 pm

If you connect a LED through a 330 Ohm pull up resistor tied to +5V, you
should get full LED bright, doesn�t matter if the PIC pin is open collector or not.
One of the best characteristics of the PIC family is that they can
source/sink up to 25mA in every INP/OUT PIN.
Re-check your hardware and if posible swap the PIC.

Humberto

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Alain's first attempt. Not working because A4 is Open Collector:

horizontech · Joined: 09 Nov 2003 Posts: 13

Thanks for you help !!!

Alain Tanguay

Guest · Posted: Sun Apr 24, 2005 1:32 pm

Can I add that although nearly all PIC pins can sink/source 25mA they can't do it all at the same time! There is a overall current limit for the device!